Talk:Photon/Archive 3

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What's still needed? Please help!

• A reference for how the photon came to be symbolized by γ, or a link to a scientific nomenclature committee that has established it so.

I don't have a ref handy, but I'm pretty sure it derives from the early days of radioactivity, when the three differently-behaving rays were named alpha, beta, and gamma. Later, when the gamma rays were discovered to be the same as X-rays and light, that is, electromagnetic, the name propagated to all EM "rays". Dicklyon 03:00, 4 October 2006 (UTC)

• Do we need more on the interactions of photons with matter? It's dangerous because it's a huge field and the article is already long. Moreover, it doesn't pertain intrinsically to photons, although this is debatable. Your thoughts are welcome!

I think not much more, if any. Depends, though; someone might write something I like. Dicklyon 03:00, 4 October 2006 (UTC)

• A detailed account of photon spin is missing. It should include the 1935 Beth experiment. It should mention the problems with a gauge-invariant expression of photon spin and with its conservation law.Aoosten 22:46, 12 December 2006 (UTC)

From encyclopedia Britannica: Six years after the discovery of radioactivity (1896) by Henri Becquerel of France, the New Zealand-born British physicist Ernest Rutherford found that three different kinds of radiation are emitted in the decay of radioactive substances; these he called alpha, beta, and gamma rays in sequence of their ability to penetrate matter.

In 1908 Ernest Rutherford received the Nobel Prize. The symbol is the Greek letter for gamma. Daron Smith

Change to article:

• The American physicist Arthur Holly Compton in 1922 was the first to suggest the name “photon” instead of a light quantum.

End of change to article:

From encyclopedia Britannica:

• In 1922, however, he concluded that Einstein's quantum theory, which argued that light consists of particles rather than waves, offered a better explanation of the effect. In his new model, Compton interpreted X rays as consisting of particles, or “photons,” as he called them.

End of encyclopedia Britannica:

Dsmith7707 15:18, 25 June 2007 (UTC)

But he didn't call them "photons" until after Lewis suggest that name in 1926. What year Britannica are you looking at? Dicklyon 19:59, 25 June 2007 (UTC)

The book "The Neutrino" by Isaac Asimov and the encyclopedia Britannica confirms Compton named the photon in 1922 what more do you want. Dsmith7707 15:57, 26 June 2007 (UTC)

Maybe reason to believe them, like a quote or a source? Hundreds or thousand of sources say it was G. N. Lewis, and they point to his 1926 letter (letter to the editor of Nature magazine Vol. 118, Part 2, December 18, 1926, pages 874-875) where he says "I therefore take the liberty of proposing for this hypothetical new atom, which is not light but plays an essential part in every process of radiation, the name photon." If Compton had already named it, surely someone by now would have found that and pointed out that Lewis had overlooked it. But, no. Let me know if you'd like me to email you a scan of it. Dicklyon 18:49, 26 June 2007 (UTC)

The section “nomenclature” is riddled with errors.

• The American physicist Arthur Holly Compton in 1922 was the first to suggest the name “photon” instead of a light quantum. This is confirmed by Asimov’s book “The Neutrino” and by the current encyclopedia Britannica.
• In 1902 the New Zealand-born British physicist Ernest Rutherford found that three different kinds of radiation are emitted in the decay of radioactive substances; these he called alpha, beta, and gamma rays in sequence of their ability to penetrate matter. The Greek letter gamma is the symbol used for light ever since. This is confirmed by the current encyclopedia Britannica.
• In the Thomson Gale registry at the library in the Macmillan encyclopedia of energy it confirms Compton was first to come up with the name photon.

Dsmith7707 11:17, 27 June 2007 (UTC)

IIn the 1927 Nobel Prize Arthur Holly Compton used the word photon in place of light corpuscles or quanta as if it was used for the first time. The original work was from years before because the approval process takes a lot of time.

Dsmith7707 20:13, 27 June 2007 (UTC) You fellas should supply a link to the sources you are citing. As a reader all I see are a few seemingly intelligent people claiming different answers to the same question. A lamen like myself reading this article has nothing to be except confused. XXLegendXx 03:21, 2 July 2007 (UTC)

Try this link for the 1926 Nature letter by G. N. Lewis. And this one for a snippet about Compton from the Asimov book (sorry, it's not the key snippet). Dicklyon 03:32, 2 July 2007 (UTC)

The first occurence of photon that I can find in Compton's Scientific Papers is on p.540, in "X-Rays as a Branch of Optics", a 1927 paper. here. Dicklyon 03:51, 2 July 2007 (UTC)

The internet is not a very good place to get accurate data it often has a conflict from one site to another. Magazine articles are a little bit better than the internet but still can be wrong. Books are the best place to get the data. Nobel Prize papers are an extremely good source of data. The Nobel Prize people use papers published in the Physical Review and so it is a good source for getting data.

Dsmith7707 12:07, 3 July 2007 (UTC)

In the 1927 Nobel Prize Arthur Holly Compton used the word photon in place of light corpuscles or quanta for the first time. Gilbert N. Lewis used the term photon in 1926 but was referring to an atom not a quanta of light. http://th-www.if.uj.edu.pl/acta/vol37/pdf/v37p0565.pdf

Dsmith7707 14:22, 6 July 2007 (UTC)

The first person to name the photon as light

In the 1927 Nobel Prize Arthur Holly Compton used the word photon in place of light corpuscles or quanta for the first time. Gilbert N. Lewis used the term photon in 1926 but was referring to an atom and even said that the photon is not light. Gilbert N. Lewis said “I therefore take the liberty of proposing for this hypothetical new atom, which is not light but plays an essential part in every process of radiation, the name photon." In a letter to the editor of Nature magazine (Vol. 118, Part 2, Dec. 18 1926)

Lewis's "atom" means "quantum". The subtle difference in conception is not as much as you're making of it. Compton clearly adopted Lewis's proposal. Dicklyon 15:28, 19 July 2007 (UTC)

Lewis said in reference 8 that the photon he was naming was not light but all dictionaries say that the photon is a quantum of light so reference 8 should be removed. It’s like including a reference to orange the color into an encyclopedia article on orange the fruit.

Dsmith7707 15:18, 31 July 2007 (UTC)

In the Physical Review the minutes of the Chicago meeting of November 25 and 26 was the use of the x-ray photon used several times and also A. H. Compton is also in the minutes.

Dsmith7707 16:59, 7 August 2007 (UTC)

Some problems in the introduction?

I understand that this article has been trhough a couple of peer-review rounds, and I'm probably a bit late in pointing this out, but I have a problem with a couple of the statements made in the introduction of this article:

"It mediates electromagnetic interactions and is the fundamental constituent of all forms of electromagnetic radiation, that is, light."

Is the term "light" really used for electromagnetic radiation in general? I have never heard it used that way. To me, "light" refers to electromagnetic radiation that can be detected by the human eye, and perhaps some wavelengths bordering on this region of the spectrum (e.g. "UV-light", and "infrared light"). Using the term "light" e.g. for radio- or microwaves seems odd to me. I would like to delete the last part of the sentence.

this is a good point - the introduction should start off not with this technical talk, but something to help the ordinary reader understand the diffrence between common usage (photons are part of sunlight...) with scientific usage (em radiation). —Preceding unsigned comment added by 24.60.137.141 (talk) 14:22, 15 October 2006 (UTC)

Furthermore:

"The photon has zero rest mass and, in empty space, travels at the speed of light;[...]"

To me, this is also quite odd. As I understand things, light/electromagnetic radiation always travels at "the speed of light", but "the speed of light" varies depending on the medium. In empty space this speed has the value which we usually identify as c (that is 299,792,458 metres per second). A more correct (or at least meaningful) statement would be "The photon has zero rest mass and, in empty space, travels at a speed of the speed of 299,792,458 metres per second;[...]" or something similar.

- O. Prytz 13:11, 1 October 2006 (UTC)

As I understand it, the photon always travels at c, even when in a medium. However, when in a medium it interacts with the other particles and hence spends some of the passage time in the interactions, and the overall bulk speed is less than c. The same happens to a certain extent in a vacuum, thanks to quantum mechanics (a perfect vacuum can never exist), so it possibly travels faster than the standard speed of light. I could be wrong, though, and this may fall into OR. Mike Peel 15:52, 1 October 2006 (UTC)

Perhaps, but wouldn't the passages I cited still be misleading? O. Prytz 21:51, 2 October 2006 (UTC)

Hi, O. Prytz, sorry for not replying immediately, I was away when you first wrote. For the reasons outlined above, we do need that little phrase "that is, light" — the definition makes the article a lot better, and the wording was worked out over several iterations to avoid making the header "clunky", i.e., to avoid breaking the flow of the writing too much.

Your point about "the speed of light" is well-taken; that could indeed be confusing to some readers, which we editors had overlooked. Thanks for improving the article! I hope you don't mind, I tweaked your wording a little, to stress its constancy of c and to replace the number with the variable; the number seemed like it could be daunting to some readers and a little distracting due to its length and units. Thanks again and welcome to Photon! :) Willow 10:08, 3 October 2006 (UTC)

Make clear that "electromagnetic" refers to two very different aspects: non charged, non magnetic photons that travel at the speed of light, and charged, magnetic particles that usually (always) have non zero rest mass. This confused me for a long time - how could the electrically neutral photon have anything to do with electricity. —Preceding unsigned comment added by 24.60.137.141 (talk) 15:47, 14 October 2006 (UTC)

Photons are responsible for magnetic fields... magnets stick to things because of photons. Also when current is flowing in a wire, while the electrons are moving very slowly, a sea charge of photons have traveled down the wire at the speed of light and set up the charge. Virtual photons are mediating the force.DavidRavenMoon (talk) 06:51, 21 May 2008 (UTC)

Wording of "light = electromagnetic radiation"

In the intro, perhaps we should change the fundamental constituent of all forms of electromagnetic radiation, that is, light to read including light rather than that is, light. Radio antennae don't emit light, and light doesn't travel across electric circuits. --Wjbeaty 04:18, 10 October 2006 (UTC)

Yes, "light" does travel across electric circuits... just not visible light. Radio waves, microwaves, magnetic fields.. they are all photons, just at different frequencies. We just happen to see a very small range of frequencies as visible light, but photons are responsible for it all. Photons also mediate the charge in an electrical circuit. Remember, all flowing current produces a magnetic field, and magnetic fields are made of photons. But I also wouldn't call all EM radiation "light", just the visible stuff. DavidRavenMoon (talk) 06:57, 21 May 2008 (UTC)

I second that. I despise the use of "that is". -Ravedave ^{(help name my baby)} 04:58, 10 October 2006 (UTC)

Hi, I'm sorry that you all don't like the use of "light" to stand for all forms of electromagnetic radiation (including radio waves!) and our little verbal finesse with "that is" instead of "i.e." (Laura deprecated my Latin inflorescences. ;) These issues have been debated at length here and in related articles, producing the article that you see here. We need to define "light=EM radiation" (which is routine anyway in scientific circles), since there seems to be no practical way of writing an FA article (which must have sparkling, easily intelligible prose) if you replace a 1-syllable, familiar word such as "light" throughout with the 10-syllable phrase "electromagnetic radiation". Aside from being more clunky and less intuitive, "EM radiation" may even be seen as less accurate by some readers; since we argue that the electromagnetic field itself is produced by photons, it might seem like circular reasoning if we seem to call photons "particles of electromagnetic radiation". It's probably better to make a cleaner break in terminology with the classical Maxwellian model, don't you agree? And "light" is simply a beautiful, direct word. Hoping that you see things in the same light ;) Willow 08:32, 10 October 2006 (UTC)

I am just against 'that is' I don't really care how it gets changed. I find that sentences using that connector are hard to read. -Ravedave ^{(help name my baby)} 13:29, 10 October 2006 (UTC)

You may be right. Laura and I had been trying to give the opening paragraph a light touch with lots of flow, so we replaced the original clunky spelling out of "light=EM radiation" definition with the "that is", or "i.e." version after some tinkering. Unfortunately, the light touch may be doomed; I foresee that many readers will be unfamiliar with scientific customs and, not understanding that we intentionally set "light=EM radiation", will feel the impulse to change "that is" to "including", or "such as", or some such. To save ourselves a lot of explanations with new users, how about we spell out the definition and its rationale in the article itself:

...all forms of electromagnetic radiation, which we call "light" for brevity.

Although it uses the first-person "we", this version seems lighter and more direct that the passive voice version

...all forms of electromagnetic radiation, which will be called "light" for brevity.

What does everyone else think? Willow 20:54, 10 October 2006 (UTC)

I like including or perhaps beeing or wich is ty

I do agree that the "this is" formulation might have been too short, so that readers might not realize that a definition was being made for the rest of the article. On the other hand, we definitely don't want to make the lead paragraph clunky and worded overly technically. So I'm trying out a solution where we raise the definition into the pre-article (italics) section; how do people like this approach? Willow 13:58, 13 October 2006 (UTC)

In my opinion the key is communicate to the layman is that visible light, radio waves, the microwaves that heat up frozen pizza, and the UV that gives you sunburn is all the same thing. I don't think its a matter of finding the right words, but a matter of saying that explicitly and prominently. If someone was only going to take one thing about light from the article, I'd vote that the most important. TRWBW 19:35, 17 October 2006 (UTC)

Can light slow down?

Can light slow down? Notice that if we define "medium" as meaning "a transparent uniform medium" in the classical physics sense, then light really does change speed, e.g. when it enters glass. In particular, EM waves slow down and change wavelength when propagating through a medium. The atomic spacing of glass is far smaller than the light wavelength, so to a first approximation the glass behaves as a uniform medium for light, and light does propagate more slowly in glass than in vacuum.

On the other hand, if "medium" refers to a more modern model where the vacuum contains a crystal array of atomic nuclei with electron clouds between, then light is *always* travelling in a vacuum, even when it travels through glass, and the very idea of "medium" barely applies. Analogy: humans cannot pass through solid wood, therefore they can never travel within a forest; they can *only* travel through space between trees! I'm not trying to be funny, in fact I've encountered many people who respond with rage when they hear the suggestion that light "slows down" within transparent materials. I suspect their trouble is with concrete thinking versus abstract thinking. The terms "EM waves" and "transparent media" are abstracts, since all that really exists are photons and subatomic particles. A purely concrete thinker might discard the ideas of light waves and glass, then insist that light (meaning photons) can never change speed. This has consequences: they're essentially denying that the "transparent material" concept and the "EM waves" concept has any utility. In their world, glass is a vacuum, "light waves" are a misleading illusion, and photons travel only in the vacuum between particles and can never be inside a "transparent medium." --Wjbeaty 04:13, 10 October 2006 (UTC)

See Wave-particle duality. Both pictures are useful for different things. Glass is never a vacuum, though - it still has nuclei in it, plus all sorts of fields, with which the photons interact. Hence why light slows down in it (or alternatively, the time between the waves/particles entering and leaving the material is increased). It's transparent as the interactions don't change the properties of the photons - i.e. it doesn't shift them in colour/wavelength/frequency. Mike Peel 05:22, 10 October 2006 (UTC)

If you think of time and space as a framework, with time moving at a constant speed relative to space, and photons "attaching" to time (like a commuter on a subway car), then several observed phenomina become understandable. For example, space is distorted near matter. Glass is matter, therefore space is distorted inside glass, time, which is moving at the same speed relative to space is also distorted. The photon does not change speed at all, but appears to the observer to slow down because of this distortion. Diffraction is caused by light passing near the edge of matter. On a larger scale, this effect around black holes and even galaxies is being used as a lens by astronimers. —Preceding unsigned comment added by 71.49.168.72 (talk) 17:03, 7 November 2009 (UTC)

Preparation for onslaught of vandalism

Today's Main Page article, Enzyme, became a featured article not long before Photon. Unfortunately, its prominence has attracted a horde of vandals, who are hitting it roughly every 15 minutes. Photon is due to appear on the Main Page in three days (Saturday, the 14th) and I, for one, am totally unprepared for the onslaught. I probably won't even have access to a computer then. What can we do to prepare for the attacks — maybe lock the article for a day or so? I had no idea that it would be so bad, and I hate to think of all the time wasted on repairing the damage. Hoping for some good ideas and quickly, Willow 14:51, 11 October 2006 (UTC)

Does not the attraction of a lot of attention include the attention of lots of people who are willing to notice and revert vandalism? Dicklyon 15:06, 11 October 2006 (UTC)

It does indeed. No worries Willow. Caffeine got hit pretty hard when it was on the front page. I asked to have it locked but apparently there is policy not to do that. Overall caffeine actually ended up gaining some good material. So sit back, install Wikipedia_talk:Tools/Navigation_popups and squash some vandals. -Ravedave ^{(help name my baby)} 15:30, 11 October 2006 (UTC)

That's true, Dick, but why should they have to? It's a horrible waste of time. If we know in advance that we're going to be vandalized, we should probably prepare for it, don't you agree?

There will be roughly 300 edits to Enzyme today, and although the new editors aren't all malicious, it's probably safe to assume that none of those edits will improve the article significantly. However, once Enzyme is off the Main Page, it seems likely that the vandals will turn their attention elsewhere, whereas those who care to improve the article will remain. So why not proactively "freeze" the article until it's faded from the memory of those who might want to damage it? We'll save ourselves and other well-meaning people a lot of pointless work that way. While the article is frozen, we could encourage people to leave suggestions here on the Talk page on how to improve the article. But other ideas are welcome as well! Willow 15:48, 11 October 2006 (UTC)

My guess as to why featured articles aren't locked is that when someone comes to Wikipedia for the first time, odds are the first article they'll look at will be the featured article. If they then go to edit that article (after all, anyone can edit any article at Wikipedia), and find that they can't, then they'll be a bit non-plussed. If it happens to be a journalist that finds that out... well, that'd be some bad publicity for Wikipedia.

In the long-term, it doesn't really matter either way - someone can always come along on Sunday or so and revert to a pre-vandal onslought version. So I wouldn't let it stress you too much - possibly consider taking the day off Wikipedia on Saturday, then repairing any damage on Sunday. Mike Peel 16:40, 11 October 2006 (UTC)

Agree with Mr.Peel. There are plenty of very very good vandal fighters. Take the day off, when you come back it will still be there in the same shape if not better. -Ravedave ^{(help name my baby)} 17:15, 11 October 2006 (UTC)

(copied from Willow's talk page)

Congratulations on getting Photon featured! It's not easy to do for any article, but for a highly technical subject, with some conflict in its history, is truly an accomplishment. You and your collaborators deserve many kudos for that! Reading over it now, I see a truly fine piece of work, and I'm glad to know my essay may have helped in some small way.

As far as protection goes, please see User:Raul654/protection, where Raul lays out his reasons why featured articles should not be protected. It is painful to watch an article you've worked hard on be vandalized, and it's frustrating if you can't watch over it constantly yourself, but rest assured that many people add featured articles to their watchlists for the day it's on the main page, even if they're not otherwise interested in the subject. Between those who already watch it and those who will add it, and the ever-vigilant bots who watch for blanking and bad words, any graffiti will be reverted in seconds. On the other hand, many featured articles gain many improvements during their day in the sun, and a great many new editors get their start by dipping their toe in experimenting with whatever's on the main page. I agree with Raul that the benefits outweigh the inconvenience, and that a dynamic article that is responsive to users (both good and bad) does a better job of illustrating "what Wikipedia is all about" than a pristine and untouchable piece of scholarship, no matter how good. And remember that everything can be undone -- even if nothing good came out of being unprotected and attacked all day, after twenty-four hours it would be exactly back where it started, with no greater harm done.

Hope that helps to calm your fears; I will add the article to my own watchlist today to help do my part. Good luck to you! — Catherine\^talk 16:51, 11 October 2006 (UTC)

(deep breath) Whew.... Thank you all for helping me calm down; sorry for getting panicked by Enzyme! But it was also good for me; I appreciate now even better the nature of Wikipedia and the collective strength of Wikipedians. It's thought-provoking and makes me realize how much I still don't understand. Willow 17:47, 11 October 2006 (UTC)

Cool. I decided to check on Enzyme to see how bad the vandalism was and I did a diff from yesterday to today. There were actually more improvements than I thought. [1] -Ravedave ^{(help name my baby)} 19:06, 11 October 2006 (UTC)

Indeed, the passing editors on Enzyme caught three typos, one serious mistake and two unclear and misleading sentences. Two figures were upgraded to SVG versions and the lead brought to a better format. Overall, a very positive experience. TimVickers 15:02, 13 October 2006 (UTC)

Thanks, everyone! I really appreciate the supportive words and the reassurance that Photon will benefit from its day on the Main Page. Enzyme improved significantly, and Photon will probably make a "quantum leap". ;) I'm at least hoping that somebody will track down the missing article titles or a good historical reference for the first time that a photon was symbolized by γ. Less than two hours to go — good luck, everyone! Willow 22:28, 13 October 2006 (UTC)

Some questions about Photons I am trying to locate answers for

Would appreciate help I have had little success finding satisfactory explanation of these, please excuse me if I am being green:

1. If photons are destroyed when detected, is it safe to assume that light actually travels like a particle at all? Because we can only know of an event creating a photon at some point in space and then measure the photon arriving at some other point, for any individual photon we make the assumption that it travelled in a straight line, but this is impossible to verify by experiment. We can test many photons in a stream and move a target to demonstrate that a light beam appears to move in a straight line, or shine a beam through smoke. However this tells us nothing about the path taken by any individual photon from its creation to its destruction. This actually throws into question whether the photo travels at all. It might just appear where detected after a time delay dependent on distance between emission and detection without actually travelling at all. Additionally if it is true that it is impossible to measure the existence of a photon without destroying it, this raises some interesting questions about the assumptions made generally, and regarding chains of observation. (If we detect a photon by measuring the impact of it on a smoke particle, will this destroy the photon?)

You're quite right that it might not make sense to talk of the "path" of the photon between its creation and destruction. Indeed, if we accept the Feynman conception, then all possible paths of the photon are tried out by Nature.

There are some disturbing consqeuences of this of course... if we accept that nothing can travel faster than the speed of light then light can only arrive from A to B at the speed of light be going in a straight line. So I would guess that there is no actual path tried at all. It kind of leads me to think that a photon is actually just information and information takes time to propogate... or something. But it also gets very confusing when you set up an experiment where dependent on the 'path' taken the photon might be detected at one of two detectors at different distances... say one at a distance of 1 light second and the other at a distance of 1 light year. The interesting aspect of this is that the probability field resolves time as well as location. If the experiment somehow succeeds in limiting the options to one or the other, I guess you can know for certain it will hit the more distant target if it does not hit the nearer in one second. Does this mean that the probability field is resolved before the photon is detected? Have we not detected the photon without destroying it in this case? Further my guts tell me that the only way to resolve this kind of problem is if the photon is already destined to resolve in a certain way at the moment of its creation, which would possibly have severe philosphical consequences regarding free will.

