Talk:Twin paradox/Archive 15

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Article lead : Inaccurate summary of the references.

There has been much discussion of the use of "incorrect" and "naive" in the lead but it seems to me that a more significant problem has been missed. The current sentence is "This result appears puzzling because each twin sees the other twin as moving, and so, according to an incorrect[1][2][3] and naive[4][5] application of time dilation and the principle of relativity, each should paradoxically find the other to have aged more slowly.". I have emboldened the relevant phrase. My understanding is that it is not the rate of ageing that is paradoxical but the total amount by which each has aged. there are five references cited, the relevant extracts are:

[1] "When the travelling twin gets home, she has aged only a few years, while her sister is now old and gray."

[2] "To Speedo, the most significant change is that his brother Goslo has aged more than he and is now 60 years of age. Speedo, on the other hand, has only aged by 34.6 years. ... This leads to the paradox: Which twin will have developed the signs of excess ageing."

[3] "An (incorrect) application of relativistic time dilation yields the paradoxical conclusion that each twin would find the other to be older."

[4] "It follows that the twin B must be younger than the twin A when they meet again. ... it should be also possible to argue that A is younger than B."

[5] "Thus Terra has aged more than Stella. If we naively apply the time dilation formula in the reference frame of Stella, we would reach the opposite conclusion."

In each case, the paradox is not that one or the other "ages more slowly", it is that the total ageing is greater for the twin who moves inertially throughout. To correct this, I suggest the end of the sentence is rewritten as "... each should paradoxically find the other to have aged by a lesser amount."

George Dishman (talk) 19:32, 6 March 2017 (UTC)

Hi George, I don't agree. Time dilation means that A measures (with his t-clock) the time (dt) between two events on B's remote t'-clock as longer (dt = gamma dt', since dx'=0, as you can check with the Lorentz transformation). A says something like "what your clock calls a second, I call 4 seconds". So indeed A measures the aging of the remote clock as "more slowly than his own". If A remains inertial between B's departure and return events, then when they meet again and compare their elapsed times, indeed B's clock has aged more slowly (or just less), and B is indeed younger than A. That is the correct, informed application of time dilation. The converse is not true, simply because B bas not been inertial between the two events. The mistake of assuming that B remains inertial is the incorrect, naive application. Indeed A finds B to have aged more slowly, but B does not find A to have aged more slowly. - DVdm (talk) 20:05, 6 March 2017 (UTC)

Ha... I see what you mean: perhaps we should just remove source [3] because they have the typo: that should be: "... paradoxical conclusion that each twin would find the other to be younger." Good find! - DVdm (talk) 20:27, 6 March 2017 (UTC)

Hi DVdm, your disagreement is exactly why I put the view here rather than editing the text so thanks for replying.

You gave for instance "A says something like "what your clock calls a second, I call 4 seconds" but it'll be easier to use v=0.8c and gamma=1/0.6. Suppose each has a clock that ticks once per second. B departs and travels for 100 seconds at a constant 0.8c as measured by A before turning round and returning in another 100 seconds again at constant speed. Time dilation says that B's clock will measure 60s elapsed time during each leg of their trip giving a total of 120s compared to 200s measured by A. So far, so good. However, B can also apply time dilation to each leg. B says "my outward leg took 60s hence in that time A's clock should have ticked 36 times". Similarly for the return leg, B measures 60s and no acceleration so during that time A's clock should have advanced by another 36s. Thus when B returns, by applying time dilation to both legs, he expects A's clock to have advanced by a total of 72s. This application of the time dilation formula is entirely legitimate and correct for each leg which is why "ages more slowly" is not the nature of the paradox. The cause of the paradox is that using only time dilation means that B predicts that A's clock will have advanced less than his own overall when they are compared at the same location on B's return, but even when resolved, it is still true that each ticks "more slowly" than the other on both legs.

What is "naive" about this is to assume that only time dilation is relevant. When B turned round (assumed to be impulsive so take negligible time), he switched from the inertial frame in which he was at rest on the outward leg to that in which he is at rest on the return leg. That switch of inertial frame needs to be included via the "relativity of simultaneity". If we number the ticks for each clock starting at 0 when B departs, tick 60 on B's clock is simultaneous with tick 100 on A's clock as judged by A but with tick 36 on A's clock as judged by B. Similarly, in the return trip inertial frame, tick 60