2. Is there a coherent explanation of Young's Double Slit experient as yet? Particularly the question of how a single particle can apparently interfere with itself? I have failed to locate one as yet.

There is no need for an explanation in terms of a more basic mechanism. Physicists now accept quantum mechanics as the way the world works, as a fundamental mechanism with no need of further explanation. The classical conception was that Nature had a secret "notebook" in which she records the position and velocity of all particles, a notebook that is intrinsically impossible to read beyond a certain refinement, as shown by Heisenberg. Experiments have shown that such hidden-variable models of Nature are incorrect. The modern conception is that position and momentum (which seem so natural to us, thanks to the philosphies of Aristotle, Newton and others) are not actually part of reality until they are measured; there is no secret notebook beyond the reach of any experiment.

This appears to be a case of "We give up". I suspected that there was no full explanation of Youngs Slits. Has it not occured to anyone that it might be possible that a grand unifying theory can not be found where there is no explanation of such a simple experiment?

Assuming a single photon to be one or more particles orbiting around each other, the degree of deflection when it encounters the warped space-time at the edge of a slit becomes a function of probability, the angle of deflections depends on the exact position of the photon particle when it passed an edge of the slit. When there are two slits, the distortion of space is changed enough to create the "interference" pattern. Simplisticly, if we roll a pair of dice 360 times, we get a normal (bell shaped) distribution with a mean of 7, but if we roll a single die 720 times, we get a linear distribution of the numbers 1-6. Similarly, one slit (which has two edges)produces a single bell shaped distribution of light intensity. Adding another slit (two more edges) creates areas that have a very low probability. This appears to be an interference pattern, but since single photons exhibit this pattern, we must assume that it is the distortion of space caused by 4 edges that is the cause.

3. Where can I find an explanation of why the speed of light changes when passing through a medium?

Ummm, doesn't the article itself discuss that? Maybe we should make that more clear.

It mentions slowing down due to "interactions" but I found this a bit vague... what interactions exactly? The only thing I can think of is that as photons do not interact with anything except gravity, unless the effect of slowing down in a medium is due to some kind of absorbtion and re-emission on collisions with particles (which I believe can be ruled out on grounds of such collisions being too rare and also because there would be a randomisation of the travel time and actual scattering of light as well) then I can only conclude that the effect is due to minute bends in space caused by the matter of the medium and that the light is actually navigating rippled space that as a consequence is a longer distance to travel. Thus the light doesn't not slow down, it just has further to travel. Does that make any sense?

Because space distorts near matter. The photon is traveling at the same speed relative to space, but there is "more" space inside the glass than there is in air.

Oops scratch all that I missed the actual explanation in the article confused it with statements in the talk. The official explanation I see is quite complicated. Are there models that predict the refractive index of a material given it's compsition? Dndn1011 17:01, 14 October 2006 (UTC)

thanks - Dino Dndn1011 12:57, 13 October 2006 (UTC)

Hoping that this helps with your excellent questions, Willow 14:18, 13 October 2006 (UTC)

Thank you very much for your help Dndn1011 15:55, 13 October 2006 (UTC)

Decay

Do photons decay? Intutivly I would say no, they last forever, but that doesn't seem right. -Ravedave ^{(help name my baby)} 16:48, 13 October 2006 (UTC)

You know that old joke about time being nature's way of keeping everything from happening all at once? Well, for photons, everything really does happen all at once, including emission and absorption. Since technically no time passes for a photon while it's in flight, there's not way to know if it would be unstable if it traveled slower and had time to decay. In any event, no particle traveling at the speed of light can decay, because in order to decay, you have to have time in which to do it. SBHarris 17:03, 13 October 2006 (UTC)

Photons are indeed stable, as far as I know; they don't decay spontaneously, although they may be absorbed or produce antiparticle pairs upon interacting with matter. Since photons can cross countless light years from distant galaxies and have survived as microwave radiation from the Big Bang itself, it seems that photons are at least very long-lived.

I understand Dr. Harris' point of view, which may very well be valid. However, I don't share his opinion that the stability of photons is an apodictic truth; the history of physics teaches us to be cautious about being certain of anything. Willow 17:12, 13 October 2006 (UTC)

Well, I think it's fair to say that since the decay half-life of particles can only be computed in particle's proper time (ie, rest frame), and since photons, gluons and gravitons (if they exist) have no rest frame, the two concepts are incompatable. Asking what the lifetime of photons would be if they didn't travel at the speed of light is like asking how massive a spaceship would be if it traveled at the speed of light. It's one of those "divide by zero" questions. The proper answer is what your programming language tells you if you attempt it: some variation of ERROR. The fact that photons travel on for light-years in OUR reference frame is not relevent to their life, any more than the fact that cosmic ray muons go through tens of km of atmophere and rock, even though their half-distance to decay would be 0.66 km without relativistic effects. Of course, there are relativistic effects. SBHarris 17:59, 13 October 2006 (UTC)

My head asplode! So to fill in the infobox would decay time be stable, 0, undefined or infinity/0? Has anyone actually done research on that exact thing? -Ravedave ^{(help name my baby)} 17:15, 13 October 2006 (UTC)

As SBHarris points out, the notion of a lifetime for photon is essentially inapplicable. They are usually called "stable" even though they can not exist for any nonzero amount of proper time. Personally, if a lifetime was called for, I'd put zero (but when I did that here a while back, it didn't survive, because the offical line is "stable", which also makes no sense at all). This problem goes away in the transactional interpretation where photons aren't really particles, I think. Those "photons" that have survived since the big bang are really just transactions, at zero interval in spacetime, between our detectors and some emitters in the big bang; those photons are absorbed by our detectors "immediately" on being emitted in the big bang, from their own point of view. They are not "stable" in any normal sense, but completely ephemeral. This viewpoint was articulatd by Gilbert Newton Lewis, I believe in 1926 before he coined the term "photon". It is consistent with everyhing we know in modern physics, just a different way of looking at it. Dicklyon 17:27, 13 October 2006 (UTC)

Cerebral asplosion — funny! Modern physics is dangerous for your mental health ;)

The Eidelman et al. table (ref #46c in the main article) says that the mean lifetime of the photon is "stable", which is the mainstream nomenclature among particle physicists. "Zero" is less than ideal, since that could be interpreted as saying that the photon decays instantly in our frame, which is evidently not true. "$\infty$" is likewise problematic for the opposite reason; the photon itself experiences zero proper time, as Dr. Harris and Dick Lyon point out. So "stable" is the generally accepted compromise. Willow 17:47, 13 October 2006 (UTC)

From a theoretical POV you can't have decaying photons, but you can have mixing interactions where photons get transformed into other zero or low mass particles (such as axions or gravitons) in the presence of strong electromagnetic fields.see e.g. here pdf of preprint. Count Iblis 18:30, 13 October 2006 (UTC)

doesn't every particle exhibit duality?

The article implies to me that there is something special about the photon's wave/particle duality. Historically the photon is important in this regard, since its ubiquity and relatively large wavelength made its wave behavior obvious, and led to the realization that all particles show duality. But that's a different then what is implied here. TRWBW 00:32, 14 October 2006 (UTC)

Yes, that's right. Perhaps one should write "Like all particles, Photons exhibit both..." Count Iblis 01:36, 14 October 2006 (UTC)

Hi, I don't want to start an edit war when Photon is on the Main Page, but that's not strictly true. Yes, electrons and photons both exhibit wave/particle duality, but their wave-particle dualities are different in kind. Electrons and other material particles are described by the Schrödinger equation; the photon is not. The photon is described instead by quantum electrodynamics, the quantum field theory (QFT) first developed by Paul Dirac; an electromagnetic wave is different in kind from the Schrödinger wave. We do not use ΔxΔp in connection with photons but, as the article states, ΔnΔφ. Unfortunately, this is a common misunderstanding and difficult to clear up, since it involves the highly technical field of QFT. We could, if we like, plunge into QFT and talk about the Dirac equation and the electron field, but (I believe) that is not what most people will understand by "wave-particle duality". Willow 10:17, 14 October 2006 (UTC)

There is a ΔxΔp inequality for photons, where x and p are the transverse momentum and position, or ΔfΔt, where f and t are frequency and time. And, although the Schrodinger Equation does not apply to photons - there is a direct conversion between it and Maxwell's equations. (e.g. the core of an optical fiber can be thought of as a finite potential well for photons). In most circumstances, you can think of photons in the same way as electrons just with a different dispersion relation. I support Count Iblis' wording.--J S Lundeen 16:25, 14 October 2006 (UTC)

Since Willow reverted my changes, I don't want to revert them back, but I stand by my point. The current wording implies that wave-particle duality is specific to photons, which is simply not true. If you are saying a photon's duality is different from other particles, okay, craft a sentence that that says how it is different. Something like "Photons, like all bosons, ..." TRWBW 17:07, 14 October 2006 (UTC)

The Count's wording is fine with me — thanks again, Count! I'm sorry for reverting TRWBW's edit, but I couldn't agree that the wave-particle duality of electrons and photons was "identical", at least not as that identity would be usually understood. (Inspired by the analogous two-slit experiments, most lay-people assume that Maxwell's equations are just another version of Schrödinger's equation.) That particular edit also had a few minor flaws that might not be obvious: its wording was historically inacccurate (quantum electrodynamics was developed around 1930, whereas the photon correlation experiments were in 1970-1980), it could be mis-read that the photon was a material particle, and it had a weak connection to its paragraph. I tried to find a compromise wording, but that apparently made the problem worse rather than better. I'm sorry for not explaining myself more on Saturday (and any hard feelings that might have raised); it was difficult for me to get computer access, so I had to be as brief as possible. I'm sure that, together, we can find a compromise that is scientifically accurate and elegantly worded. One element of a solution might be to substitute "quanta" for "particles" in a few key places, so that we don't end up saying "particles have wave and particle properties", which might sound bogus to lay-people. Anyway, other ideas are welcome! Willow 11:04, 16 October 2006 (UTC)

It's been a while since I studied quantum, I could be totally wrong on this. But is there a way to explain it to folks in the middle who understand a curvature tensor but may not be up on the latest? Where do photon's currently fit in the theory? Is QFT enough? If you are distinguishing the wave-partical duality of photons from other particles, where is the line? Bosons? Gauge bosons? Or are photons in a class by themselves? TRWBW 02:59, 17 October 2006 (UTC)

Of course photons are in a class by themselves ;) You don't see editors lavishing so much love on those massive gauge bosons, do you? We're special. :D

Kidding aside, I should say upfront that I'm not an expert and would welcome other viewpoints and insights. I guess I was distinguishing between the wave/particle duality of first-quantized theories (exemplified by the Schrödinger wave equation for persistent, massive particles) — which is what most people understand by wave/particle duality — from the wave/particle duality of a second-quantized quantum field theory and, more specifically, from the QFT of gauge bosons. But I didn't want to go into too much detail in the article, because we'd already been criticized for being too technical. The best solution might be to clarify the issue in wave-particle duality. What do other people think? Willow 12:30, 17 October 2006 (UTC)

Intro

Hiya. I feel it should be mentioned (especially since this article is on the front page) that the speed of light is 3x10^8 m/s. Or however you want to measure the velocity. Wikipedia should appeak to a non-geeky audience, if you are going to say its speed is "c" you might as well say what c is. Kinda an important quantity, and should be in the bit thats featured on the main page. 24.222.116.79 03:54, 14 October 2006 (UTC)

Hiya, 24.222.116.79, thanks for the suggestion! We thought about that, but in the end we chose not to give the numerical speed precisely because we have a non-geeky audience. Instead, there's a link to the excellent article speed of light. The numerical value isn't essential for the Photon article, so we don't need to give it much "air time" in the lead. Willow 10:35, 14 October 2006 (UTC)

Hiy'all. True, but the current text does this in a bit underhanded way: "The photon ... travels at a constant speed c." Won't the readers think: "I know what constant speed means; that's one link I don't need to follow"? I suggest this:

The photon ... travels at a constant speed c, the speed of light.

 --Lambiam^Talk 09:31, 16 October 2006 (UTC)

Thanks, Lambiam, that's a good suggestion! I'll make the change right away. We had an objection earlier to wording similar to yours, roughly: "Why shouldn't a light particle move at the speed of light? Isn't that a tautology?" which led to the, umm, presently understated version. ;) BTW, thanks very much for fixing up those references; that was one of the happiest things on Saturday! :) Willow 11:12, 16 October 2006 (UTC)

Newton/Maxwell

I reverted this edit because it seems to say that corpscules are part of the "modern concept of the photon," which I don't think is accurate. The "modern concept" seems to refer to the photon as a quantum of light which exhibits wave-particle duality. Newton's idea are worth mentioning, to be sure, but I don't think that was the right place. -- SCZenz 17:24, 14 October 2006 (UTC)

Thanks for opening this on the talk page. I was prepared to back off for a few days, but the photon concept really does embody many of the main tenets of the corpuscular theory, notably the quantization of energy and momentum (which Newton predicted but nobody could measure until the turn of the 20th century). Einstein's work is notable in a historical context mainly because it unified the wave and particle theories of light, which had both existed since Huygens & Newton's time. I don't like the intro (and wish I'd noticed it before it was FA/FP) because, like so many physics discussions, it fosters ignorance of the long history of the concept being explained. For light, that is particularly tragic as the story is so interesting. I'd like to see at least a short reference in the intro. The wave/particle history is covered moderately well in the wave/particle duality article; rather than putting a full historical section in Photon it would be better to refer to an augmented history there. zowie 17:35, 14 October 2006 (UTC)

I think improving the history is a great idea, and noting that light had previously been seen as a particle, and then a wave, is also very good for the intro. My concern is with the idea that corpscules really share many properties with the modern photon. The intro says the modern concept was invented by Einstein, and that's right; obviously the "modern concept" was an outgrowth of older ideas, but that's not quite what the edit said. I think it was just the wording I objected to, and there's something similar that we can all agree on. Any ideas? -- SCZenz 04:33, 16 October 2006 (UTC)

I'm fond of history, too, and almost too eager to honor the dead by remembering their contributions. But I also feel that we should be historically accurate and specific to photons, not light in general. To my (admittedly limited) knowledge, the corpuscular theory of Newton does not include the energy quantization equation E = hν, which is for me the quantitative cornerstone of the modern theory. Also, Newton was not the first to pose the qualitative hypothesis that light is composed of particles; most theories of light up to that era did that, even ones in the ancient world. As I recall, Newton proposed that the light particles had some quasi-gravitational interactions with matter to explain Snell's law of refraction (then recently discovered); that law seemed at odds with the naive idea that light should travel more slowly in denser media, not faster. The Maupertuis article that was translated recently has a short discussion of the scientific debate surrounding light in that era. But is it Newton's contributions specifically that you'd like to add, or something more general about earlier particle theories of light? Interested and open to other ideas, Willow 11:53, 16 October 2006 (UTC)

Photons in matter - group velocity misleading?

This claim appears vaguely misleading:

In some cases, the dispersion can result in extremely slow speeds of light.

Has it been scientifically peer reviewed? My problem is that the section starts giving some views on the observed reduced speed of photons through matter (a wave classical and a particle view). The polariton is introduced and before the equation there is:

The polariton propagation speed v equals its group velocity, which is the derivative of the energy with respect to momentum.

Although a new concept to me, it makes sense so far, even for a single photon. However the link to slow light seems misleading because it describes a "group property" (if that is the right jargon), and not a property of separate photons. In other words the slow light article appears merely to describe the optical equivalent of the audio beat effect. I can visualise two beams of coherent photons, being phase shifted in such a way that they create such a beat effect, but would that affect a separate photon (or a cascade of non-coherent photons) passing through? -213.219.184.15 01:14, 15 October 2006 (UTC)

While certainly good science, non-technical descriptions can use "speed" and "velocity" in ways that are misleading. As in the famous thought experiment, the blades of a pair of scissors can close faster than the speed of light, but that doesn't mean anything is travelling faster than light. TRWBW 02:39, 15 October 2006 (UTC)

Proof

"further experiments proved Einstein's hypothesis that light itself is particulate." Is it right to say the experiment proved the hypotheisis? AS a science article should we not be more careful with such a word? Would it not be more accurate to say that "futher experiments provided evidence which supported Einstein's hypothesis that light itself is particulate over the others."? Or something like that? Phoenixis 15:24, 15 October 2006 (UTC)

I changed it to "light itself has particulate properties". --HappyCamper 15:26, 15 October 2006 (UTC)

There is a fine line between keeping it simple and dumbing it down. Either saying light has particulate properties or light has wave properties doesn't hit the key fact. Both the wave model and the corpuscle model are approximations of quantum theory. We should find some wording that captures this. Quantum theory is a contender for the greatest achievement of mankind, and we shouldn't downplay it. TRWBW 01:33, 16 October 2006 (UTC)

I have a worry that too many readers won't understand that "particulate" is just an adjective for the noun "particle" and not a particulate synonym of the word "particular". Also, the flow of thought and how what is connected to what is perhaps not as clear for neophytes as it could be. A suggested replacement text:

..., further experiments confirmed Einstein's hypothesis that it is light itself that is quantized. The quanta constituting light are what we now call "photons".

In particular the last sentence is a missing link. (I know it comes back later, under Nomenclature, but by the time the reader is there they may have lost the relevant connection with Einstein's hypothesis.)  --Lambiam^Talk 09:52, 16 October 2006 (UTC)

Again, excellent wording, Lambiam! :) I had been trying to contrast the semiclassical hypothesis (light itself is continuous, but systems that emit/absorb light are quantized) with the "light itself is quantized" hypothesis of Einstein. Since all semiclassical theories were disproven by those elegant photon-correlation experiments mentioned in the text, it seems fair to say simply that the "light itself is quantized" — "confirmed" is much better than "proved". Willow 15:12, 16 October 2006 (UTC)

Would it be fair to say that the resolution of the classic wave versus particle argument was the discovery that both are approximations of quantum theory, just like the ideal gas law is an approximation of statistical mechanics and newtonian gravity is an approximation of general relativity? TRWBW 06:02, 17 October 2006 (UTC)

That sounds fair to me at least, although one needs to specify what is meant by "quantum theory". Semiclassical models are quantum theories, but do not require that light itself is quantized. Such theories do a good job of accounting for most phenomena, so it wasn't trivial that they were ruled out in the 1970's.

As I understand it, the only first-principles quantum theory of light is quantum electrodynamics, begun by Paul Dirac and amplified by so many others. It's a quantum field theory, so it's an extension of the normal Heisenbergian quantization of a one-dimensional simple harmonic oscillator to the infinite-dimensional case. The photon is not a persistent particle with position&momentum operators in the same sense as the electron is in the Schrödinger equation; instead, it's a quantum of an electromagnetic mode, one of those infinitely many simple harmonic oscillators used to describe the electromagnetic four-potential field.

I usually think of valid 19th-century theories as limiting cases of some variable, such as c→infinity (Galilean relativity) or h→0 (correspondence principle). But the creation and annihilation of a photon seem like discontinuous processes that would not disappear until h=0, not in the continuous limit h→0. So I'm not sure if I understand the sense in which you're using the word "approximations"; maybe "aspects" is better? I guess I haven't really thought it through :( What do other people think? Willow 12:07, 17 October 2006 (UTC)

I meant approximation, not limit. The older theories get the right answer too, mostly, most of the time. But on second thought, my phrasing could be misleading, since it could imply that newer theories are the final truth. They are all approximations, the new ones are just more better approximations. Saying phrase wave/particle duality without explaination bugged me, since it could imply that particles behave one way sometimes and another way at different times. Aspects is pretty good. TRWBW 02:25, 19 October 2006 (UTC)

External links

At the moment, we have three external links. I'd like to see two of them removed. The links we currently have are:

• The Nature of Light: What Is A Photon? OPN Trends, Oct 2003
• MISN-0-212 Characteristics of Photons (PDF file) by Peter Signell and Ken Gilbert for Project PHYSNET.
• How to entangle photons experimentally

The first one of these points to what looks like an interesting document, however it seems that you need to be a member of the Optical Society of America to read it. That probably isn't the case for most readers, so it's a bit pointless having it there. The second one of these looks useful, although the link needs cleaning up a bit. I'd suggest it changes to:

• Signell, Peter. "Characteristics of Photons" (PDF). Project PHYSNET. {{cite web}}: External link in |publisher= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

i.e. using a cite template. The final link points to part of a Physicsweb article, but is very short - the content from that could easily be integrated into the article, if it isn't there already. Does anyone have any objections to me removing the two links (the first and third)?

Also, are there any other relevant links that could be added? They don't have to be web links - if there's useful journal articles, or any other media, that would be useful to readers, then it can be added into this section (possibly changing the title to "further reading"). Are there any relevant chapters in Feynman's Lectures, for example? (sadly, I don't have a copy to hand, so I can't check. ) Mike Peel 19:46, 15 October 2006 (UTC)

Lots of journal articles on the internet have restricted access. They are generally not available to the public, but if you go directly to a university library to request them, they will give it to you... --HappyCamper 20:48, 15 October 2006 (UTC)

Can we put it (with a full bibliographic citation like "OPN Trends (special issue), Guest editors: Roy & Roy") under Additional references, adding something like "Online version (OSA membership required)"?  --Lambiam^Talk 10:01, 16 October 2006 (UTC)

I haven't seen the OPN article, but it doesn't seem true to the spirit of Wikipedia to list a subscriber-only source, don't you think? Perhaps we should see if it has good content before suggesting that others pursue it? The other two sources don't seem to add that much to the article, so I'm OK with their deletion if you'd like. Feynman mentions photons a few times, esp. in Chapter 2 in volume I and Chapter 4 in volume III, but they're unfortunately very brief. There must be a good elementary discussion of photons out there somewhere, though! I looked at the Britannica and a few other encyclopedias a few weeks ago, but their treatment of photons was surprisingly meagre, nothing worth referencing. Willow 16:05, 16 October 2006 (UTC)

I added that OPN article as a reference originally. Then later I added the direct link to a page which took $15 credit card orders for a paper copy of that backissue. Now it appears that the website is no longer selling it, but instead only offers a download to OSA members. Also, I don't recall moving the ref into External Links. It's not a link, it's a reference. If such a thing is too confusing, then we should turn it back into a reference only, with no URL. BTW, it's an excellent set of articles discussing cutting-edge photons concepts, written by physicists and aimed at the technical public. Some of the articles are the sort of thing we'd have seen in Scientific American back in the 1960s, others are a bit more advanced. A valuable reference for anyone who wants to know what physicists now think. --Wjbeaty 01:24, 17 October 2006 (UTC)

I followed HappyCamper's advice and got ahold of the journal at my local library. I haven't studied the articles in depth, but they do seem very good and aimed at the technical public, as Wjbeaty says. In particular, they have some thought-provoking ideas about the wave-particle duality of photons, complementary to the usual presentation of quantum electrodynamics. We should definitely digest these articles (which at first blush don't seem perfectly consistent with one another) to improve the article. They also point to some articles by Willis Lamb that seem like they should be considered as well. Overall, though, they do seem to agree with the article as it is now (whew! ;) Willow 16:45, 17 October 2006 (UTC)

I think the OPN issue is a worthwhile read. The reason the articles 'don't seem perfectly consistent with one another' is because they are all written from a different physics subdiscipline's perspective. Each sub-discipline views the photon differently: the particle physicists view is often quite different from the quantum optician's view. Part of what makes this wiki article difficult to write is incorporating all these ideas coherently. Overall, though, I would say you guys (especially Willow) have done an excellent job making the article clear and educational. Waxigloo 19:08, 17 October 2006 (UTC)

Quantization of electromagnetic radiation proven?

I have been trying to find the experimental evidence which is cited as reason for believing that emr is quantized, rather than it being some classical kind of fluctuation in a field which deposits energy (for reasons we don't necessarily understand) in a way which we find impossible to interpret as other than discrete quanta.

This article currently contains: "Nevertheless, all semiclassical theories were refuted definitively in the 1970's and 1980's by elegant photon-correlation experiments.[28] Hence, Einstein's hypothesis that quantization is a property of light itself is considered to be proven."

This may be a valid statement about mainstream opinion, but it doesn't help the reader understand exactly what these experiments are. The footnote leads to four references: Clauser 1974, Kimble, Dagenais and Mandel 1977, Grangier, Roger and Aspect 1986 and Thorn et al. 2004. Only the Thorn paper is available to people without library access: http://people.whitman.edu/~beckmk/QM/grangier/grangier.html

Regarding "photon" and "quantization of electromagnetic radiation": I assume both mean that emr is ejected as a discrete packet from one piece of matter and that this entire packet is absorbed by another piece of matter. The idea that two photons can interfere is not compatible with my understanding of the emr itself being quantized.

My impression is that this original theory of Dirac has been adopted by most people in a modified form to account for interference, such as between two lasers or radio transmitters. However I don't see how any such modified theory is compatible with quantization of emr itself, which to me is a separate issue from the quantization of emr's interaction with matter. I can't explain why the (to me) apparently diffuse fluctuations in the electromagnetic field results in what we observe as discrete, intense, energy deposition. I still think that the observed interference between two separate sources disproves the hypothesis of actual quantization of emr.

Chasing the first 4 additional references in the Photon article, I put together a bunch of papers, most of which are not accessible to people without library access to journal websites. These papers include the Clauser and Kimble papers, the 1967 Pfleegor and Mandel paper, and a number of more recent papers citing Pfleegor and Mandel. Please contact me at [email protected] if you would like a zip file of these papers. I am reading the Glauber lecture and I haven't been able to access the Optics and Photonics papers.

Also, I found a part of a longer discussion critiquing experiments which are generally interpreted as proving the quantization of emr: http://groups.google.com/group/sci.physics.research/msg/7ee770990a31bd3f Robin Whittle 13:10, 30 December 2006 (UTC)

There are alternatives (to the current paradigm of quantized emr and probabilistic QM) that might be of interest. In particular, Carver Mead's book Collective Electrodynamics: Quantum Foundations of Electromagnetism shows how it is possible to do all EM interactions as collective wave functions of electrons (and other charged particles), with no appeal to fields or photons. See also transactional interpretation, which is basically the same idea. Dicklyon 14:46, 30 December 2006 (UTC)

Two States of Polarization

This article states, "a photon has two possible polarization states", and then fails to make it clear what those are. What are they? If you go to the polarization and photon polarization articles, you get lots of information about linear, elliptical, and circular polarizations; and polarization angles; and left- or right-handed circular polariztion. It's clear that polarization is quite complicated. So, would somone who knows all about this stuff please clarify what is meant by the "two possible polarization states" thing?

Yeah, polarization is a bitch, being subject to all of QM's weirdness. What the two states thing means is that however you analyze it, a photon has to come out in one of two states. Those states can by X versus Y polarization, or left versus right circular, or other dichotomies. But it can't be both at once (that is, it can't br X and right circular, since determining that it is X causes it to be 50% chance of right and 50% chance of left if that's what's measured next), nor somewhere in between except probabilistically, even though a beam of many photons can have a macroscopic polarization that's more nuanced. I'm not up to writing a coherent explanation myself. Dicklyon 00:23, 9 January 2007 (UTC)

This will have to be brief, since I need to go, but here's the scoop. The two states are best thought of as the right-handed and left-handed states of circular polarization. However, there are other, equivalent ways of analyzing the two states of polarization, such as the polarizations of the electric field along the two directions (say, the x and y directions) that are perpendicular to the direction of the photon's propagation (call it the z direction). The ability to resolve the two polarization states in different but equivalent ways (e.g., circular vs. linear) is related to linear superposition in quantum mechanics; which representation is relevant depends on what experiment one makes to measure the polarization. If it's still unclear, I'll check in again either late tonight or early tomorrow to help explain it. Thanks for being patient! :) Willow 00:26, 9 January 2007 (UTC)

Three States of Polarization not two

I couldn't enter my own post,so I am hopping in on this one, sorry

http://link.aps.org/abstract/PRL/v95/e203905 Snowflakeuniverse 18:25, 10 January 2007 (UTC)

That's no surprise. That article talks about random fields. I can always get polarization in any direction if you allow me to sum up two modes with linearly independent optical axes. It describes a characterization technique, not a violation of light as a transverse field. 128.223.60.87 (talk) 20:24, 7 June 2009 (UTC)

Polarization question

I was just looking into light polarization. It would seem that the behavior of polarisation filters is that a polarizing filter lets through some proportion of photons according to their relative angle of polarisation to the filter, but all emitted photons are strictly polarised to precisely the angle of polarisation of the filter. Any other behavior would counter the physical evidence. This is easy to find references on, it is high-school physics. But what is interesting about this is that it means that polarising filters must not only block a certain percentage of non-aligned photons, but must also change the polarisation of photons which pass through it. It is this change of polarisation which interests me. I am just seeking clarification if this is a correct analysis, sorry for doing it here a search on google revealed no confirmation of the rotation of the polarisation. Thanks. Dndn1011 16:29, 2 February 2007 (UTC)

You'll get nothing but paradoxes if you insist on photons having an exact polarization. If you define your axes, each photon is each polarized one way or the other; only two states are distinguishable. That's all you can say. If you want to talk about exact polarization directions, you need to use E & M fields, not photons. Getting the classical and quantum views to align is sometimes tricky, but well understood, I think. Dicklyon 17:55, 2 February 2007 (UTC)

I'd say your analysis is correct, although there might be other ways of thinking about it that are for some purposes more useful. --Art Carlson 21:11, 2 February 2007 (UTC)

Thanks, although I am sticking with what is observed experimentally at this time. Some kind of exactness does exist in the sense that one can very accrurately measure the polarisation of a stream of photons, although I understand that underlying theories get complex... thanks for the help. Dndn1011 21:33, 2 February 2007 (UTC)

Light–Photon merge proposal

Can someone please explain to me the difference between light and photon? Because if not then definately the two articles should be merged. —Preceding unsigned comment added by 68.252.36.83 (talk)

Oppose. A photon is a low-level quantum mechanical explanation of light. Light has many ways to be understood at other levels, and most people who want to know about light will not need much about photons. Dicklyon 19:27, 8 February 2007 (UTC)

Feynman Diagram: Repulsion of an electron and a positron?

I am not an expert in quantum physics, but the Feynman diagram showing the repulsion of a positron and an electron by the exchange of a virtual photon cannot possibly be correct. Can it? DRHagen 21:51, 22 March 2007 (UTC)

I think not, since they obviously don't repulse. Better change something here. SBHarris 22:02, 22 March 2007 (UTC)

There's nothing formally wrong with it, since a Feynman diagram is just a symbol for an integral, not an actual depiction of a physical process. That said, the image might be confusing to lay-people, and I've never liked its position; it seems prematurely advanced for such an early section of the article. Besides, we already have another Feynman diagram, later in the article. I'd be in favor of deleting this image altogether, or replacing it with another image that's more pertinent to that section. What do others think? Willow 22:15, 22 March 2007 (UTC)

I'm not an expert either but since in feynman diagrams an e+ is practically the same as an e- moving backwards in time, having two identical arrows (for example: top-left, bottom-right) with different labels is probably incorrect. Also, assuming that time is from bottom to top, an e+ and an e- anihilate each other and produce a photon which latter is split again into an electron/positron pair. Assuming time runs from left to right, an e+ and an e- should not repel each other User:sharhalakis Sun Nov 2 19:13:47 UTC 2008 —Preceding undated comment was added at 19:15, 2 November 2008 (UTC).

Couldn't we just reverse the direction of the positron? At least that's the convention I think I was taught at university. Niel.Bowerman (talk) 17:36, 14 November 2008 (UTC)

There are at least two conventions; the diagram shown as the right arrows to be an electron and positron in one of them. Pretty much all of particle physics is awash with different conventions, it gets frustratingly confusing. Stannered (talk) 23:47, 16 November 2008 (UTC)

Liquid Light

I found an interesting article on photons being condensed to a liquid state. It is in the November 2002 issue of Discover magazine. I will be adding a section on this unless anyone objects (and I can't think why they would?).Meson man 21:52, 10 April 2007 (UTC)

Hey Meson man, welcome to Wikipedia! :)

I like your name; you're a "man in the middle"!

Can we Talk about this liquid light before you work too hard on writing it up? I'm a little worried that the writers at Discover may have misunderstood something; it'd be a pity if you put a lot of effort into writing up something that wasn't actually in the scientific literature. It'd be great if you could sketch the ideas out (maybe with references?) so that we're all on the same page. Thanks! :D Willow 22:13, 10 April 2007 (UTC)

I think that maybe Discover was a bit mixed up. They said "Drops of liquid light" and then said that(in the same sentence)it would be a entirely new form of matter, dose that imply that it would not really be a liquid? The physist is Humberto Michenel of the University of Vigo in Orense,Spain. He modeled what happened when a laser beam passes through a cubic-quintic nonlinear medium. The drops aren't that big, only 1/500 of an inch across. They would use a gigawatt laser to condense the photons. Hope this helps. ;-)Meson man 22:38, 10 April 2007 (UTC)

Thanks for your quick reply, Meson man! :) I'm not sure, but perhaps what they mean is that super-intense light might cohere into "balls of light" inside a medium with a highly nonlinear dielectric constant. You know how you can confine light within a glass rod (like an optical fiber) by total internal reflection? I think this would be similar except that the light would be confined in every direction, not only laterally. The "balls of light" would look like "droplets of light", which might explain the "liquid light" name. The high light intensity would be needed to give the spatial non-linearity in the dielectric constant.

If that's true, the photons themelves aren't really attracting each other, at least not the way that molecules do when they cohere into a liquid (I think). They're just being continually emitted and reabsorbed by the medium in a way that herds them in the same space. If this picture is true, it would be nifty to know how these "balls of light" move around inside the crystal, what happens when they hit each other, and how long they can last before they bleed all their energy into heat and die. Thanks for sharing such an interesting article with us! :) I'll have to see if I can find the same article at my local library. Willow 22:52, 10 April 2007 (UTC)

Please do, it would make an interesting addition to the article. :-! Meson man 23:24, 10 April 2007 (UTC)

Atomic transitions

Hi Arnero,

I was a little confused by your recent addition, and I was worried that others might be, too. At an earlier time, we had a section called Semiclassical radiation, which was removed as being too technical for this article. Is this perhaps what you wanted to introduce? Thanks for being patient with us, Willow 11:54, 19 April 2007 (UTC)

Wording of "light = electromagnetic radiation" revisited

I see this was discussed earlier, but speaking as a physicist, I do not like to see a special non-standard definition of the word "light" just for this article. Consider the following sentence: This symbol for the photon probably derives from gamma rays, which were discovered and named in 1900 by Villard[8][9] and shown to be a form of light in 1914 by Rutherford and Andrade. I'm afraid that makes me cringe. And the first sentence of the linked article on light completely contradicts this sentence, saying (correctly) that light refers to visible radiation.

I am not suggesting a large change to the article. In most places where "light" is used, it is indeed referring to visible light. In fact, the only exception I can see is the gamma ray example mentioned above. Another would be the Compton effect, which involves x-ray photons, but the word "light" is avoided in the sentence that mentions it.

Therefore, I suggest that the definition in the preamble be removed and we rewrite the open sentences as follows:

In modern physics, the photon is the elementary particle responsible for electromagnetic radiation of all wavelengths, including gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves and radio waves.

Comments? Timb66 13:05, 2 May 2007 (UTC)

Thank you for coming here, Timb66! I hoped that you liked the article in other respects, aside from the one cringe. Perhaps we could fix that by replacing "light" there with "electromagnetic radiation"? But I'm not sure whether I agree with your other points:

• Is it indeed more correct to say that the only valid scientific definition for "light" is "visible electromagnetic radiation"? There are international agreements on scientific nomenclature, but I'm unaware of one that defines "light" that way. Perhaps you could point us to a reference? To me, it would be strange to define a physics term based on foibles of human physiology that may differ from person to person. My impression is that, for most physicists, "light" is synonymous with "electromagnetic radiation", regardless of frequency; think of the T-shirt "And God said, <Maxwell's equations>...and there was light!" :)

• The full quote from the light article is

“ Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength. ”

This definition doesn't seem to contradict our article, since "photon" is a pretty technical/scientific context, as our article makes clear.

• Our article does use the word "light" to refer to EM radiation of unspecified wavelength, and even when the wavelength is likely invisible. For example, ultraviolet or higher frequency radiation is often necessary to initiate chemical reactions. We also want the article to be completely general, not treating visible photons as qualitatively different from those of other frequencies, right? To me, that suggests that, if we define "light" as only "visible light", then we have to change every instance of unqualified "light" in the article to "electromagnetic radiation". Not a good idea, in my opinion, because...

• ...as we discussed above, there are some strong encyclopedic reasons for using "light" instead of "electromagnetic radiation". Some key ones were the quality/pace of writing, the intelligibility to lay-people and the potential for confusion. By these less scientific, more expository criteria, I think that "light" is much better; if you agree, then we should minimize usage of "electromagnetic radiation" in favor of "light".

Maybe we can compromise by changing that one cringey sentence to "electromagnetic radiation"? Anyway, those are my impressions, but we should also hear from others. :) Willow 21:13, 2 May 2007 (UTC)

Hi Willow. Yes, I did like the article -- it is excellent! Sorry if I sounded a bit grumpy, that comes from editing late at night. Also sorry for not reading the light article more carefully. However, I don't agree with it: I don't think "light = EM radiation" is in general use. The reference cited in that article (maintained commercial suppliers of light sources[[2]]) is hardly authoritative! When I see the T-shirt you mention, I don't think "let there be light, which includes gamma rays and radio waves". I think "let there be light and all the other forms of EM radiation". And I think that when pressed, most physicists (and textbooks) would say the same.

I agree completely that it is best to minimize usage of "electromagnetic radiation" and I also don't think we have to change every instance of "light". My concern is that right now, the first few lines of this article don't read as well as the rest. I realise that a lot of thought has already gone into this, but I think it can still be improved. Timb66 23:16, 2 May 2007 (UTC)

Willow, you say it would be strange to define a physics term based on foibles of human physiology that may differ from person to person. I used to think the same, until I read the definition of candela! :-) That said, I have the impression (just from experience, sorry, no references...) that "light" usually means visible, plus sometimes UV and IR. The others are usually called waves, rays, or radiation. The all-encompassing term is electromagnetic radiation, although I agree too that it is a bit verbose and "unfriendly". --Itub 08:04, 3 May 2007 (UTC)

Thank you both for your nice messages! :) For my part, I'm not completely happy with the present "light" usage, although I've seen it as the best general solution for this encyclopedia article. The preamble is definitely a clunker, writing-wise, although it does help by spelling out the rules and preventing "ghettoization" of visible photons. I like your sentence as the lead-off sentence, although I might mention the electromagnetic interactions as well

In modern physics, the photon is the elementary particle responsible for electromagnetic radiation of all wavelengths, including gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves and radio waves. The fundamental forces of electricity and magnetism are also mediated by photons. The photon has zero...

Maybe the best thing for us to do right now is to get a feel for the usage among practicing physicists, from textbooks and agreements on scientific definitions (if any exist). Then we can decide whether to add something to the lead clarifying that "unqualified light=EM radiation of any wavelength" in this article. I really would like our conclusions in the article to be general — i.e., pertain to all wavelengths, not just visible ones — but maybe we can express that thought in a better, more elegant way. Time for brooding! ;) Willow 09:59, 3 May 2007 (UTC)

P.S. Since Photon, I've been fitfully trying to get a few other physics articles up to Featured Article status. My most recent project, along with many other editors, has been the equipartition theorem, which maybe you know already? If you have any suggestions for the article, or would like to comment at its FAC page, I would be delighted and grateful! Thanks muchly for all your help, Willow 10:05, 3 May 2007 (UTC)

Please remember that this article is not aimed at physicists, nor even at scientists. Its primary focus is the layman. I would dispute the suggestion that "light" is a physics term; it's much more than that, even in the context of an article about the photon. The layman will relate to "light" as per its dictionary definition rather than any obscure technical definition. While most readers would be happy to accept IR and UV as light, and might even allow their imagination to stretch to the inclusion of X-rays (X-ray light is a term I've come across on numerous occasions) and maybe even gamma rays, I doubt they would be impressed by the claim that radio waves are a form of light. Personally I find the suggestion that all EM radiation is light to be mildly irritating, and I think its use in this article does nothing to enhance the article's quality; it probably diminishes it. Most readers will have no problem whatsoever with "electromagnetic radiation". 86.31.75.148 22:42, 5 May 2007 (UTC)

I have reworded the Intro to reflect the above discussion. Note that I removed the sentence that "light = EM in this article". i looked at every occurrence of the word "light" and convinced myself that the page is clear and correct without the need for that (non-standard) definition. Comments? Timb66 12:07, 12 June 2007 (UTC)

I have nothing to add, except that I feel that this is a mistake, mainly for the reason outlined already: it lowers the generality of every statement about "light". I also disagree that "light=EM radiation" is a non-standard definition. However, I think your change should stand; although I wrote most of this article, I don't own it, and the change seems relatively innocuous. It's probably better for Wikipedia that this debate close quickly, so that we can move beyond minor changes to Featured Articles and start writing/improving other physics articles to the same standard. Willow 12:53, 12 June 2007 (UTC)

Hi Willow. Thanks for that - I appreciate your attitude. Perhaps our disagreement indicates that there is something notable here that is worthy of a mention in the article, with a few references? Concerning other physics articles in need of work: if you have the time, please look at mass in special relativity, mass-energy equivalence and inertial mass. Timb66 23:18, 12 June 2007 (UTC)

Thank you in turn for your nice note! :) I'll think about adding something, but it's not clear how to do it right away. As for mass in special relativity, I'm reminded of Parmenides in Plato's dialogue of the same name, who, when asked to resolve a debate, said he felt like an old race-horse who trembled before the race, knowing what lay before him. I'm not an old race-horse, but even I tremble at trying to disentangle all that. Before I would even try to do that, I would want to do a complete survey of the scientific literature from 1895 to the present to assess the fraction of articles in which "relativistic mass" has been used in the sense championed by some Wikipedians. My own impression is that that concept has never been used in that literature in that sense, not even once in over a century; however, one should test that hypothesis and be properly quantitative about it, don't you agree? It will probably be another century before I'm willing to commit that kind of effort to exorcising those spectres, though. I'm not bothered as much by nonsense articles as others at the Physics Wikiproject; I tend to focus instead on writing good articles about classic topics. My latest was equipartition theorem, which you might enjoy as an astrophysicist. Nowadays, I'm half-heartedly toying with X-ray crystallography and some classical mechanics articles such as Appell's equation of motion, Liouville dynamical system and Kepler problem in general relativity (still a stub; want to help?). I'm a little out of my depth, though, and I keep having to go to the local library to fetch books. :) Willow 01:08, 13 June 2007 (UTC)

Nice work! Those pages you are working on are not in my area of expertise, so it would take more time than I have available to get up to speed in order to add anything useful. I am trying to limit my time, fun though it is, by sticking to subjects in areas of my work, either in research (stellar astrophysics) or teaching (various physics courses, such as relativity). This last one is interesting, and there are certainly still plenty of books that use the concept of relativistic mass. Timb66 05:58, 13 June 2007 (UTC) ....................

I would like to see some explanation as to how photons can be viewed, represented or explained in the case of long wave radiations, that is, radio waves, and most particularly very long waves and below. Theoretically, even 0.1 Hz can be defined as "electromagnetic radiation," and could be expressed as "photons," but I never see any discussion of photons or particles below, let's say, the infrared range. That is, how can "particles" be conceived at radio wavelenghts? I would like some of a layman's discussion on this (I'm not a physicist). Daniel_C (talk) 14:12, 17 May 2009 (UTC)

Mass again

There seem to have been recent edits that disturb the established compromise on the usage of the term "mass". The footnote, as of this version, claims that assigning the photon a relativistic mass is a "common mistake". But it is not a mistake; it is just a different use of the term "mass", a different bookkeeping convention, if you will, and one that has been used by some very reputable physicists. Nothing has changed in the understanding of physics to refute the usage, only fashions in how the physics is described.

I am going to try to clean it up now. I may just have to revert a couple of days and let people re-add their intermediate contributions. --Trovatore 18:13, 9 May 2007 (UTC)

Trovatore: I am not aware of any different use of the term mass that is justified for the photon in terms of special relativity, much less "relativistic mass". Even though "relativistic mass" has been used in loose ways for particles in the past, by reputed authors, this is not the case for photons. If you disagree, please provide a mainstream reference otherwise. Thanks! Edgerck 18:42, 9 May 2007 (UTC)

addendum: I saw that you changed it again. Well, if you see the definition of "relativistic mass" you will see that it has a *singularity* at v=c. So, the definition does NOT even apply for the photon. That's why it is incorrect. I am not aware of any "compromise" in physics that would make the formula valid for v=c. Just because it has been here in wikipedia for some long time it does not mean that it is correct or even acceptable. Please revert the change to the last one, as I don't want to clash edits. Thanks! Edgerck 18:53, 9 May 2007 (UTC)

I'm looking for the ref -- it was an article by three authors, among whom Gell-Mann, and appeared in Scientific American sometime in the 1950s-1970s, I think. I'm sure I mentioned it somewhere in this discussion but I can't seem to find it in the archives. It's true that they didn't call it "relativistic mass", but simply "mass". The problem with your point about the singularity at v=c is that you're asserting that that formula is the definition of relativistic mass; it is not necessarily the only one. An alternative definition is that the relativistic mass of any system is simply its (relativistic) energy divided by c², and I believe that this is the definition that was tacitly used for much of the last century. It has the advantage, over invariant mass, that the mass of the whole is the sum of the mass of the parts, which is a property that adheres to the intuitive notion of "mass".

It is, of course, entirely redundant with the notion of "energy", which is presumably why it's not used much anymore. But it's not WP's place to rewrite history or to help contemporary physicists enforce their linguistic reform. Saying that it's not used (or at least very little used) in contemporary physics is fine; eliminating all mention of it is not fine. --Trovatore 19:06, 9 May 2007 (UTC)

Travatore: Thank you for your reply. I am glad you recall that Gell-Mann did not call it relativistic mass. Regarding your assertion above that there is an *alternative* definition for relativistic mass, I find it to be incorrect for a couple of reasons. First, this is simply not in the literature. And there are other two undisputed reasons to edit out the phrase you inserted. The phrase says that the "relativistic mass" of the photon is ascribed to be m = E/c^2. Well, the mass in E/C^2 is actually called the "rest mass" -- not the "relativistic mass" (which is the formula with the gamma factor). That phrase is saying that the photon has a rest mass that is not zero. Once you catch a photon at rest, of course, which is not possible and that's another reason why M= E/c^ (with p = 0) does not apply to the photon. Please revert your last edit, as it is incorrect any way you look at it -- either as relativistic or rest mass. Now, regarding Gell-Mann, when you find the text you will probably he was talking about the increase/decrease of system mass when a photon is absorbed/ or emitted/created/. Thanks! Edgerck 19:20, 9 May 2007 (UTC)

point by point:

• It's at least implicitly in the literature. The article I mentioned was in a compilation called "Particles and Fields"; I can't seem to lay my hands on my copy at the moment.
• When you say "the mass in E/C^2 is actually called the 'rest mass' " -- this is ahistorical. That works only if you interpret the E to be the energy in the center-of-momentum frame. Historically it meant the energy in the laboratory frame.
• No, that's not what Gell-Mann (or his co-authors) were talking about. It was a table of elementary particles with their various properties, one of which was "mass", and the photon was included with a nonzero mass. Some accompanying text (can't remember if it was a footnote or an aside) said something about the photon having zero rest mass (again I can't remember if that was the exact term used) but that it had energy, and therefore mass, "according to the celebrated formula E=mc²" (quote may not be exact). --Trovatore 20:21, 9 May 2007 (UTC)

• Well, that would have been a screw-up on Gell Man's part (or somebody's), because if you define it that way, a photon can have any mass you like. Which is a pretty dumb system of doing things. Physics seeks for invariants, and the only invariant about a photon, when veiwed by different people in different frames, is that its invariant mass is always zero. And every other particle ALSO has an invariant mass (which happens also to be the rest mass, for particles which don't move at c), and that mass is the same when calculated by means of this invariant mass equation. So what are we to call this new invariant quantity which the equation outputs, if not "mass"? If we want to use "mass" for "total energy/c^2" that makes it a non-invariant, and leaves us with no term for the honest-to-god invariant which is what we always measure in the lab whenever we measure the "mass" of things on scales. So historically, that's not a good idea, either. Historical "mass" (what you measure with scales) is closest to invariant mass, not "total-E"/c^2. So let's just let "mass" continue to be that invariant quanity which is closest to what mass has always been understood to be. Okay? That's the way modern physics has decided to do it. You're right that the old way of using relativistic mass wasn't a "mistake", but it was inefficient, and it did end up leaving invariant mass (a genuine and new invariant) out in the cold, with no word to associate with. When we already had a good historical one to use: mass. SBHarris 20:50, 9 May 2007 (UTC)

It did not of course have a fixed value for the photon mass. It probably said something like E/c². As to whether it's a good idea to use the word "mass" in this way, that's entirely beside the point. We can report that contemporary physicists think it's not a good idea, but it's not our role to aid them in getting people to talk the way contemporary physicists want them to.

Of course that's our role! We're an enclyclopedia. You can continue to call it "dephlogisticated air" if you like, but I'm going to use the modern agreed on IUPAC terms here. And it is our job to encourage everybody else to do likewise. SBHarris 21:04, 9 May 2007 (UTC)

As I say, we can absolutely report the usage preferred by contemporary physicists. But no, it is absolutely not our role to assist with language reform, justified or not. This is a reference work. Calling well-attested and equivalent systems of terminology "incorrect" is itself incorrect. --Trovatore 21:10, 9 May 2007 (UTC)

It would be different, maybe, if there were an actual error in the earlier understanding of the physics. But there wasn't; it was exactly the same, not only instrumentally but even noumenally, for the simple cases we're discussing. This is just terminology and bookkeeping; we need to explain the various conventions that people are likely to have encountered, without calling any of them "mistakes" (because they're not mistakes), and reporting which are the preferred conventions today. --Trovatore 20:59, 9 May 2007 (UTC)

Travatore: As you say now (and contrary to your edit note), Gell-mann did not say that the photon has relativistic mass -- so, your text "some older treatments" is not supported.

Photon is a basic article. We don't want to confuse readers with some footnote that is at the same time: (1) not mainstream; (2) undefined (relativistic mass for v=c); and (3) a misnomer (E/C^2 as equal to relativistic mass). Wikipedia is not physicspedia but incorrect attribution of E/C^2 as "relativistic mass" is just against everything else in wikipedia as well. I am willing to take your point on "popular science" in consideration but only if it is qualified as incorrect. Thanks. Edgerck 20:57, 9 May 2007 (UTC)

It is not incorrect to refer to E/c² as "mass". It is historically well-attested. WP is not a textbook; our purpose is not per se to present the material in the way that is easiest to learn without confusion (though of course that's a good thing if other things are equal). No, I cannot accept "incorrect", because it's not incorrect. --Trovatore 21:02, 9 May 2007 (UTC)

If you look up the many definitions of enclyopedia, you will find that they do indeed encompass the idea of presenting material in a way that is easiest to learn without confusion. "Informative" is the word you'll see most in such definitions, and it's pretty hard to be informed while you're confused. Also the idea of providing "overview, background information, summary, a synthesis of information from a variety of sources, and a selective bibliography of authoritative books and articles on a topic." Nobody wants to do overview and sythesis in a way which other than that which is least confusing. I hope you don't. If you have additional historical material, put it later in its own section, along with the alchemical terms, mesotrons, thorium G, and other stuff from the era of less completely understood science. SBHarris 21:15, 9 May 2007 (UTC)

That's a false analogy. Unlike in the case of alchemy versus chemistry, the different terminologies here do not correspond to any difference in the fundamental understanding of these elementary concepts. All that has changed is the terminology and bookkeeping. There is no warrant to describe the earlier terminology as "incorrect", even if it is less convenient when learning from scratch.

We want to present material in the way that is least confusing subject to the constraint that it be accurate and comprehensive. Describing the earlier terminological systems as "incorrect" is inaccurate; ignoring them entirely is a sin against comprehensiveness (and may be more confusing for some readers who have learned the material according to these systems; they need at least a map to the newer phraseology). --Trovatore 21:24, 9 May 2007 (UTC)

• Well, here's an analogy you may like better, then. It's perfectly possible to present things in English rather than metric units. Indeed, it's possible to use older units like minims and pecks, without any fundamental loss of understanding, since interconversion remains perfectly possible. As it does with Arabic to Roman numerals. Would you like Roman numerals to come back, based on the fact that they are historical, and Arabic numerals don't really represent any increase in understanding? As you say, all that has changed since MCVIII is terminology and bookkeeping (this is literal, as Arabic numerals were originally introduced as a bookkeeping device to save use of the abacus). Roman numerals are not incorrect, and nether (for that matter) are speeds in furlongs per fortnight, and energies expressed in dram-furlongs^2/fortnight^2. It's just that we'd rather keep this old stuff out of the energy articles, if it's okay with you. If you really have a fetish for furlongs, you can go to the appropriate specialty article. Otherwise, energy's going to be given in joules, based on meters. I already said I'm not for labeling the earlier stuff as "incorrect", or removing ALL reference to it. But there's really no reason to present much of it-- other than a place to look for historical refs, now that have people have moved on to more convenient notation.

  You say: We want to present material in the way that is least confusing subject to the constraint that it be accurate and comprehensive. Sorry, but just there you slip in a suggestion that we can do something that we cannot generally do. It's like saying we want to know the momentum of a particle "subject to the constraint" that we know its position. Things don't work that way in this universe. Truth and clarity are conjugate variables in writing, just like mercy and justice in law. You can't have unlimited amounts of the one without degrading the other. Fully "accurate AND comprehensive" history (what comes out of document archives) is nearly impossible to make sense of, as it turns out. That's one reason we require historians. Fully accurate and comprehensive treatment of subject material in the sciences is also very confusing, which is one reason we have encylopedias which in order to upgrade comprehension degrade comprehensiveness by means of summary, and also degrade strict historical accuracy (in the case of material which isn't itself explicitly historical) by ignoring many of the twists and turns that historical science actually took to understand it as concept.SBHarris 22:56, 9 May 2007 (UTC)

Travatore: While it is certainly not incorrect to refer to E/c² as "mass" (actually, rest mass), it is incorrect to refer to it as relativistic mass. Note that the term "relativistic mass" is well-defined as mass.gamma where gamma is the Lorentz factor. So, with pun intended, saying that E/c² is the 'relativistic mass' of the photon is a real mess.

Sbharris also makes the point that wikipedia is for learning, hence readers should not be misled, even by omission, and left to sort out by themselves what has already been sorted out. There simply are no "earlier terminological systems" or language support, from gell-mann or other physicists, to the idea that E/c² should be used as the "relativistic mass of the photon".

Any physicist will say that relativistic mass depends on velocity relative to an observer, whereas no velocity is to be found in that formula from "popular science". Its contradictions are so many that we really should not mention it in a scientific, basic article as photon should be. If there are any further questions on this, I'll respond to comments in my user page. Thanks, in the hope that this all proves helpful. Edgerck 21:36, 9 May 2007 (UTC)

E/c² does in fact depend on velocity, because E here refers to energy in the lab frame. I had already responded to that point. --Trovatore 21:37, 9 May 2007 (UTC)

Oh, and by the way, Wikipedia is not for learning, though it's a valuable learning tool. It is a reference work, a place to look things up. This is a subtle but important distinction. If you want to lead readers through your preferred way of learning something, rather than simply presenting the facts, the place to do it is Wikibooks. --Trovatore 21:50, 9 May 2007 (UTC)

We've already discussed this at length and it pains me to read the exact same arguments being repeated over and over and over again. Please re-read Archives 4 and 5 above, particularly Archive 4, which presents my summary of the usage in actual published physics textbooks. As noted then, the clear consensus in the physics community is that relativistic mass is not a useful scientific concept, especially given the need for transverse and longitudinal masses. However, a few non-physicists see a use for the concept, and so we worked out the compromise. This is really not worth arguing over, since no actual physics is being discussed; there is no experimental difference between the two positions, as far as I remember. Please, let's not get into yet another edit war over such an airy trifle! Why don't you all devote your energy to writing new physics articles instead and leave this be? Willow 22:07, 9 May 2007 (UTC)

Trovatore: it just occurred to me that there is a solution to this disconnect! I don't believe in pushing 'compromise' solutions in exact sciences. So, it's not that. It's a real solution and one in which the popular science idea for "mass of a photon" may be presented clearly and usefully. The idea is quite simple. What we are calling "relativistic mass" has two different meanings and needs disambiguation:

-For popular science, "relativistic mass" is the mass that can be calculated using the special relativity theory mass-energy equivalence relation m = E/c². The relativistic mass, so defined, gives the mass-equivalence of the particle energy in the laboratory frame of reference. It can be well-defined for any particle, including photons, and represents the mass increase if the energy E is entirely converted to mass.

-For physics, "relativistic mass" is given by m' = m.gamma, where gamma is the Lorentz factor. The relativistic mass is not defined for photons.

One could have part of the text above in the footnote. The main part of the 'relativistic mass" for popular science could be expanded there.

I hope this is helpful. Thanks. Edgerck 23:01, 9 May 2007 (UTC)

Hmm, getting closer. However I'm unconvinced that the E_lab/c² formulation was only "popular" -- I hardly think Gell-Mann would have signed off on that, even in an article in SciAm (remember that back then SciAm was serious science for scientists in different specialties, not the slightly-better-than-Discover thing it has become today). And I was under the impression that contemporary physics didn't make much use of relativistic mass at all, so that's a problem with the second part of your formulation.

On a side note, I can't see any advantage at all to making use of "relativistic mass" if you exclude massless particles. The version that includes them at least has the advantage, as I mentioned, that the mass of the whole is the sum of the masses of the parts, and that momentum equals mass times velocity (note that this works even for the photon). --Trovatore 00:18, 10 May 2007 (UTC)

As you recollected (up above), gell-mann did not sign off on a relativistic mass for the photon. And I'm not the one excluding massless particles that move at speed c from having a physics-defined relativistic mass -- mathematics excludes them, as gamma becomes undefined at the singularity with v=c.

The second part of my suggestion is mainstream physics (I did not invent it). The first part is my attempt to clarify what may be a popular intuition about what you are describing above -- the "mass of a photon" in relativistic theory. BTW, the wikipedia entry for popular science has no derogatory connotation -- it's worth reading it. It is science in popular terms and it should NOT be incorrect. Both parts of my suggestion are physically sound.

It's not just a matter of terminology. Please see above that the two notions of "relativistic mass" are completely different. They are apples and speedboats. For the photon, one is definable while the other is undefined.

My suggestion is to avoid language interference with communication. If tell you I am going to send you a GIFT and you think that I will send you a poison (Gift is German for poison), then we are not communicating! Let's avoid the trap and disambiguate. Edgerck 01:03, 10 May 2007 (UTC)

• The reason that Gell-Mann did not refer to a "relativistic mass" for the photon is the article simply called it "mass". The contraposition was rest-mass/mass for what would now be called mass/relativistic-mass. If you prefer to say that older sources ascribed a mass to the photon, but that what they meant was what would now be called "relativistic mass", that would be OK.
• As to "mathematics excludes them" -- sorry, I don't buy it. mv/(1-v^2)^0.5 is a formula for relativistic mass, not its definition, and obviously it applies only for v!=1. The correct interpretation for the relativistic mass of a particle traveling at the speed of light is obvious; it's the value of m that makes both the formula E=mc² and the formula p=mv true, just the same as for massive particles.
• I have nothing against disambiguation. But I am not convinced that physicists did not use this terminology freely around the middle of the last century. The Gell-Mann reference was trivial to find; I just saw it lying around, thought "let's see what they say about photon mass", and there it was. I've seen other such casual reference that I don't recall now. I think you may be too influenced by contemporary usage to have noticed these things. --Trovatore 02:09, 10 May 2007 (UTC)

Relativistic mass

Edgerck attempts a bifurcated definition above, which I repeat here since that section was too big to edit it:

• -For popular science, "relativistic mass" is the mass that can be calculated using the special relativity theory mass-energy equivalence relation m = E/c². The relativistic mass, so defined, gives the mass-equivalence of the particle energy in the laboratory frame of reference. It can be well-defined for any particle, including photons, and represents the mass increase if the energy E is entirely converted to mass.

• -For physics, "relativistic mass" is given by m' = m.gamma, where gamma is the Lorentz factor. The relativistic mass is not defined for photons.

Now, please correct me if I'm wrong, but doesn't the second definition follow from the first in all cases of particles with a nonzero rest mass?

No. But it doesn't matter because this is the talk-photon. I'd think more progress could be made in less time if the discussion stays on the topic: photon. Edgerck 08:34, 11 May 2007 (UTC)

That would be fine; but I was responding to your claim that the photon is an exception in its relation to Einstein's E=mc2 formula. I say it's not. Your "No" here is baseless. Dicklyon 15:40, 11 May 2007 (UTC)

Dick -- Oh, please! You would think that if it were baseless I wouldn't say it. Now specifically to your reply, I did not say that the photon is an exception to E=mc²; I wrote that the relativistic mass is not defined for photons.

Einstein used the equation m=E/c² to define the rest mass of a particle (when p=0). Some mainstream physicists (around 1934, I can provide the references if you need) then "generalized" it and applied that equation to cases when p>0, using it to redefine a notion of "relativistic mass" (that was already defined around 1910 as M=m.gamma). But the equation is incorrect for p>0. If you suppose otherwise, then you will arrive at the conclusion that the more general equation m = (E/c²) sqrt [1 - (pc/E)²] is incorrect. Both give the same result iff p=0 as you can see by inspection. What's happening here is that the validity condition for m=E/c² was forgotten in the hopes of a "generalization".

Now, quite an interesting aspect and one which the photon article does not mention, is that many mainstream physicists today think that unless every photon in the system has the same momentum (which is statistically unlikely), a system of more than one photon will have mass. This is quoted as a simple result of the facts 1) that such a system's energy will be greater than c times its momentum, and 2) that the invariant mass of a system is given by m = (E/c²) sqrt [1 - (pc/E)²]. For example, consider two photons of identical energy E and opposite momentum. In this case, the system has no momentum so its invariant mass is 2E/c².

So, mainstream physics today says that while one photon has zero (invariant) mass, two photons of identical energy E and opposite momentum have (invariant) mass of 2E/c².

Now, going back to that formula m=E/c² for "relativistic mass", if you would apply it to the system of two photons of identical energy E and opposite momentum you would get the very same values for "relativistic mass" and invariant mass! In other words, the photons must be at rest. Since this is absurd, it shows that you cannot apply the formula m=E/c² as a "relativistic mass" of the photon. In fact (as shown above), the contradiction is more general and the formula m=E/c² simply is not correct except in the rest frame, where it gives the invariant mass.

Hope this all proves useful.Edgerck 19:59, 11 May 2007 (UTC)

Hm? The system consisting of the two photons is "at rest", so to speak. More precisely, it has zero total momentum in the lab frame. How is this supposed to be an absurd outcome?

Try this: "In other words, the photons must be at rest always". In still other words, the absurd outcome is that always (i.e., for any total momentum), the formula m=E/c² gives the same number for "relativistic mass" as if the photons had zero total momentum.

BTW, perhaps one should add that example of two photons, that together are not massless, to the article footnote on the massless photon. (more below)

For more complicated systems with particles going every which way, it's not so clear what would be meant by the velocity of the system as a whole, and I don't expect this simple outcome would be reproduced for any obvious definition of that velocity. But this particular example hardly seems to establish your point; quite the opposite. --Trovatore 20:05, 11 May 2007 (UTC)

As Trovatore gets and Edgerck doesn't, the notion of a photon at rest is nonsense, but the notion of a system at rest, some of whose rest mass is in the form of photon energy, is not. It's not that complicated. E=mc^2 applies perfectly consistently to the total energy and the total relativistic mass of artibrary systems and arbitrary observers. It's just not the same same definition of E and m that Edgerck and many others prefer to use; but it is one definition that is still worth mentioning in a footnote. Dicklyon 22:07, 11 May 2007 (UTC)

and yet, whatever the velocity of the system as a whole might be, the formula m=E/c² gives the same number for "relativistic mass" as if the photons had zero total momentum. Isn't that strange? Edgerck 20:45, 11 May 2007 (UTC)

Really not following you here. In a different frame, in which the photons do not have zero momentum, the value of E will be different, so the formula m=E/c² will not in fact give the same number. Is your point simply that the formula doesn't show explicit dependence on v or p? If so, so what? If not, what did you mean? --Trovatore 21:35, 11 May 2007 (UTC)

If so, then the former is more general, and no less correct; for massless particle, the factorization would be zero*infinity, which is not well defined, though the energy and mass and velocity all are. And from where comes the idea that one is more "popular" and the other more "physics"? References, please. Dicklyon 04:37, 10 May 2007 (UTC)

Mass and energy may be equivalent but they are different concepts. Ask any physicist (I am one) and they will confirm that photons have energy but zero mass, and this applies to both rest mass and relativistic mass, in whatever form the latter may be defined. I find it unlikely that Gell-man wrote otherwise (I can't find any articles by him in Scientific American). If any physicist does say that photons have relativistic mass, then they are part of an extreme minority point of view. Timb66 05:28, 10 May 2007 (UTC)

What does it means that they are equivalent, but are different concepts? Didn't Einstein show a unification of the concepts? If you accept relativistic relationships between mass, velocity, and energy, why go to the trouble of making a special-case exclusion for the photon? I'm not a physicist per se, so I can't attest to what language they use in their journals, but there's got to be some way to talk about the energy-mass of relativistic particles without excluding photons, yes? Willow has proposed a narrow criterion below, based on what she knows the right answer is, which is to ignore this problem; I can't comment there, since she demands only data in the form of physics journal references. But, as she notes, the term relativistic mass in not uncommon for this purpose outside the physics journals. It's easy to find lots of good examples in google book search, including a physics journal article that appeals to physicists to stop using it that way; that's got to mean... what? Dicklyon 06:09, 10 May 2007 (UTC)

Perhaps this will address your question?

• "Mass" m is considered to be a scalar, a single number that is an intrinsic property of a particle. Mass has no direction and does not depend on the apparent motion of the particle, which can be changed without affecting the particle at all, merely by changing your own frame of reference. In other words, the mass of a particle is the same for all observers, no matter how they are moving; in that sense, it's part of objective reality.

• "Energy" E is part of a four-vector p^μ with momentum p. The energy of a particle does not have objective reality, since it can be changed without affecting the particle at all, by changing your own frame of reference. The energy of one particle at one event in space and time is different for different observers. This is true for the components of all four-vectors; their components differ with different observers. However, you can construct an invariant scalar from those components that is the same for all observers; this scalar is part of objective reality, in our sense. And guess what? For the p^μ four-vector of a particle, that invariant scalar is its mass squared

$p^{\mu }p_{\mu }\equiv {\frac {E^{2}}{c^{2}}}-\mathbf {p} \cdot \mathbf {p} =m^{2}c^{2}$

times the speed of light c, squared. That's why m is sometimes called "invariant mass". In the particular frame where the particle is at rest, p=0, so that the above equation becomes

$p^{\mu }p_{\mu }={\frac {E^{2}}{c^{2}}}=m^{2}c^{2}$

Taking the square root of both sides and rearranging yields the familiar equation E = mc². That's why the invariant m is sometimes called the "rest mass".

In some cases, the invariant scalar formed from p^μ equals zero, as it does for the photon. Thus, we say that the mass m of the photon is zero.

Does that clarify the relationship between energy and mass? Hoping that this helps, Willow 07:05, 10 May 2007 (UTC)

Willow, I know enough physics to understand all that, but thanks anyway. The issue here, however, is what to call the energy-equivalent mass E/c2, which as you note depends on frame of reference and is not an intrinsic property of the particle. Maybe if we all agree that we all understand the same physics, we can focus on the question. Dicklyon 15:02, 10 May 2007 (UTC)

Edgerck, on the other hand, seems confused. He took out the note again, essentially implying that the notion of a relativistic mass is inapplicable to the photon. That's because he doesn't see how Einstein's E=mc^2 applies in this case, and wants to only acknowledge his formula that multiplies rest mass by gamma, which is of course not going to work for the photon. I think he should recuse himself from the discussion, since he doesn't acknowledge the same physics as the rest of us do. Dicklyon 00:09, 13 May 2007 (UTC)

One more thing: I did provide a link to a GBS search showing a bunch of books that talk about the relativistic mass of the photon, so it is easy to provide a reliable source for the "some authors" statement. Should I go ahead and embed one or more of those book refs into the footnotes, or are people OK with it as it is? Dicklyon 00:12, 13 May 2007 (UTC)

Edgerck, here's the simplified one-page version from The Physics Companion By Anthony C. Fischer-Cripps, which says it's for college undergraduates:

File:Relativistic Mass of Photon.png —The preceding unsigned comment was added by Dicklyon (talk • contribs) 03:33, 13 May 2007 (UTC).

Dick: Nice find. Unfortunately it's a copyright violation; it will have to be deleted from image space. It isn't really necessary to provide the image here anyway, since the link works and is free. --Trovatore 19:13, 13 May 2007 (UTC)

All: I hope that wikipedia helps such things not to be published again. But, referencing it? I don't think so. Why value what's wrong? An author that did not do a basic search on the material published?

Further, I do not think that a pre-college level book qualifies as a counter reference of a widespread notion for a basic article such as photon, which references (specially for a counter-reference) should a par with the other references that are cited in photon.

I am also making a call for this discussion to be more helpful in style, without the forceful language that I see in a few examples above (unfortunately, as frequently used when the subject is language of relativity theory as popularly discussed). Thanks. Edgerck 00:19, 14 May 2007 (UTC)

The point is that it isn't wrong. It's simply a different use of terminology and a different bookkeeping method. The predictions are exactly the same; even the underlying noumena are exactly the same (it's not even a different interpretation, like many-worlds v Copenhagen in QM). It's not your preferred use of language; we get that. It is certainly not now, and may never have been, the preferred usage of any large contingent of physicsists. But it can't be ignored, for the reasons Count Iblis details below, and it shouldn't be called "wrong", because it isn't wrong. --Trovatore 01:21, 14 May 2007 (UTC)

As I wrote below, I think this discussion is off-topic for photon. I don't know anyone in physics who said that the photon had relativistic mass. So, let's drop this discussion in photon-talk and continue it elsewhere (Trovatore's talk page?) where it is pertinent. Edgerck 01:51, 14 May 2007 (UTC)

Add only data to this section

There's a way out of this painful labyrinth. We need to distill the dispute into questions that can be answered definitively, and then answer them so either by reasoning, citations or data. The present dispute seems to revolve around two issues: the correctness and mainstream-ness of "relativistic mass" in actual physics research.

Correctness of "relativistic mass"

It is possible — as shown by Lorentz originally, I believe — to re-jigger Newtons's 2nd law to make it reproduce results consistent with the energy-momentum 4-vector approach. Although there is no new prediction in using such a "relativistic mass" approach — i.e., no new physical content — it is a feasible representation of relativistic mechanics, especially if one uses Lorentz's longitudinal and transverse relativistic masses. Likewise, one may say consistently that the relativistic mass is defined as E/c², although that definition would require a reference to an article in a primary physics research journal to allow that usage in Wikipedia. Since it makes no false predictions, "relativistic mass" is not "incorrect" per se. However, it seems to be a fringe concept used mainly for explaining relativistic physics to non-physicists, at least in my survey of the literature.

Mainstream-ness of "relativistic mass"

The sixteen physics textbooks surveyed in Archive 4, as well as Einstein's own works, show that relativistic mass has never been a mainstream concept used by practicing physicists. This survey included many of the major contributors to relativity and relativistic field theories, including Einstein, Pauli, Sommerfeld, Weyl, Landau & Lifshitz, Feynman, and Steven Weinberg as well as many standard textbook authors such as Jackson, Goldstein and Misner, Thorne and Wheeler. This survey covers eight decades (1905-1984), including the "middle of the last century". Despite a sincere search, I have not found even one instance in the physics literature where "relativistic mass" is used by that name in the E/c² sense; in particular, I have never found that concept applied by name specifically to photons.

I think it's fair to ask anyone who argues otherwise provide a similar amount of data in support of their position. Speaking for myself, I would like to see 10 research articles in Physical Review, Physical Review Letters, Annalen der Physik and/or Zeitschrift der Physik that use "relativistic mass" by name in the E/c² sense, and another 10 that apply it specifically to photons. If such articles cannot be produced within the next ten days, I believe the honorable thing to do would be to withdraw the assertions that "relativistic mass" has ever been a mainstream concept in physics, and that it has ever been applied to photons. Conversely, if the 20 are found, I suggest that we find some compromise wording that puts the usage in perspective.

For the record, Scientific American has never been a physics journal, since it does not publish original research in physics. The same is true for "Physics Today" and other popularization journals, and online resources, lecture notes, etc. Only primary research journals in physics are acceptable for the following 20 citations.

Research citations that define relativistic mass explicitly as E/c²

• Ref 01:
• Ref 02:
• Ref 03:
• Ref 04:
• Ref 05:
• Ref 06:
• Ref 07:
• Ref 08:
• Ref 09:
• Ref 10:

Research citations that apply relativistic mass (with that name and definition) to photons

• Ref 11:
• Ref 12:
• Ref 13:
• Ref 14:
• Ref 15:
• Ref 16:
• Ref 17:
• Ref 18:
• Ref 19:
• Ref 20:

Millions of articles have been published in the primary research physics literature over the last century. Hence, it is fair to ask that twenty be supplied to show that a controversial usage was accepted at some point by at least some practicing physicists. If you need more than 10 days to collect your references, or if you feel that twenty is too high a number, let's discuss it. I am determined that this debate should be resolved now and for good. There are too many other physics articles demanding our collective attention. Hoping that this vampire will stay dead this time, Willow 05:19, 10 May 2007 (UTC)

I do not agree that the references need to be in primary physics journals, nor do I think twenty citations is remotely reasonable. A couple good attestations by respected physicists should be good enough, whether they're in research journals or not. Note that I'm not asking that the article claim that this is the preferred usage of physicists. I just want it acknowledged that it exists, without claiming that it's "wrong". --Trovatore 07:57, 10 May 2007 (UTC)

First off, let me apologize for my cranky tone above. You caught me at a bad time, and I hope that you forgive me for my lapse in gentleness. However, I hope that you'll appreciate that this ever-resurgent conflict is painful to me and that you'll make an effort to resolve it quickly. We need to establish in what sense the definition m_γ = E/c² "exists" for photons.

As noted in Archive 4, the definition of relativistic mass as m_r = γm has appeared in a few textbooks (2 of 16) for material particles. Therefore, the questions remaining to resolve are

• Has relativistic mass been defined explicitly as E/c² in any publication, such as a pedagogical introduction to special relativity for non-physicists? Has that definition been applied to photons and other particles with zero invariant mass?

I believe the answer is "yes". However, we should establish which "authors" have stated this definition and applied it to photons, and in what sources. I propose the following threshold: the "relativistic mass of the photon" should be stricken from this Wikipedia article unless the definition m_γ = E/c² has been made explicitly, accepted as valid and applied to photons by at least two notable physicists in paper publications.

• Has relativistic mass been defined explicitly as E/c² in any published physics research? Has that definition been applied to photons and other particles with zero invariant mass?

If the answer to this is "no", then it's fair to say that the "relativistic mass of the photon" is not a physics concept. Physics is defined by what has been published and confirmed in its primary research literature. My own diligent search has suggested that the answer is indeed "no", but it's easily possible that I missed something and would be glad to learn better.

I would like these points to be established, so that we don't waste each other's time by arguing past one another without listening to one another or respecting each other's hard data and arguments. Personally, I feel that 10 articles for each assertion is appropriate, but five articles might be OK if they came from more than one researcher. Wikipedia requires reliable sources and I am formally asking you to provide them. I don't feel that this is too great a burden; anyone can go to a friend's house or the local library, no? Willow 12:21, 10 May 2007 (UTC)

Willow, your equation of "reliable sources" with "research journals" is simply not part of Wikipedia policy. WP relies on many publications that are not research journals. Often these later sources are more reliable than research journals, because there has been time and perspective to find the errors that may have appeared in research papers.

So no, I simply cannot accept your formulation that "the 'relativistic mass of the photon' should be stricken from this Wikipedia article unless the definition m_γ = E/c² has been made explicitly, accepted as valid and applied to photons by at least two notable physicists in paper publications." I say instead that the Gell-Mann paper, by itself, should be enough to attest this usage (if I can find it). I agree that it can be qualified as having appeared in a publication for non-physicists. --Trovatore 21:15, 10 May 2007 (UTC)

Are we still just talking about a mention in a footnote? Is there any reason to NOT acknowledge that some authors ascribe a relativistic mass to a photon? The relativistic mass article is linked, and more depth on usage patterns can be presented there for anyone who cares to go further. Dicklyon 05:31, 11 May 2007 (UTC)

I've changed the footnote to reflect more precisely where the two definitions are encountered in practice. In the scientific literature you will almost never encounter the relativistic mass. But if you read popular books, e.g. Paul Davies or read some high school physics book where relativity is heuristically explained, you will encounter this definition. Count Iblis 22:49, 11 May 2007 (UTC)

It's not really verifiably "precise." You said "In the popular literature and in high school textbooks one sometimes encounters an alternative definition of mass, the so-called relativistic mass, which is always positive. This definition of mass is never used in the scientific literature." The part about high school textbooks, which sounds like it was intended to be perjorative and limiting, does not really cover the scope of usage revealed by the google book search that I linked above. And the assertion about "never" is obviously not verifiable, just your impression. Dicklyon 03:43, 13 May 2007 (UTC)

Ok, but we should write in the article what is (at least roughly) the situation in the real world. If, in an effort to be "100% verifiable" we come up with some vague sentence that would be consistent with a hypthetical situation in which, say, 30% of all scientific articles use the relativistic mass concept, then that would be misleading and the wiki policy Wikipedia:Ignore all rules should be invoked.

I think this situation here is comparable to the problem in the global warming article about how to mention the fact that there are a "few" scientists who dispute the scientific consensus on climate change was resolved. No one has actually counted the dissenting scientists, the word "few" is a weasel word etc. etc. So, it seems to violate some wiki rules to write that "a few scientists..." don't agree with the consensus. However, a sentence like "there are scientists..." would be very misleading about the extent of the consensus.

So, I suggest that we should think first about what we want to tell to the readers and only then look at the wiki rules (if necessary). I.m.o. what you want to do is:

1) Clear up possible confusion in the minds of lay readers who have read that the photon has a mass of E/c^2 but read here that it is zero.

2) Make clear that "invariant mass" is the standard "default" definition in the scientific literature. Now, one can argue about the number of articles that do use "relativistic mass", but it is a fact when writing an article in say Phys. Rev. D., you don't define what you mean by mass, because the invariant mass is the "default" definition. If you really wanted to use the word mass in the sense of "relativistic mass", you would have to write a clear sentence explaining that you are using the non standard definition.

3) Write that outside the scientific world (at least today) "relativistic mass" is still used quite a lot.

Points 1), 2) and 3) together pretty accurately explain what is going on in the real world, i.e. the apparent contradictory facts about the mass of the photon and where you are most likely to encounter invariant mass and relativistic mass.

B.t.w., I didn't mean to be "perjorative and limiting", just as accurate as possible. Most high school physics books do use relativistic mass and a lay reader reading this article may well have studied from such a book and he would then think that this article is wrong or that the sentence about invariant mass being used in science is wrong (isn't his high school book a "science book" too?) Count Iblis 13:59, 13 May 2007 (UTC)

Count Iblis, I think that's an excellent summary. The comparison with global warming is a bit misleading because in that case the argument is over substance, whereas here it's just over terminology, but other than that your comments should be the basis for a fine solution. --Trovatore 19:05, 13 May 2007 (UTC)

I think this discussion is off-topic for photon. While the term 'relativistic mass' was used in physics (1912-1950~1980), hardly anyone in physics said that the photon had relativistic mass. So, it's not the case to mention it in photon.Edgerck 01:48, 14 May 2007 (UTC)

Obviously I disagree with you. The usage is well attested, even if not in research journals, and must be dealt with. Since it is not incorrect, we also can't call it incorrect. --Trovatore 01:51, 14 May 2007 (UTC)

Ok, let me bifurcate this into verified and unverified. I edited the article to the following change, which I dissect below, inlined for the quote:

(VERIFIED, current)

The mass of the photon is believed to be exactly zero, based on experiment and theoretical considerations, as referenced in this article. The same references do not use the concept of relativistic mass for the photon.

This first part is supported by the article's references, including disclaiming relativistic mass for the photon. For those who do not like it, please live with it -- it is true that well-respected experts have affirmed that one should not talk about a relativistic mass for the photon. For a popular science explanation why this is the case see "What is the mass of a photon?"

(UNVERIFIED, current)

The following assertion is unverified and not in conformance with wikipedia rules -- do not reference or denote: Some references, purported in the past that the photon has a relativistic mass defined as E/c², where E represents the photon's energy.

Verifiable, peer-reviewed source and date for relativistic mass of photon should be cited or this last sentence should be deleted per wikipedia rules.

Hope this proves to be helpful. Edgerck 02:34, 14 May 2007 (UTC)

What is Edgerck's point?

Edgerck, you wrote in the footnote that "According to the same references, it is incorrect to say that the photon has relativistic mass." Can you point that out please? Which references? If not online, please quote. If some references do actually say that, it's new to me, and important. I'm puzzled, because you seem to disagree with all those who DO acribe relativistic mass to the photon, as if you believe in a different physics, not just a different terminology. I'm wondering what sources are leading you to think that way. Dicklyon 03:09, 14 May 2007 (UTC)

Dick: That's quite an interesting attempt to turn the table. Instead of you providing a quote for the unverified claims in the second part of that footonote, you now request me to prove that mainstream physics today does not ascribe a relativistic mass to the photon. I also found it interesting what you wrote: all those who DO acribe relativistic mass to the photon -- as if "all those" would be any significant following.

I have provided references above; have you looked at them? Would you like me to point out more of them explicitly? I'll be happy to do so if that's what you're asking for. You are the one who added a statement to the article that you refuse to point out the support for. Dicklyon 04:21, 14 May 2007 (UTC)

Masterful. But, I don't bite. You know, physics is not a matter of opinion or "my reference against yours". It's an exact science governed by experiments. Yes, language creates problems to communicate knowledge and that's why we, more and more, prefer to use abstraction and mathematics. Ever wonder why the quark is called "quark" and has "flavors"? Because it's difficult to ascribe a false meaning to the names and then misuse those meanings as having any significance for the referents.

As far as I know, I don't have any differences with anyone here on the physics. That's why I'm asking for a clarification of your point. If you think we differ on the physics, which is the feeling I'm getting from some of your comments, please clarify and make explicit that difference, so we can resolve it. If it's just a matter of what language people use, that's a matter for references. Dicklyon 04:21, 14 May 2007 (UTC)

So, finally, can you please tell me What is your point?. Do you want to question basic facts in physics? Why don't you, instead, please verify adequate references for that UNVERIFIED statement that we all know (by wikipedia rules) should not even exist in the article as it stands.

No, I don't want to question basic facts in physics. I want to acknowlegde that basic fact, and acknowledge authors who do, that the total mass of a system includes the relativistic mass of the photons in the system, as measured in the system's frame of reference. Remember the "photon in a box" discussion from many months ago? The mass of a box includes the energy mass of any photons in the box. Dicklyon 04:21, 14 May 2007 (UTC)

I hope this continues to be helpful. Edgerck 03:52, 14 May 2007 (UTC)

To skirt discussion, the VERIFIED part has been changed to:

VERIFIED

The mass of the photon is believed to be exactly zero, based on experiment and theoretical considerations, as referenced in this article. The same references do not use the concept of relativistic mass for the photon.

Thanks. Edgerck 04:11, 14 May 2007 (UTC)

Yes, that's very helpful. Thanks for retracting the assertion that I was questioning. Dicklyon 04:21, 14 May 2007 (UTC)

I reverted your latest chatty edit to the footnote. That's not a good place for the arguments about what is verifiable. Are you saying that the statement that some authors ascribe a relativistic mass to the photon is unverified? The physics is not in question, is it? The language is not so popular perhaps, but certain some authors, not just high-school and popularization authors, do exactly that. I provided refs above, and can embed them into the footnote if that's what you're askig for. Dicklyon 04:29, 14 May 2007 (UTC)

What you did is vandalism, as you deleted a valid request for citation on the text that had "some" as a reference. I reverted the vandalism. The unverified quote can stand, mind you, but please do edit WITH references or DO NOT delete the request for same. Please refer me to the wikipedia FAQ if you have questions. Edgerck 04:37, 14 May 2007 (UTC)

That's rather disingenuous of you, don't you think? The fact tags were on the strange stuff you wrote, which I replaced with a different version. You had "Some references, purported in the past that the photon has a relativistic mass defined as E/c², where E represents the photon's energy." Since you're the one who wrote about "some references" and "purported in the past" and expressed your doubt by putting fact tags on your own writing, I just took it back to the last sensible version. But since I now understand that what you're askig for is a source embedded in the footnote, I'll go ahead and do that.

In fact the E/c² meaning is well attested; see the Google search Dickylon provided, for one example. For another there's the article I mentioned where Gell-Mann is one of the co-authors. I still can't find the collection, but I found the title: It's Particles and Fields. Readings from Scientific American. W. H. Freeman, San Francisco. 1980. ISBN 0-7167-1233-4.. I'll try to find my copy or find it in the library; if I can't I'll order it. --Trovatore 04:31, 14 May 2007 (UTC)

Trovatore: I would be happy in keeping the request for references there until you can provide them in a reasonable time. Until then, please do not edit out the valid request for references. Edgerck 04:37, 14 May 2007 (UTC)

In the mean time we can put in the one Dicklyon found, The Physics Companion. Out of curiosity, exactly what do you mean by "denote"? I do not think it means what you think it means. --Trovatore 04:41, 14 May 2007 (UTC)

"denote" is commonly used in publication guidelines. Denote: announce: make known; make an announcement; "Denote a reference at the appropriate place in the text by an underlined or italicized Arabic numeral in parentheses, e.g., (2) or (2)."; Edgerck 04:49, 14 May 2007 (UTC)

OK, I added two sources to the note. By the way, Edgerck, this energy mass matters in situations such as described at photon#Contributions_to_the_mass_of_a_system. That is, this mass we're talking about is already acknowledged in the article, and has been for a very long time. Dicklyon 05:05, 14 May 2007 (UTC)

Dick -- The "mass" you are talking about is just a formula, that is not even valid for v=c. Regarding actual mass and photons, I observed this in one my previous replies to you (some place above): "mainstream physics today says that while one photon has zero (invariant) mass, two photons of identical energy E and opposite momentum have (invariant) mass of 2E/c2". Hope this helps. Edgerck 20:13, 14 May 2007 (UTC)

I have not followed this mass/massless discussion in detail, but it seems to me that

1. one can make a consistent description involving (high-energy) photons even if one substitutes the letter m' for "E/c^2", as long as you know what you're doing.

1. if you make this substitution and then try to apply other laws of physics that involve a mass m, you can get into trouble if you don't know what you can and cannot do after this substitution.

What is relevant in a mathematical description of physics is whether or not it can describe empirical facts. If a couple of authors find the maths more elegant by assigning a mass to the photon, by all means mention it somewhere in a footnote. However, I think most physicists prefer the description with a massless photon, so that interpretation of reality should be focused on in the article. The subject of relativistic mass is already tricky enough. (Put photons in a reflective box and the box will be heavier than when it is dark inside because of the relativistic redshift of the photons when they travel against the gravitational field.) Disclaimer: theoretical high-energy physics is not my field of expertise. Han-Kwang 12:24, 14 May 2007 (UTC)

The formula m=E/c^2 does not apply when the particle moves with v=c. So, while its use is questionable (google "okun mass") for particles with mass (that travel at v<c), it is actually undefined for the photon. That's why it is incorrect to talk about a relativistic mass for the photon (wikipedia notwithstanding). Nonetheless, the energy-momentum equation does apply to the photon, and for a system of photons.Edgerck 19:54, 14 May 2007 (UTC)

Finally: a point of physics to argue!. Edgerck, do you have a source for this exception that you are asserting? All the books I've seen suggest that there is no such exception, that photons obey the same law of momentum and energy and mass (both kinds) as all other particles, massless or otherwise. Dicklyon 20:01, 14 May 2007 (UTC)

Dick -- I have said and referenced this very point some 5x above. Most books and articles I've read, as well as my own conclusions, are clear on this same point. So, if you want to talk about it and since it's off-topic for photon-talk, I'd be happy to reply in your talk page. Perhaps you could prepare a fresh topic item there, using my paragraph above as my summary for the start and stating your points as well. Thanks.Edgerck 20:21, 14 May 2007 (UTC)

I can imagine that it makes certain types of mathematical analysis a bit more simple. However,

If you are using gravitational field arguments, you're outside the domain of special relativity. The example you cited can be explained, however, in special relativity. Similar arguments apply to an an atom in a mirrored box. Edgerck 19:54, 14 May 2007 (UTC)

Thanks for you comments, Han-Kwang. It's really much simpler than that, though, as nobody disagrees that photons are massless. The citations that talk about the relativistic mass also talk about the zero rest mass. It's just a question of whether an author prefers to mention the total mass equivalent of the total energy, or not; and if so, what do they call it. I agree that the modern trend among physicists is to not mention the relativistic mass, since it is completely redundant with energy. Nevertheless, that relationship is still mentioned by a fair number of texts, and there's nothing wrong with it. It is not an indication of any underlying disagreement on what the physics is, as Edgerck seems to want to make it. Dicklyon 14:11, 14 May 2007

(UTC)

The book by "Frederick J. Bueche" cited by the dissenting minority in this community actually negates the use of relativistic mass for the photon. That book needs to be deleted from the supporting documentation to the notion of a relat. mass for the photon. The formula given there for relat. mass is undefined for v=c. The book also correctly says that there is a (invariant) mass equivalence relation for the photon. I will wait for the corresponding editor to delete that reference, to prevent edit clashes. Thanks. Edgerck 19:54, 14 May 2007 (UTC)

It says the photon is massless (which we all agree on), but that it has an associated mass given by the mass–energy relation. Do you remember what that is? It's E = mc^2, or m = E/c^2. Shall I add "associated mass" to the list of things it is called? I don't think they really meant that as a name for it, just a generic term for this mass that's usually called energy mass or apparent mass or relativistic mass. Dicklyon 19:49, 14 May 2007 (UTC)

By the way, who is with you in the "silent majority"? I haven't been able to identify anyone. Please, guys, speak up if you want to be heard. Dicklyon 19:50, 14 May 2007 (UTC)

speed of light in vacuum

These were removed/changed from the article:

• "... always travels at the speed of light in vacuum"
• "Although a photon travels with the speed of light in vacuum, the speed of light in a medium can be lower. In the quantum-mechanical description of light and photons, this is due to photons being absorbed and re-emitted by the medium."

I see that it was a bit ambiguous, but I meant it to be parsed as "travels at the (speed of light in vacuum)" rather than "in vacuum, it travels at the speed of light". Speed of light without additional specification can refer both to c and to c/n where n is the refractive index. Any objections against placing it back, rephrased in order to prevent this ambiguity? Han-Kwang 11:44, 14 May 2007 (UTC)

Photons always travel at the speed of light. It does not matter where. So, why would one add a restriction (vacuum)? Thanks. Edgerck 19:39, 14 May 2007 (UTC)

I put the "in vacuum" back in. Photons indeed travel at lower speeds in media. The picture of them being absorbed and emitted is a bit simplistic and misleading, because it suggests that the real speed at which photons travel inbetween atoms is still c. Photons in vacuum are ultimately defined as excitations of the electromagnetic field, but in media the physical photon is defined differently, because the electromagnetic field will perturb the media (this is explained later on in the article). The naive picture of absorption and emission breaks down because the wavelength of the photon is much larger than the distance betwen atoms. Count Iblis 20:56, 14 May 2007 (UTC)

I mostly agree with Count Iblis. I think the problem is that there are two somewhat different interpretations of the photon concept. The first, that I tried describing in the intro, is excitations of E.M. field modes. However, in this picture, photons do not have a speed at all, since the modes are delocalized. One could say that the phase velocity of the field modes is the "speed of the photon", and that indeed varies with the medium. However, in the other picture, photons are the carriers of all E.M. interactions, including electrostatic interactions, and as such, they do travel at speed c. I don't know enough about quantum field theory (if anything at all) to judge to what extent these two pictures are related. Han-Kwang 21:06, 14 May 2007 (UTC)

Modes are delocalized, but they have a well defined momentum... And yes, one can look at different aspects of the electromagnetic field, ultimately one has to define very clearly what one exactly means with "speed", "mass" etc. Count Iblis 21:48, 14 May 2007 (UTC)

Iblis and Kwang: photons always travel at the speed of light in the medium, be it vacuum or not. Adding vacuum is unduly restrictive. If you really believe that "Photons always travel at the speed of light." is not clear English in that regard, then you can use "Photons always travel at the speed of light in the medium." But, usually, less words is better. Hope this helpful. Edgerck 21:22, 14 May 2007 (UTC)

OK. Han-Kwang 21:28, 14 May 2007 (UTC)

Ok, about the language comment, but not on the physics. Photons do not travel at the speed of light in media at all. The interactions with the media causes the photon to gain an effective mass, as explained later on on the article. If you formally write down the physical photon propagator in a medium in terms of the vacuum photon propagator you have to sum over all the interaction it can have with atoms, which leads to the physical propagator. The pole shifts from zero to a finite value so it's massive. You cannot isolate the individual terms in the summation and give it a well defined physical interpretation... Count Iblis 21:37, 14 May 2007 (UTC)

The photons speed is not constant it is c divided by the refractive index of the material it is in. Leave the above line in the article. There is no pure vacuum even in space. There is a lot of dark matter in the universe. @6/15/2007 by Daron Smith

Does the mass–energy relation apply to the photon?

Edgerchk has invited me to make a new thread from these bits from above:

The formula m=E/c^2 does not apply when the particle moves with v=c. So, while its use is questionable (google "okun mass") for particles with mass (that travel at v<c), it is actually undefined for the photon. That's why it is incorrect to talk about a relativistic mass for the photon (wikipedia notwithstanding). Nonetheless, the energy-momentum equation does apply to the photon, and for a system of photons.Edgerck 19:54, 14 May 2007 (UTC)

Finally: a point of physics to argue!. Edgerck, do you have a source for this exception that you are asserting? All the books I've seen suggest that there is no such exception, that photons obey the same law of momentum and energy and mass (both kinds) as all other particles, massless or otherwise. Dicklyon 20:01, 14 May 2007 (UTC)

Dick -- I have said and referenced this very point some 5x above. Most books and articles I've read, as well as my own conclusions, are clear on this same point. So, if you want to talk about it and since it's off-topic for photon-talk, I'd be happy to reply in your talk page. Perhaps you could prepare a fresh topic item there, using my paragraph above as my summary for the start and stating your points as well. Thanks.Edgerck 20:21, 14 May 2007 (UTC)

It appears that we have a genuine physics disagreement here. Edgerck believes that massless particles are exceptions in considering Einstein's energy–mass relationship E=mc^2. I say they are not exceptions, and that they fit all formulas the same way (except that the indefinite m0 * gamma, zero times infinity, the relativistic mass, needs to be clarified, in the case of a massless particle, to be the general energy mass, which is E/c^2 for any particle massless or not). Anyone case to say whether one of us in confused? Dicklyon 20:52, 14 May 2007 (UTC)

Given the amount of talk and archived talk devoted to this subject I don't think I want to get too much involved. I came to this page because someone complained that the article, including the intro, was totally incomprehensible. Somehow the discussion reminds me of self-taught physicists that claim to prove that Einstein was wrong, although I'm not sure to who is in that role here. Han-Kwang 21:22, 14 May 2007 (UTC)

Dick: My suggestion was to use your talk page but I'll be brief. Why the formula E=mc^2 (which is NOT the formula for the rest energy E₀ where the particle is at rest) is incorrect, and why one should not talk about a 'relativistic mass' for the photon? A good, accessible summary was written by Lev Okun, in The Concept of Mass, Physics Today, June 1989, free online at http://www.physicstoday.org/vol-42/iss-6/vol42no6p31_36.pdf. Hope this proves to be helpful. For questions, please write to the author. Edgerck 21:34, 14 May 2007 (UTC)

Thanks, that looks very interesting, and I will finish reading it. My impression is that he is making a great case to do what you're saying all physicists do, which is to focus on rest mass and ignore relativistic mass. Probably a good idea. But why did he need to write this? Dicklyon 21:48, 14 May 2007 (UTC)

Indeed, quite a fascinating paper. Maybe I've learned something. But it clearly supports the statement that "some" sources ascribe a relativistic mass to the photon, even as it argues that that can lead to confusion. He admits that a photon can transfer mass from one emitting object to another absorbing object; it's important to note that the sizes of the mass differences are not always equal, and can depend on relative velocities and masses of the objects. To me this seems more complicated, not simpler, than evaluating all masses as relativistic masses in a common reference frame, but it's OK if he thinks it's easier the other way. So what are you hoping I'll get from it? Dicklyon 19:35, 15 May 2007 (UTC)

And what's up with adding back the phrases "referred in the article. The same references do not define a "relativistic mass" for the photon." Did you ever point out which references you have in mind? Isn't the fact that some refs don't use relativistic mass already implicit in the statement that some do? Is there a reason we need to care that some particular refs don't? Dicklyon 19:35, 15 May 2007 (UTC)

Why?

Could someone, in a way a non physicist can understand, tell me exactly what makes photons so special? Why can nothing travel faster than them? The laws of the universe ought to still apply right?

That's because of causality paradoxes. I just started an article about this topic a few days ago, see here. Count Iblis 18:52, 10 June 2007 (UTC)

I'd be a liar if I told you I had any idea what a Tachyonic Antitelephone is. Are you BSing me?

Well, the idea is that if you can send signals faster than light you can also send signals back into your own past. So, you could talk to yourself using that "Antitelephone" to tell what the stockmarket will do and what stocks to buy.

However, the problem is that you would tell yourself to do things you haven't done, which is not possible. In particular, you could make a machine programmed in such a way that it will send a message to itself some time T ago if and only if it didn't receve such a signal at that time.

There is one known effect that causes photons to travel at a speed larger than the speed of light, see here, however, this effect cannot be used to create causal paradoxes. That is explained here Count Iblis 21:40, 10 June 2007 (UTC)

Photons have no mass, so they can travel the speed of light (in a vacuum). As you increase in speed, so does an objects mass, and then it requires more energy, and so on.DavidRavenMoon (talk) 07:18, 21 May 2008 (UTC)

Wave-particle duality

According to this article all objects experience wave-particle duality. I previously made this change [3] and then was reverted [4]. I'm not a physicist, so I wouldn't know which is correct. Either way, one of the articles needs to be changed since they contradict each other. --Android Mouse 18:18, 19 June 2007 (UTC)

The most massive object for which wave-like properties have been observed and reported is the Buckyball (C60 or C70, around 800 proton masses), and even that hasn't been replicated or widely accepted. There's really no good theoretical or experimental justification for extending wave-like properties to larger objects except insofar as they can be coaxed into showing quantum behaviors. It is popular to say "all objects", but it is less meaningful than "all quantum objects", in my opinion.

I just reviewed the article you linked. I think that if you re-read the first two sentences, you'll find that there's no contradiction. Dicklyon 18:35, 19 June 2007 (UTC)

Thanks for the informative reply! Although I've read over the lead section in the wave-particle article and I still think there is a contradiction. These sentences: (emphasis not mine)

"Through the work of Albert Einstein, Louis de Broglie and many others, current scientific theory holds that all objects have both wave and particle nature (though this phenomenon is only detectable on small scales, such as with atoms)"

"The reason why we don't notice wave properties of macroscopic objects around us is their small wavelength."

I think is the most contradictory. It may be that it is only refering to quantum objects, but such wording leads me to believe quite the opposite. I don't understand how it would be possible for larger objects to not display some degree of duality, when their building blocks, atoms do. --Android Mouse 19:21, 19 June 2007 (UTC)

I agree, those statements may be unsourced and flaky; or maybe a lot of sources do say that, out of ignorance of what it takes for "object" to make sense in this context. Why don't you take it up on that page, or call for a citation on it? Dicklyon 20:18, 19 June 2007 (UTC)

Ok, I've created a section on the wave-particle talk page, and added the citation tag on those sentences. --Android Mouse 22:40, 19 June 2007 (UTC)

It's all about the uncertainty of an objects location at any given time. Large objects, like people, have a very small uncertainty, so while we vibrate, it's too small to notice (smaller than your Schwarzschild radius). —Preceding unsigned comment added by DavidRavenMoon (talk • contribs) 07:20, 21 May 2008 (UTC)

Recommended changes

The American physicist Arthur Holly Compton was the first to suggest the name “photon” instead of a light quantum. —Preceding unsigned comment added by 68.22.26.73 (talk)

that's been added and reverted already, since it contradicts the sources info the article and has no source of its own. Dicklyon 14:36, 20 June 2007 (UTC)

The source of the above line is Isaac Asimov The Neutrino from Avon Books Copyright 1966 by Isaac Asimov.

Can you quote the relevant passage from Asimov for us? I think you're misinterpreting what he said. If Compton had used the word before Lewis, the evidence of that should be easy to find; but he didn't. Dicklyon 15:25, 20 June 2007 (UTC)

The page number is 61 in the book Isaac Asimov The Neutrino in chapter 4 Mass-Energy subtital Photons.

I don't have the book handy, which is why I asked you to quote the relevant passage. All I can see is this. Dicklyon 18:45, 20 June 2007 (UTC)

The lepton number is one not zero as the article states.

How do I get a user name. Give me a link to help me. If no link exists then it should be added to help new editors.

added June 20 2007 by Daron Smith

In the upper right, the sign in/create account link. Dicklyon 15:25, 20 June 2007 (UTC)

A table showing frequency HZ versus wavelengths in meters and radio waves, microwaves, infrared rays, visible rays, X rays and gamma rays would be nice. —Preceding unsigned comment added by 68.22.26.73 (talk)

It's in electromagnetic radiation, and not so relevant here. Dicklyon 14:36, 20 June 2007 (UTC)

Photon in a vacuum have a velocity of “c” but in a transparent medium instantly slow down but then instantly go back to “c” upon entering a vacuum again. —Preceding unsigned comment added by 68.22.26.73 (talk)

that an OK view of light wave propagation, but photons themselves travel only at c; the interaction with electrons in matter cauess them to be absorbed and re-radiated a lot, according to the particle view. Dicklyon 14:36, 20 June 2007 (UTC)

The lepton number is one not zero as the article states. One of the most important physical properties of the photon is its energy. 1.77 eV is red light 3.10 eV is blue light

I'm confused by this insistence that the lepton number of the photon is one. The photon is not a lepton; it has spin 1 (not 1/2) and, more specifically, it's not a neutrino, electron, muon or the tau lepton. Since it's not a lepton, the photon's lepton number is zero, right? Willow 20:11, 20 June 2007 (UTC)

The photon is a lepton so the lepton number would normally be one but since the photon is its own antiparticle the lepton number has to be zero. The conservation of both baryon and lepton number would otherwise be the same with just zero and one interchanged. Daron Smith

FYI, the photon is not a lepton; as described on that page, there are exactly six leptons and the photon is not one of them. But you're right, the photon's lepton number is zero. As an aside, "lepton" means "light thing" in Greek, not "small thing", which would be "micron". Here, "light thing" means "not heavy thing"; lepton is contrasted with "baryon", which means "heavy thing". Their names come from the fact that leptons are less massive (lighter) than baryons. Having a little Greek and less Latin, Willow 18:55, 22 June 2007 (UTC)

Physical questions

Would it be correct to say that a photon is a wavelet in the E/M fields?

If so, what is the transverse spread of the photon? (Or if this length is not defined, root-mean-square peak-width or something like that.)

If the transverse spread is zero, then the photon cannot interact with other particles unless through the E/M fields (action at a distance); but I understand the photon to be the force carrier. Where is the flaw in my understanding?

Thanks! —Preceding unsigned comment added by 24.12.103.158 (talk) 03:32, 21 June 2007

No, it is not correct to say that a photon is a wavelet in the E/M fields. Timb66 04:49, 21 June 2007 (UTC)

Long quote from Asimov

Photon

This was requested to fix a problem with the person that coined the term photon and it may help in other places. Don’t use this copyrighted data directly. This is from the book “The Neutrino” by Isaac Asimov chapter 4 Mass-Energy subtitle Photons as follows.

Let’s move into reverse now. Having considered mass in terms of energy, let’s consider energy in terms of mass. A photon of light, for instance, possesses a certain amount of energy and this must, in turn, be equivalent to a certain amount of mass.

According to Plank’s quantum theory, one can easily determine the energy of a photon of light from the wavelength of that light. In order to express that energy in terms of electron-volts, one must divide the quantity 0.000124 of 1.24X10-4 (obtained by a chain of mathematical reasoning I need not go into) by the wavelength of light in centimeters. The longest wavelengths of visible light (deep red in color) are roughly 0.00007 or 7•10^-5 centimeters in length, while the shortest (deep violet in color) are about half that, 0.000035 or 3.5•10^-5 centimeters (see Figure 7).

If 1.24•10^-4 is divided by 7•10^-5, we get a quotient of nearly 1.8. We can conclude then that the photon of the longest wavelengths of visible light has energy of 1.8 ev. As the wavelength of light decreases, the energy of the associated photon increases in proportion. The shortest wavelengths of visible light, having half the wavelength of the longest, have photons twice as energetic-3.6 ev.

Since chemical reactions liberate up to about 4 ev of energy per atom, it is not surprising that the photons produced in the course of such reactions are frequently in the energy-range of visible light.

Less energetic photons are also produced, photons of light of longer wavelength than red light. We cannot detect such photons of infrared radiation by eye, but our skin can absorb them and feel them as heat. Still less energetic photons are associated with microwaves which are longer in wavelength than the infrared radiation, and with radio waves which are longer still in wavelength. Radio waves used in ordinary transmission may have wavelengths as high as 55,000 centimeters. This would be equivalent to a photon possessing energy of about two-billionths of an electron-volt.

We can work in the other direction, too. Some chemical reactions emit light more energetic than those of the shortest visible wavelengths. These photons of ultraviolet radiation, although invisible, can easily be detected by their effect on a photographic plate. Ultraviolet light can be produced with wavelengths so short that the associated photons possess energies up to 1000 ev or 1 Kev.

Beyond the shortest-wave ultraviolet light is a region of still shorter wavelength, where the radiation is referred to as X radiation, or X rays, X-ray photons, with energies from 1 Kev up to 100 Kev, can be found. Finally, there are the gamma rays, with shorter wavelengths and more energetic photons still. The energy of gamma-ray photons can work their way well into the Mev range.

It is not surprising, then that nuclear reactions, which liberate energies of millions of electron-volts per reacting atomic nucleus, result in the formation of gamma rays. What of the mass equivalence of those photons? I have already explained that energy of 938.905 Mev is equivalent to the mass of a hydrogen nucleus. This surpasses the energy content of even very energetic gamma-ray photons.

An electron is an easier mark to shoot at. The electron is 1/1836.11 the mass of a hydrogen nucleus and is therefore equivalent to 938.905 divided by 1836.11, or about 0.51 Mev (which can also be expressed as 5100.000 ev).

A visible light photon, with the typical energy of 2.5 ev, would have a mass-equivalence equal to about 1/200,000 of an electron. Such a tiny mass is quite negligible and even on an atomic scale no great error is produced by considering the photon of visible light to be mass-less.

However, as one progress down the electromagnetic spectrum toward shorter and shorter wavelengths, the photons become more and more energetic and equivalent to more and more mass. A gamma ray with a wavelength of 0.00000000024 or 2.4•10^-10 centimeters is made up of photons possessing a mass just equivalent to that of an electron. The same devices that detect the particlelike behavior of an electron ought, therefore, to be able to detect the particlelike behavior of gamma-ray photons also.

This was demonstrated in 1923 by the American physicist Arthur Holly Compton. He found that an X-ray photon, with a mass equivalent to rather less than that of an electron, could strike an electron and make it rebound. The electron had gained energy and the photon had lost energy, precisely as thought two colliding electrons had been involved. More than that, the photon acted as though it was a particle, carrying momentum, and the law of conservation of momentum was observed in the interactions of the photon and the electron.

Under the circumstances, it seemed clear that light and its related radiations had to be viewed as possessing the properties of particles as well as of waves. It was Compton, then, who suggested the name “photon” for a light quantum, making use of the “-on” suffix which had become the hallmark of the names given to subatomic particles since the discovery of the electron a quarter century earlier.

The particlelike properties of gamma-ray photons are even more marked than are those of X-ray photons. When gamma rays are emitted in the course of nuclear reaction, their momentum must be taken into account. What’s more, photons may be regarded as possessing spin and, therefore, angular momentum. In applying the laws of conservation of momentum and of angular momentum to nuclear reactions, the momentum and angular momentum of photons must be included in the calculation.

Although a gamma ray and an electron may be equivalent in mass, there is nevertheless a difference between them as far as that property is concerned. This is not a paradox, for equivalence is not necessarily identity. (A check for ten dollars may be the equivalent of ten dollars in cash, but it is not identical with it)

Consider the mass of an electron, for instance. An electron may move at any velocity, relative to an observer, from 0 cm/sec (when it is at rest) to 3X10+10 cm/sec (the velocity of light in a vacuum). The mass of an electron, or of any material object for that matter, varies with its velocity in such a way as to be a minimum at rest and to approach the infinite at the velocity of light. (1)

The mass of an object at rest relative to an observer is its rest-mass, and it is the rest-mass which is usually referred to when one speaks simply of “mass.” When the mass of the electron is given as 9.1091•10^-28 grams, for instance, it is well understood that that refers to its rest-mass. Electrons are often encountered which travel at velocities equal to or more than 0.99 times that of light in a vacuum and their masses are then seven or more times as high as their rest-mass.

A photon moving through a vacuum, however, travels always at the velocity of light, 3•10¹⁰ cm/sec, relative to all observers. (2) This is the cardinal point of Einstein’s Special Theory of Relativity. Since a photon can never be at rest relative to any observer, one cannot measure its rest-mass directly. However, physicists find it convenient to consider the rest-mass of photons to be zero.

Despite the fact, then, that a photon can be considered as possessing the equivalence of mass, it is generally spoken of as a massless particle, the adjective referring to its rest-mass of zero.

The photon is not the only massless particle, as we shall see, for the core of this book deals with massless particles that are not photons. We can make the generalization that all massless particles, whether photons or not, travel at the velocity of light from the moment they are formed to the moment they are absorbed.

(1) Einstein’s Special Theory of Relativity predicts this fact and it has been amply verified by experiment. The increase in mass is quite negligible until speeds of thousands of miles per second are reached. In ordinary life we are quite safe in considering mass to be constant.

(2) In a transparent medium other than a vacuum, photons travel at lesser velocities. Even air slows them down very slightly. When photons leave a transparent medium, however, and enter vacuum again, their velocity accelerates at once to 3•10¹⁰ cm/sec once more.

The end of the book. Daron Smith

You could have just quoted the relevant sentence: "It was Compton, then, who suggested the name “photon” for a light quantum, making use of the “-on” suffix which had become the hallmark of the names given to subatomic particles since the discovery of the electron a quarter century earlier."

Too bad Asimov got it wrong. In the case of the G. N. Lewis claim, there's a well-known paper to point at. Did Asimov give a source to Compton using the word "photon"? I doubt it.

I think it's best to just ignore Asimov in this case, even though he would ordinarily be regarded as a reliable source, because there's no supporting evidence for him, and a preponderance against. Or find the evidence, if it exists.

Dicklyon 17:38, 22 June 2007 (UTC)

Dear Daron, please keep your messages more-or-less chronological; the conversation is hard to follow otherwise! :) Also, please don't put such long quotations into Wikipedia; we don't want to run afoul of copyright laws! Finally, please consider getting an account here and signing your posts with four tildes. We should indeed examine Compton's papers from that time to see whether he did coin the term "photon", but as Dicklyon says, it seems unlikely given the existing evidence. Even Isaac Asimov can make mistakes sometimes. ;) Willow 18:46, 22 June 2007 (UTC)

In 1922, however, he concluded that Einstein's quantum theory, which argued that light consists of particles rather than waves, offered a better explanation of the effect. In his new model, Compton interpreted X rays as consisting of particles, or “photons,” as he called them. From Encyclopædia Britannica Article by Daron Smith I have a usedID but how do I sign using (UTC)time

NEWS, THIS MONTH: valid for basic bosons, gravitons and photons

Fairness principle?

Sorry Willow to your last rv, but WIKI demands fairness - or not? Showing here that zero-mass is not (was never?) considered as a solid fact. Are then different meanings in WIKI allowed to be written? - Or not? Make the judge, please fair, but read seriously what serious experts mean and allow to make in WIKI a mirror of those opinions, please:

• Local Photon and Graviton Mass and its Consequences, M. Tajmar, ARC Seibersdorf research GmbH, A-2444 Seibersdorf, Austria; C. J. de Matos†, ESA-HQ, European Space Agency, 8-10 rue Mario Nikis, 75015 Paris, France. "Abstract: We show that non-zero masses for a spin-1 graviton (called graviphoton) leads to considerable gravitomagnetic fields around rotating mass densities, which are not observed. The solution to the problem is found by an equivalent graviphoton mass which depends on the local mass density to ensure the principle of equivalence. This solution, derived from Einstein-Proca equations, has important consequences such as a correction term for the Cooper-pair mass anomaly reported by Tate among many others. Similar results were obtained for the photon mass which is then proportional to the charge density in matter. For the case of coherent matter the predicted effects have been experimentally observed by the authors."

• Already in 2002 On the Graviton Mass, Andrei Gruzinov, Physics Department, New York University, 4 Washington Place, New York, NY 10003 - Gruzinow: "What’s wrong with a non-zero graviton mass? Zakharov (1970) and van Dam & Veltman (1970) (ZvDV) suggested that observations of the solar system tell us that the graviton mass must be rigorously zero. One would usually expect that observations can give just an upper limit, but in the linearized gravity the ZvDV theorem is indeed correct as we explain below using a non-covariant linearized massive gravity. In §2 a covariant version of the ZvDV theorem is given in the quasi-massive (induced) gravity of Dvali, Gabadadze & Porrati (2000)".

• De Matos and Tajmar found within a superconductor experiment that they had to set the graviton mass to be 10^-54 kilograms (Physica C, vol 432, p 167).

Newest stuff, publ. this month

You should read newest Braneworlds, conformal fields and the gravitons showing quite clearly for all BRANEWORLDS (affecting gravitons as photons) that not an Einstein's 4D-space but only a 5D-space works for calculating such particles and how:

The result, confirmed explicitely above also for photons (other not yet found in hurry) is for those particles:

• "The eigenvalues m2 are positive or zero leading to m=0. The set is infinite but discrete, mi, i = 0, 1, . . . + ∞. There is just one massless graviton, m0 = 0. It has a positive wavefunction ψ0 in the fifth dimension. This wavefunction is localized near the positive tension branes. The massive gravitons have oscillating wavefunctions ψi, i = 1, . . . + ∞. Their masses and mass splittings decrease when rc increases."
• This means: A normally only considered zero mass of a graviton as of a photon was/is correctly called:
• intrinsic mass (please look there - or original German old and new texts "Ruhemasse", the solution with eigenvalue=zero) of a "black" photon without frequency (see Planck's black body radiation) also at Schwarzschild radius.
• "A conformal 5D energy–momentum tensor Tµν and its Sturm–Liouville problem result in positive or zero (m=0) eigenvalues. The set is infinite but discrete, mi, i = 0, 1, . . . + ∞, (this means:) there is just one massless solution with m0 = 0".

Massless solution is correctly called (photons) intrinsic mass

A normally only considered zero mass of a photon is correctly called its intrinsic mass - original German texts "Ruhemasse" = "not moving" (anyhow) with eigenvalue zero - of a "black" photon without frequency (see Planck's black body radiation) or at a Schwarzschild radius, see below. A conformal 5D energy–momentum tensor Tµν and its Sturm–Liouville problem result in positive or zero (m=0) eigenvalues. The set is infinite but discrete, mi, i = 0, 1, . . . + ∞. This means: there is just one massless solution with m0 = 0.

A disambiguous Photon mass by old-fashioned "Einstein-Planck relation"

An anciently "Planck mass" called result by "Einstein-Planck relation" gives an old-fashioned "Einstein-Planck mass" with Planck's constant (6.6262 x 10-34 J-sec) by combining

• Eintein's equation E=mc² and Planck equation E=h $\nu \$ (see e.g. in Relativistic Momentum).
• "Planck's work led Einstein to propose that light was particulate and exists as a collection of particles called photons. They have "zero rest mass but non-zero relativistic mass".
• That kind of mass of a photon moving at the speed of light is given by m = h $\nu \$/c², e.g. [Textbook: Chemistry, Fourth Edition], Steven S. Zumdahl, Houghton Mifflin Company, 1997.

That kind of a "Planck mass" (the now taken is more correctly a Schwarzschild mass for the second Schwarzschild solution) was since "ever" rigidly taken to compute the frequency of of other elementary particles like electrons and others, because by Einstein calculated "photon's mass" was confirmed since 1905 by two different Einstein effects (see there) and was generally used, especially to declare Tired light by photon's gravity.

Prior reason for Planck's name

Originally related only to Planck as prior to Schwarzschild's generally accepted theory. Albert Einstein, himself obviously did not understand, but accepted again Plancks work in 1950:

• "Even the Greeks had already conceived the atomistic nature of matter and the concept was raised to a high degree of probability by the scientists of the nineteenth century. But it was Planck's law of radiation that yielded the first exact determination - independent of other assumptions - of the absolute magnitudes of atoms. More than that, he showed convincingly that in addition to the atomistic structure of matter there is a kind of atomistic structure to energy, governed by the universal constant h, which was introduced by Planck.
• This discovery became the basis of all twentieth-century research in physics and has almost entirely conditioned its development ever since. Without this discovery it would not have been possible to establish a workable theory of molecules and atoms and the energy processes that govern their transformations. Moreover, it has shattered the whole framework of classical mechanics and electrodynamics and set science a fresh task: that of finding a new conceptual basis for all physics. Despite remarkable partial gains, the problem is still far from a satisfactory solution."
• Einstein did not understand Planck effects, as shown by famously derided quantum entanglement as "spukhafte Fernwirkung" or "spooky action at a distance."

Don't worry, be happy: Even FEYNMAN not understood

Feynman.R.”Q.E.D.- the strange story of light and matter”, Penguin,London,1990 p76 (ref. also 1985 (same theme) Princeton University Press): He describes by Quantum mechanics the transmission of light through a transparent medium simply as “photons do nothing but go from one electron to another, and reflection and transmission are really the result of an electron (remark: in molecules) picking up a photon, ”scratching its head”, so to speak, and emitting a new photon.” In transparent solid matter - good glass produces no blurring and fuzzy light at all, as objected against Zwicky - the density of molecules is huge compared to thin interstellar gas. Like Photons continually absorbed and re-emitted by electrons in atoms of good, tansparent glass the photons of light travelling through interstellar molecules are very, very seldom but continually absorbed and re-emitted by electrons in the very thin interstellar plasma, withhout visibly losing speed like in massive glass. Feynman, in this book: "What I’m going to tell you is what we teach our physics students in the third or year of graduate school... It is my task to convince you not to turn away because you don’t understand it…. You see my physics students don’t understand it that is because I don’t understand it. Nobody does."

Fairness-hope

• We hope that you will kindly revert now our (now visibly?) well-founded, related but erased section by newest researches?
• We find more and more that not only one solution of GR may be true. Mathematically is valid each possible single result and superposition results for Big bang, Tired light, Intrinsic redshift etc., especially considering different clocks by different intrinsic gravity at different places of the universe and different Potentials at different places. Only beeing with the photon it has always a same intrinsic zero-value...

84.158.222.180 23:12, 30 June 2007 (UTC)

Willow replies

Viellieber 84.158.222.180,

Na, schön wär' es, wenn Sie mal uns Ihren Namen nennen würden! Wie sollen wir gemütlich schnacken wenn wir nicht wißen wie Sie heissen? Ich heisse Willow, aber meinetwegen können Sie mich auch "Weide" nennen. ;)

Ich hab' das alles noch night durchaus überlegt, aber vorläufig find' ich das alles nicht überzeugend. Ich bestehe darauf, daß wir hier nur beschreiben, was für das Photon schon bestätigt ist, und zwar konservativ bestätigt. Als Enzyklopädie können wir es uns nicht gönnen, allerneuste Forschung zu beschreiben, was vielleicht nicht zuverlässig ist. Aber ich werde mir Mühe machen, das alles anzuschauen, ob nicht da Juwelen zu finden sind.

Ich schreibe ja auf Deutsch, um das uns bequem zu machen. Verzeihen Sie es mir, Sie scheinen einige Schwierigkeiten mit der englischen Sprache gehabt zu haben. Ich will Misverständnisse vermeiden, damit die anderen nicht meinen, Sie verstehen nichts, bloß weil Sie die Sachen in einer Fremdsprache schwierig erklären können.

Vielen Dank für Ihren Geduld mit mir, ich werde wahrscheinlich in etlichen Tagen wiederschreiben, Willow 00:08, 1 July 2007 (UTC)

Translation of Willow's note

Dear, dear 84.158.222.180,

Dude, it would be cool if you would tell us your name! :) How can we have a cozy chat if we don't know what to call you? My name is Willow, but as far as I'm concerned, you can call me "Weide" (German for Willow). ;)

I haven't considered everything in depth, but after a preliminary look, I don't find these ideas convincing. I maintain that we should describe only things that have been confirmed for the photon, and that means conservatively confirmed (i.e., experimentally). As an encyclopedia, we can't allow ourselves the luxury of describing cutting-edge research that may not be reliable. But I will take pains to look over everything, to see if there are gems there.

I'm writing in German to make it comfortable for us. Forgive me for saying so, but you seem to have had a few difficulties with the English language. I want to avoid misunderstandings, lest the others think that you don't understand stuff, just because you express your ideas with difficulty in a foreign language.

Thank you for your patience with me, I'll probably write back in a few days, Willow 00:08, 1 July 2007 (UTC)

Walli replies, also in German

• VORAB: Danke vielmals!!! - Wir sind "Astronomischer Club" (3 Clubs asoziiert), bis 2006 Big-Bang Fans. Ich, nun Club-Writer, habe Physik und Elektrotechnik studiert, war Systemprogrammierer, zuletzt 2 "Weltpatente" (Ton, Bild-, Signal-Analyse). Nach Lizenz schwerer Unfall, zeitweise gelähmt, usw. nun relativ zufriedenstellend. Seither Probleme beim flüssigen Schreiben, diese Woche auch müde wg. neugeborenem Baby.

Zero rest mass, non-zero relativistic mass

RV was good, now was better found and declared:

• PROBLEM, EINSTEIN: Photons have a "zero rest mass but non-zero relativistic mass"!!!
• DECLARED "zero rest mass but non-zero relativistic mass", e.g. [5], same better lookout: [6]: "The problem is simply that people are using two different definitions of mass. The overwhelming consensus among physicists today is to say that photons are massless. However, it is possible to assign a "relativistic mass" to a photon which depends upon its wavelength. This is based upon an old usage of the word "mass" which, though not strictly wrong, is not used much today. The old definition of mass, called "relativistic mass," assigns a mass to a particle proportional to its total energy E, and involved the speed of light, c, in the proportionality constant: m = E / c^2. ...For all photons this (remark: Rest mass) is zero. On the other hand, the "relativistic mass" of photons is frequency dependent. UV photons are more energetic than visible photons, and so are more "massive" in this sense, a statement which obscures more than it elucidates."

OWN WIKI-"PHOTON", sect. References and footnotes:

• "BELIEVED to be exactly zero-mass": "3. ...The mass of the photon is believed (### !!! ###) to be exactly zero, based on experiment and theoretical considerations referred in the article. The same references do not define a “relativistic mass” for the photon. However, some sources ascribe to the photon a relativistic mass (or apparent mass or energy mass), equal to E/c2, where E represents the particle's total energy (kinetic energy, plus rest-mass energy, if any); among these sources are Anthony C. Fischer-Cripps (2003). The Physics Companion. CRC Press. and Frederick J. Bueche (1988). Principles of Physics. McGraw-Hill Education. and Paras N. Prasad (2004). Nanophotonics. Wiley-IEEE. ISBN 0471649880."

• REMARK: "have others no Right to BELIEVE (!!!) as well simply the contrary as well"? - THE FACTS:

ESA, Europen Space Agency:

• [[7]]: "1. Introduction: It is well known that the mass of the photon and graviton in vacuum must be nonzero. The first limit is given by Heisenberg’s uncertainty principle1 and the second by the measurement of the cosmological constant in our universe2-4." - (cf. NASA Pioneer-problem, solved by Tired light)

Internet search PHOTON and MASS

• Old sources": Limit on photon mass, less than 10-63.
• Physics News Update: "A new limit on photon mass, less than 10-51 grams or 7 x 10-19 electron volts, has been established by an experiment in which light is aimed at a sensitive torsion balance; if light had mass, the rotating balance would suffer an additional tiny torque. This represents a 20-fold improvement over previous limits on photon mass.
• THE SAME: De Matos and Tajmar found within a superconductor experiment that they had to set the graviton mass to be 10^-54 kilograms (Physica C, vol 432, p 167).
• Photon Mass Gets a Boost, Phys. Rev. Lett. 89, 101301 16 August 2002 - simply see there!
• Photon mass; magnetic photons Rod Lakes, University of Wisconsin: "Photon mass; magnetic photons, Rod Lakes, University of Wisconsin " corr, added, put together 84.158.253.90 11:32, 2 July 2007 (UTC)

ZU HIER UND WEITER OBEN:

Viel aus Originaltexten einkopiert (bitte Links sehen, dann wäre ich nicht allein an allem schlechtem Englisch schuld?)! ACHTUNG: Was oben zur Photonenmasse steht ist und war (NUR DAHER!?) immer kontrovers, schon zu seit Zeiten von Planck, Einstein, Schwarzschild, bis brandneu Monat Juni.

SCHLECHTE WIKI-ERFAHRUNGEN?

Alter bekannter französicher Professor und John Dobson kamen auf eine große Astro-Messe und machten uns "Big-Bang-skeptisch". Seither sehen wir alles relativ, finden z.T. sogar: Vereinigung der Theorien scheint möglich statt z.T. "religiöser Wahn" mit Kastenbildung (gerade wir in Deutschland müssen kritisch geworden sein). Leider haben oben Genannte und locker Assoziierte, wie (anderer) Prof. Assis und besonders Dr. Halton Arp negativ erlebt: Partiell rein statistisch fundiertes Intrinsic redshift - zumindest als lokaler Sonderfall im Universum doch ernst zu nehmen!?ARP wird stattdessen mit seinem Werk unfaßbar herablassend verächtlich gemacht. Ähnlich wie fast überall Tired light).

• FAZIT DAZU: Alle oben Genannten haben (z.T. exrem) schlechte Erfahrungen mit WIKI gemacht, hatten den (subjektiven?) Eindruck daß alles was sie publizieren von zu einseitig geschulten "Physikern" sofort wieder eliminiert wird.

• POSITIV: Viele unserer Verbesserungen von WIKI-Artikeln blieben. Ein vor zwei Monaten noch mangelhaft bezeichneter (quasi STUB-) Artikel ist bei einfachster Suche mit 2 Begriffen + Wikipedia in MSN und ALTAVISTA nun auf die erste Suchseite (gestern 2. und 5.Stelle) hochgeschnellt.

• NEGATIV, GOOGLE: Alte WIKI-Kopien fremder (umgleiteter) Links davon fangen ab Stelle 15 an! WIKI-Original kommt nur bei exakterem Zusatzbegriff...

Good night at nearly 4 o'clock here - Walli (dynamische Club-IP)84.158.234.15 01:57, 1 July 2007 (UTC) rv by Walli 84.158.212.247 12:43, 1 July 2007 (UTC)

Although I can read and understand most of it, I think Talk Page discussions on English Wikipedia should be in English so that everyone can participate. See WP:TPG#Good practice So please consider providing a translation and in any case continue in English. Han-Kwang 13:35, 1 July 2007 (UTC)

I don't really understand the parts written in German (please switch back to English), but I think I support Willow's decision here. See also what I wrote in Talk:Graviton/Archive 1#Edits concerning massive graviton. Yevgeny Kats 20:17, 1 July 2007 (UTC)

It's OK for someone to comment in German, but in order for the comments to have any sensible impact, someone who agrees with them is going to have to interpret and advocate for the point of view. I think that's unlikely to happen in this case, from what I can understand of the comments. The POV is a bit too non-mainstream. It makes no sense to have a "criticism" section in an article on photons, and the relativistic mass alternative viewpoint is already given enough attention in footnote 3 and the relativistic mass article (we do however need to continue to guard against the fanatics who want to stamp it out even in those locations). Dicklyon 20:24, 1 July 2007 (UTC)

I'm very sorry for violating that policy and I'll try to provide a decent translation above. The goal was not to alienate everyone else, but rather to create a confortable space for a newcomer. Like you all just above me, I feel that the issues raised are either (1) misunderstandings or (2) too cutting-edge theoretical and insufficiently validated by experiment to be included in this article, although conceivably they might find a home elsewhere. But I'm willing to take the time to weigh the ideas one-by-one and see if we might not be guided by them to improve the article. Unfortunately, I'm busy getting ready to a charity event at my house (well, in my garden), so I'm really, really short on time for the next few days. As you may read above, Walli is pressed for time himself, having just become a new father. :D Please, let's all be patient for a few days to sort this out; thanks! :) Willow 22:28, 1 July 2007 (UTC)

Please do not forget

Difference of rest mass and relativistic mass is not found - only here in engl. WIKI please look e.g. quite correctly (not only) in German WIKI link, [MASSE].

• And [[8]]

Trans. "A confirmation of Einstein’s photon’s-mass was succeeded by physicists Rebka and Pound in an "earthly" experiment, as reported in the year 1960 in the following text passage...":

• ORIGINAL: "In 1960, R. Pound and G. Rebka, Jr. at Harvard University conducted experiments which photons (gamma rays) emitted at the top of a 22.57 m high apparatus were absorbed at the bottom, and photons emitted at the bottom of the apparatus were absorbed at the top. The experiment showed that photons which had been emitted at the top had a higher frequency upon reaching the bottom than the photons which were emitted at the bottom. And photons which were emitted at the bottom had a lower frequency upon reaching the top than the photons emitted at the top. These results are an important part of the experimental evidence supporting general relativity theory which predicts the observed "redshifts" and "blueshifts."

• And e.g. cited ESA, Europen Space Agency are certainly no stupids if they have to calculate satelites' way in space and have learnt from Pioneer anomaly in
• [[9]]: "1. Introduction: It is well known that the mass of the photon and graviton in vacuum must be nonzero. The first limit is given by Heisenberg’s uncertainty principle1 and the second by the measurement of the cosmological constant in our universe2-4."

I want to go in vacancy if sun comes (since 1 month for the first time today): Greetings from Walli 84.158.222.230 16:36, 13 July 2007 (UTC)

Multiple definitions

Let’s say we are writing an encyclopedia article on the word orange instead of photon for demonstration purposes. The dictionary has the definition of orange the color and orange the fruit. We pick orange the fruit for the encyclopedia article. All references to orange the fruit can be included but references to orange the color can’t be used. All references to a definition of orange that is not in the dictionary can’t be used for an encyclopedia article on orange the fruit or orange the color. Compton’s definition is like orange the fruit. Lewis’s definition is like orange the color and should not be in this article. Lewis’s definition did not even make it into the dictionary.

Dsmith7707 12:42, 23 July 2007 (UTC)

Lewis's and Compton's definitions of photon are generally considered to be the same. That's why Compton adopted Lewis's proposal. They used slightly different words, and Lewis thought that photons were conserved, so there are some differences, but both were talking about the quanta that carry what we call light. Dicklyon 14:58, 23 July 2007 (UTC)

"I therefore take the liberty of proposing for this hypothetical new atom, which is not light but plays an essential part in every process of radiation, the name photon."

-Gilbert N. Lewis, 1926

Lewis's new atom is not a photon and he said it is not light so it is not the same at all.

Dsmith7707 17:23, 23 July 2007 (UTC)

Lewis's photon concept is generally interpreted as the quantum of EM interaction; if you want to interpret it differently, find a source that interprets it differently. Dicklyon 05:40, 24 July 2007 (UTC)

Compton and Lewis are both in the Physical Review. Compton is 1923-2, 1924-3, 1925-4, 1928-1, 1929-1 and 1930-1. Lewis is 1930-2. Compton’s Nobel Prize was in 1927 look at the banquet speech by A. H. Compton.

Dsmith7707 12:33, 24 July 2007 (UTC)

I don't have that handy. If it's online, send a link. Or if it says something relevant, quote it here for us. Dicklyon 15:09, 24 July 2007 (UTC)

Physical Review

A Quantum Theory of the Scattering of X-rays by Light Elements May 1923 Volume 21 Issue 5 by Arthur H. Compton

The Spectrum of Scattered X-Rays Nov 1923 Volume 22 Issue 5 by Arthur H. Compton

The Recoil of Electrons from Scattered X-rays Apr 1924 Volume 23 Issue 4 by Arthur H. Compton

A General Quantum Theory of the Wave-Length of Scattered X-rays Aug 1924 Volume 24 Issue 2 by Arthur H. Compton

Measurements of β-Rays Associated with Scattered X-Rays Mar 1925 Volume 25 Issue 3 by Arthur H. Compton

Directed Quanta of Scattered X-Rays Sep 1925 Volume 26 Issue 3 by Arthur H. Compton

Good source Physical Review http://prola.aps.org/

Good source Nobel Prize for physics http://nobelprize.org/nobel_prizes/physics/laureates/

Bad source but used to prove it’s bad http://www.nobeliefs.com/photon.htm

Dsmith7707 11:31, 25 July 2007 (UTC)

Thanks, but where is there something that says Compton's photon is not the same as Lewis's? Can you be specific, or quote a passage? Dicklyon 15:25, 25 July 2007 (UTC)

"I therefore take the liberty of proposing for this hypothetical new atom, which is not light but plays an essential part in every process of radiation, the name photon."

Quote by Gilbert N. Lewis, 1926 Nature Magazine letter to the editor.

Lewis's new atom is not a photon and he said it is not light so it is not the same at all.

Dsmith7707 11:54, 26 July 2007 (UTC)

You may recall I already quoted that one to you. What I'm asking for is a source or quote in support of your analysis that he wasn't talking about the same thing as Compton was. Dicklyon 14:35, 26 July 2007 (UTC)

The 1926 Nature magazine letter to the editor can’t be used for the reason I explained above. The 1930 Physical Review can be used but Compton’s Nobel Prize speech published for the 1927 prize is first with the correct definition of the word. Words sometimes have many meanings.

Dsmith7707 16:52, 26 July 2007 (UTC)

Your opinion can't be used without a source to back it up. Most sources agree that Lewis coined the term photon. Dicklyon 16:59, 26 July 2007 (UTC)

I am using a quote from the 1926 Nature magazine letter to the editor not my opinion to show that it should not be used because the definition is not the same.

Dsmith7707 19:10, 26 July 2007 (UTC)

Since most published works treat the same quote differently that you do, we can stop paying attention to your diatribe now. If you find a source worth quoting, let us know. Dicklyon 21:58, 26 July 2007 (UTC)

I now see why wikipedia is so bad using a letter to the editor as a reference that no professional encyclopedia would use. Even the ridiculous example to prove my point of orange the color instead of orange the fruit is in wikipedia.

Dsmith7707 14:47, 27 July 2007 (UTC)

Your personal analysis has long since stopped being interesting. If you can't bring a reliable secondary source that agrees with you, then please just stop. Dicklyon 15:24, 31 July 2007 (UTC)

Oops. Footnote on mass again.

I'm sorry; I had not realized to what extent this whole issue has been causing such angst, or else I would not have done it.... I have modified the footnote on mass of a photon. Previous version on the left, my change on the right.

The mass of the photon is believed to be exactly zero, based on experiment and theoretical considerations referred in the article. The same references do not define a “relativistic mass” for the photon. However, some sources ascribe to the photon a relativistic mass (or apparent mass or energy mass), equal to E/c2, where E represents the particle's total energy (kinetic energy, plus rest-mass energy, if any); among these sources are Anthony C. Fischer-Cripps (2003). The Physics Companion. CRC Press. and Frederick J. Bueche (1988). Principles of Physics. McGraw-Hill Education. and Paras N. Prasad (2004). Nanophotonics. Wiley-IEEE. ISBN 0471649880.

The mass of the photon is believed to be exactly zero, based on experiment and theoretical considerations described in the article. Some sources also refer to the "relativistic mass" concept, which is just the energy scaled to units of mass. For a photon with wavelength λ, this is h/λc. This usage for the term "mass" is no longer common in scientific literature.

Basically, I was trying to keep it short and sweet. It did not seem necessary to give references, and looking back I see that they are really only there because there was some dispute that the term "relativistic mass" was ever used of a photon; and so a justification was considered necessary. As a newcomer to the article, I thought it was not necessary. I'm somewhat stunned that this has aroused such passions, and I am very apologetic if I have stirred them all up again. For what it is worth, I don't use the term "relativistic mass" myself. It it is always better to speak of the energy and momentum directly, IMHO. —Duae Quartunciae (talk · cont) 07:04, 13 August 2007 (UTC)

I thought it was mostly OK, but since you spoke of rescaling the energy, it made more sense to leave it in terms of E, instead of changing to lambda; so I stuck that in, too; you might want to use just E. Dicklyon 07:09, 13 August 2007 (UTC)

Yes, I agree. In fact, the term I gave was inferior, because it does not work in a refractive material. Thanks. —Duae Quartunciae (talk · cont) 07:13, 13 August 2007 (UTC)

And you're right you did step in a big one here. One of the vocal proponents of pretending there's no such comment has been so out of control that he managed to get banned permanently from several sites, several times. Hopefully he won't be back when he sees it's being touched again. Dicklyon 07:21, 13 August 2007 (UTC)

WHO REALLY COINED THE WORD PHOTON

The books “The Universe From Flat Earth To Quasar” and “The Neutrino” both say that Compton coined the word photon. Reference 8 is a letter to the editor that anyone could have sent in so it does not represent a proof of anything.

Dsmith7707 12:00, 30 August 2007 (UTC)

OK, I added Asimov's opinion with these refs. Hopefully that's satifactory to all. Dicklyon 16:54, 1 September 2007 (UTC)

No, because the photon was named in a letter by chemist Gilbert N. Lewis to Nature in Oct. 1926. Exact reference is Nature 118, 874, 1926. Wups, I see that ref is in the article already. So why do we need Asimov's mistake referenced? SBHarris 19:06, 13 January 2008 (UTC)

I don't mind if you take that back out. I was just trying to shut up Dsmith7707; the rest of the world agrees on Lewis. Dicklyon (talk) 20:23, 13 January 2008 (UTC)

Who first named the symbol for light

From the book “THE UNIVERSE FROM FLAT EARTH TO QUASAR” is the following quote “The three varieties of radiation from radioactive substances (discovered first in 1896 by Becquerel) were named by Rutherford after the first three letters of the Greek alphabet: alpha rays, beta rays and gamma rays. Of these, the gamma rays proved to be electromagnetic in nature; a form of radiation with wavelengths even shorter than those of X-rays”.

Dsmith7707 16:06, 1 September 2007 (UTC)

The cited origin of the term gamma rays is considerably older. Apparently without Google, Asimov had a harder time finding such things in the 1960s. Dicklyon 16:55, 1 September 2007 (UTC)

If Google could only eliminate all of the garbage on the internet with some sort of rating system with ten very reliable to zero very unreliable. Just remember garbage in garbage out.

Dsmith7707 11:26, 4 September 2007 (UTC)

From a Google search: http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radact2.html Radioactivity was discovered by A. H. Becquerel in 1896. The radiation was classified by E. Rutherford as alpha, beta, and gamma rays according to their ...

Dsmith7707 19:16, 11 September 2007 (UTC)

Villard worked with gamma rays in 1900 but Rutherford named them because of Becquerel’s work in 1896.

Dsmith7707 19:47, 11 September 2007 (UTC)

Einstein

Einstein received the Nobel Prize in 1921 for his discovery of the law of the photoelectric effect. Some internet articles say incorrectly that he discovered the photon but he called it light quanta. This being the most important Nobel Prize should be in the Photon article.

Dsmith7707 13:05, 4 September 2007 (UTC)

I agree. So I added it. Dicklyon 15:37, 4 September 2007 (UTC)

size

How big is a photon? I have read before that an electron is considered to have a radius of about 2.818 x 10(^-15) m. Is there a similar figure for the size of a photon? Widsith 09:53, 11 September 2007 (UTC)

The size of a particle is not a well defined concept. They are not like little billard balls with a well defined radius. The figure you have quoted is called the Compton radius, and is really just a kind of associated length, and not properly a size. The most natural size quantity for a photon would be its wavelength; which behaves a bit like size in some respects. But thinking of a particle with a specific size is not really useful. —Duae Quartunciae (talk · cont) 10:11, 11 September 2007 (UTC)

OK yeah, that makes sense to me. I was just confused because this doesn't seem to be specified anywhere. Is it worth pointing this out in the article? Widsith 10:48, 11 September 2007 (UTC)

I don't think so. It's already there, in a sense. The wave length is defined, and it is also described as a "point like" particle for various interactions, in the section Wave–particle duality and uncertainty principles. There is a bit more detail, which is frankly beyond my comfort level, in the section Photon structure. —Duae Quartunciae (talk · cont) 12:14, 11 September 2007 (UTC)

Could there at least be some sort of comparison in size to the atom? just to help give an idea of how it reacts with them. saying that it is quanifiable or what ever means that it occupies space so even a rough estimate if anyone has one, thanks. Fists (talk) 01:43, 24 February 2008 (UTC)

The wavelengths vary over all sizes, much larger and much smaller than atoms. But photons never exist in a place, or take up space. They are the transfer of energy from one place to another. Dicklyon (talk) 06:24, 24 February 2008 (UTC)

The interaction between photon with antimatter and matter

If an antimatter(atom) combines with a matter(atom), it produces one photon. Is it possible to make(create) two atoms, one antimatter and one matter, just by using a photon? I hope somebody can solve this problem for me. --Tohyf 14:00, 24 October 2007 (UTC)

I'm no expert, but when matter and anitmatter collide, they produce at least two photons, perhaps so that (angular?) momentum can be preserved, or perhaps for some deeper reason. See Positron emission tomography for an example. DRLB 17:55, 24 October 2007 (UTC)

Yes, that's true; when a particle and antiparticle annihilate one another in empty space, at least two photons must be produced to conserve momentum. However, a single photon can degenerate into a particle-antiparticle pair (such as an electron-positron pair, see pair production) if there's something else (such as an atomic nucleus) with which it can interact to satisfy momentum conservation. If I'm not mistaken, pair production is the main way that high-energy photons such as X-rays and gamma rays are absorbed when passing through matter, e.g., through a sheet of lead. This manner of absorption isn't open to lower-energy photons, since the photon energy must be greater than twice the rest-mass energy of the electron (i.e., greater than roughly 1.022 MeV). I hope this helps! :) Willow 19:14, 24 October 2007 (UTC)

This is not really the place for this, but here goes. The conversion of two particles into a single photon, or vice versa, cannot happen in isolation because it does not conserve momentum. To see this, consider the centre-of-mass frame of the two particles. In that frame, the total momentum is zero. But the momentum of a photon is never zero in any frame (since it always has speed c). So you either have two particles converting to two photons (or vice versa), or you need something else to balance the momentum budget (such as an electromagnetic field, which has momentum). Timb66 19:09, 24 October 2007 (UTC)

Nomenclature

Ernest Rutherford in 1903 coined the word gamma that defined the symbol for light used today. From the Encyclopedia Britannica concise and Isaac Asimov’s book “The Neutrino”.

Dsmith7707 13:40, 5 October 2007 (UTC)

There seems to be a controversy there that would be worth mentioning, with this book ref. Dicklyon 17:43, 5 October 2007 (UTC)

EM-wave diagram error

The Light wave diagram on the Photon main page is WRONG! The E and M amplitudes should be phase-shifted by 90 degrees, since they are |E|sin(w*t) and |M|cos(w*t) functions of time. Instead, they have been drawn in-phase!!! The graphic is obviously a mindless copy-paste from another page: <http://en.wikipedia.org/wiki/Sinusoidal_plane-wave_solutions_of_the_electromagnetic_wave_equation>

where it appears alongside the equations for polarization, with the obvious intention to illustrate the Ex and Ey components, not the E and M component.

I'm horrified that such an error was not detected by the editorial team. If this kind of evident error has been overlooked, how about more subtle ones? 82.192.62.79 (talk) 14:38, 13 January 2008 (UTC)

Diagram is correct, B_x is in phase with E_y Count Iblis (talk) 16:59, 13 January 2008 (UTC)

Properties

Just a thought: in the properties box at the top of the page, it lists the charge and mass of the photon, relating it to gravity and electromagnetism. Couldn't we make a more complete picture by relating it to the other two interactions as well (i.e.: list its color charge, flavor, weak isospin, etc.)? —Preceding unsigned comment added by 69.65.219.204 (talk) 10:55, 3 March 2008 (UTC)

A little question..

I am taking a chunk out of the article; "In empty space, the photon moves at c (the speed of light) and its energy E and momentum p are related by E = cp, where p is the magnitude of the momentum."

does the magnitude of the momentum mean the speed at which the proton is traveling? What unit is it measured in?? Fejj the ritual (talk) 15:43, 11 June 2008 (UTC) -Thanks in advance

The magnitude of the momentum means just that - the value of the momentum without a direction attached to it. Momentum is not speed (or velocity) so it would not be the value of the speed. The units depend on whatever units you are using in the equation. If you are using SI units, then momentum has units of kg m/s. PhySusie (talk) 14:56, 24 June 2008 (UTC)

Is it the word "magnitude" that's bugging you? PhySusie addressed that but only in passing. The point is that momentum strictly speaking is a vector quantity; two momenta can be the same size, but if they're going in different directions then they're different. The magnitude of the momentum is determined just by how massive the thing is and how fast it's going--the direction doesn't matter. --Trovatore (talk) 20:08, 25 June 2008 (UTC)

overwikilinking

This article is far too heavily wikilinked. Wikilinks should be used when there is a high probability that the reader will want to follow them, not for everything that some hypothetical reader might want to look up. Right now the article is a sea of blue -- distracting and unprofessional. --Trovatore (talk) 23:53, 20 July 2008 (UTC)

I've added new wikilinks in the past few days (before seeing your comment), and all were appropriate and even necessary in my opinion. I think there should be a link for every technical term, person's name etc, and that new links should be made to subsequent appearances of the same terms only when very far from the previous link (or, say, in another section of the article). I examined the article just now and I think only few of the links can be deleted without compromising the average reader's understanding of the text. Barak Sh (talk) 01:41, 21 July 2008 (UTC)

If the text is so full of jargon that an average reader has to follow the majority of the links in order to understand the text, then the text should actually be rewritten. I actually rewrote the intro section a year back [10] after off-wiki complaints that this article was incomprehensible, but I see that in the meantime a lot of cruft has been added again to the intro section. For example instead of clarifying 'electromagnetic radiation' with two or three examples that everybody knows, it now covers the whole spectrum. Hello! This is not an article about electromagnetic radiation! Proton (which most educated people will vaguely recognize) as an example of elementary particle was replaced by 'quark'. Maybe calling a proton an elementary particle is not completely correct, but then just delete proton as an example rather than replacing it by something that makes the intro less readable. From WP:LS: In general, specialized terminology should be avoided in an introduction. Where uncommon terms are essential to describing the subject, they should be placed in context, briefly defined, and linked. The subject should be placed in a context with which many readers could be expected to be familiar. There is still plenty of work to be done in the lead section of this article, especially towards the end. This article is likely to be read by a lot of nonphysicist people. Han-Kwang (t) 10:02, 21 July 2008 (UTC)

I agree. The introduction should be a bit less technical. Also, I'm currently reviewing the article and adding some clarifications in the text where I think they are needed. Barak Sh (talk) 12:20, 21 July 2008 (UTC)

I think that the "wiki convention" of mentioning the term in the first sentence of the article and then attempting to define it is to blame. What is good for most wiki articles may not be good for physics articles. I also made this point in discussions about the entropy article. Count Iblis (talk) 13:35, 21 July 2008 (UTC)

I must agree with the point that the article is sea of blue some not so important internal links should be removed otherwise the reader will get wandered about here and there loosing interest in the real purpose he was reading the article for. Kalivd (talk) 06:44, 22 July 2008 (UTC)

photon spin and angular momentum

Some people[11] claim that a single photon is in one of 2 distinct spin states, and (simultaneously) can be in one of many -- in principle, infinitely many -- distinct orbital angular momentum states. Which of these two properties ("spin (physics)" or "orbital angular momentum") causes macroscopic electric field polarization? Are there any macroscopic effects caused by the other property? I see that "orbital angular momentum" redirects to the "azimuthal quantum number" article, which seems to be talking about something else -- not something that applies to a photon, but something that only applies to an electron bound to some atom. Are these people talking about the same thing as described as "orbital angular momentum" article, or something else that only applies to photons? --68.0.124.33 (talk) 00:05, 4 November 2008 (UTC)

Photons don't have orbital angular momentum. The angular momentum of photons is exclusively due to spin.­­Headbomb {^ταλκ_{κοντριβς} – WP Physics} 21:42, 11 November 2008 (UTC)

Errors in lead

(1) Lead says: "as a particle, it can only interact with matter by transferring the amount of energy

$E={hc \over \lambda },$"

I say: This isn't true, a photon can transfer only part of its energy, e.g. Compton scattering.

(2) Lead says: "The laws of quantum mechanics require that a photon's energy, momentum, and polarization have probable values (not definite values). So, it is impossible to definitely predict which molecule a photon will excite."

I say: Well the Heisenberg uncertainty principle (HUP) says that a localized photon comes from a photon field containing multiple wavelengths, hence momenta, hence energy. But there's no HUP for polarization, as far as I know. Moreover, the second sentence doesn't follow from the first, even though both are independently true.

(3)Lead says: "For visible light the energy carried by a single photon is around 4×10^–19 joules; this energy is just sufficient to excite a single molecule in a photoreceptor cell of an eye, thus contributing to vision."

I say: This is misleading, the word "sufficient" makes it sound like the photoexcitation is governed by a lower energy threshold. In fact, photons with too much energy won't work either. It should say something like "the energy is just right to excite the photoreceptor..."

(4) I hope someone corrects these. Thanks! :-) --Steve (talk) 20:31, 11 November 2008 (UTC)

(1) Interesting point. Do you have a better wording? I must say though that the Feynman diagram on Compton scattering suggests that one photon is fully absorbed and another one is emitted.

(2) Agreed.

(3) Hmm. It was supposed to mean that the photon energy may seem very small, but it's still enough to excite a molecule.

(4) WP:BOLD.

Han-Kwang (t) 21:21, 11 November 2008 (UTC)

I have fixed these, in the sense that I rewrote and shortened the introduction and considered some of these facts a bit too random and inessential -- so they're not there anymore. ;) -- SCZenz (talk) 18:24, 12 November 2008 (UTC)

Thanks, it's much better now. :-) --Steve (talk) 22:35, 12 November 2008 (UTC)

spelling correction

http://translate.google.com/translate_t#el%7Cen%7C%CF%86%E1%BF%B6%CF%82%0A%CF%86%CF%89%CF%82 James thirteen (talk) 11:58, 26 November 2008 (UTC)

Confusing Sentence

...is described by exactly three continuous parameters: the components of its wave vector, which determine its wavelength λ and its direction of propagation.

So are these the three parameters? It sounds like one parameter that is itself composed of two parameters.--Matt D (talk) 10:21, 27 November 2008 (UTC)

...I have a theory...

Ya, I'm just a kid, but still, hear me out, I am highly perceptive. I had this theory the other day, and people have probably already tried it, but what if the photon doesn't reflect off of the matter, but it gets absorbed and that gives the matter energy enough to emit the energy in the form of light. I'm not even in high school, so don't laugh at me college students who are taking physics classes. I just want to know if this is true and if it isn't how physics makes it impossible.

VFD642 (talk) —Preceding undated comment added 03:08, 10 April 2009 (UTC).

In fact reflection of a photon can be considered as equivalent to absorption of a photon followed by emission of a photon in the direction observed. The word "reflection" suggests that the same photon arrives at the matter and then leaves, while the words "absorption" and "emission" suggest that one photon arrives and another leaves. However in reality there is no experimental way to tell whether or not the arriving photon and the leaving photon is/are "the same photon", so the distinction actually has no meaning. Dirac66 (talk) 18:19, 10 April 2009 (UTC)

But such a process is different because the emitted photon would be in a completely different state. If you do an interference experiment in which photons bounce off a mirror, then the reason why you can see fringes is because the state of the mirror (and the rest of the universe) is not affected by the photon that bounces off the mirror.

So, while we cannot directly verify that the photon is the same, we can verify that the state mirror is the same before and after reflection, which implies that the photon was not absorbed and replaced by another photon (otherwise unitarity would be violated ) Count Iblis (talk) 18:37, 10 April 2009 (UTC)

Yes, I forgot about the interference between the incident and reflected light, which does indeed provide experimental proof that we have the same photon before and after in the case of reflection. One experimental setup is the Michelson interferometer.

I am not sure, however, about the theoretical argument that the reflecting surface remains in the same state. There is momentum transferred from the photon so that the state is not exactly the same.

It is also interesting to consider other photon-in, photon-out phenomena in which interference is not observed. Can one say that the emerging photon is merely the incident photon in a new state for the following processes?

1. Rayleigh scattering, in which the light emerges in all directions

2. Raman scattering, in which there is also energy transfer from (or to) the photon

3. Fluorescence, in which there is also a short time delay

4. Phosphorescence, in which the time delay is longer, in some cases several hours.

Many authors avoid this question and merely say that a photon is emitted, without specifying whether it is the same or another photon. Dirac66 (talk) 00:52, 12 April 2009 (UTC)

Yes, the absorbed momentum must be considered. What happens is that the following two states cannot be orthogonal:

1) Photon bounces off the mirror

2) Photon does not bounce off the mirror

because then you could in principle get the "which path information", which would destroy the interference pattern. What happens is that while momentum is conserved, the mirror's center of mass wavefunction is very broad in momentum space. Of course, the mirror's center of mass position is quite well defined, so by the uncertainty relation the wavefunction in momentum space must be very broad.

Then, what happens in the cases 1) and 2), is that the wvefunctions in mometum space certainly differ in the sense that in 1) it shifted by the absorbed mometum, but thish shift is far smaller than the width of the wavefunction. So, the overlap of th wavefunctions (which gives the visibility of the fringes) is almost equal to 1.

There is a more elementary way to see this. To get an interfence pattern, the mirror's surface must be located to within a fraction of a wavelength of the light used, otherwise the photons arriving at the same spot of the screen would arive with random phases and no interference pattern will be seen. Then if you apply the uncertainty relation to the mirror, you then see that this implies that the uncertainty of the mirror's total momentum must be larger than the momentum will absorb by deflecting the photon.

Due to decoherence, an object of mass M will have a wavefunction that is almost precisely localized at some fixed position. The spread in position space is of the order of the thermal de Broglie wavelength. If you translate that to the spread in momentum space, you find that the spead in momentum space is of te orser sqrt[M k T] where T is the temperature of the environment with which the mass interacts.

Now, this means that the spread of the speed tends to zero for M to infinity, so the object does behave clasically in the large M limit. However, the spread in momentum space does increase with increasing mass, and that ultimately explains why you can bounce photons off macroscopic objects like mirrors without the wavefunction of the mirror being significantly affected. Count Iblis (talk) 02:07, 12 April 2009 (UTC)

Yes, I see. Reflection is from a macroscopic mirror so quantum effects are negligible. And presumably this is why reflection is simpler than the other phenomena which I mentioned. Dirac66 (talk) 02:46, 12 April 2009 (UTC)

Comments on First Feynman Diagram

A Feynman diagram of the exchange of a virtual photon (symbolized by an oscillating line labelled γ (gamma) between a positron and an electron.

The first Feynman diagram seems to shown electron and positron both flowing forward in time, to the right in this diagram. The diagram shows these oppositely charged particles repelling, when actually an exchange of a photon should cause the particles to attract. This diagram, thusly, needs some editing or clarification. My suggestion would be to simply change both particles to electrons, but I also do not know the original intent of the picture, so I'll leave it to the community to decide if an edit or clarification is necessary. —Preceding unsigned comment added by 131.110.112.62 (talk) 12:23, 4 June 2009 (UTC) 420ftjesus (talk) 05:40, 6 June 2009 (UTC)

I removed the photon picture from the article for now. I placed it in this discussion for convenience. Headbomb {^ταλκ_{κοντριβς} – WP Physics} 14:03, 4 June 2009 (UTC)

two comments

I am a PhD molecular biologists, but not a math person, and I find this article full of jargon and virtually incomprehensible. NOw, it could be that I'm pompous and stupid; but I understand the relation between wavelength and energy, for instance, so I'm not a total idiot. IMHO, the article needs an intro that is geared to a general college graduate. use of words like "boson" "guage" "vector" etc are guarenteed to turn people off. One thing that has confused me for years is that the photon is non charged, but electromagnetic..I know this is really simple to the experts, but I would advocate a phrase along the lines of the following: "The photon is massless and does not have an electrical charge (electrically neutral) yet is is the carrier of the electromagnetic force - electricity and magnetism. Most of us are familiar with electricity as something that flows thru wires in our house or appliances; what flows is electrons, which are charged and which have mass. The relation between the photon and the elctron is (this is where i get stuck...sorry)Cinnamon colbert (talk) 14:50, 1 July 2009 (UTC)

Photons in matter

In the page section called Photons in matter it says "Figure at right" but I do not see any figure. --JWSchmidt (talk) 02:21, 7 June 2009 (UTC)

I came here to try and find the answer to the same question. This sentence has made reference to this non-existent figure for months. What this a copy and paste job? Was there ever a figure? Sorry I don't have time to dig into this further at the moment but I was hoping someone familiar with this article knew the answer off the top of their head. Stangbat (talk) 15:27, 28 August 2009 (UTC)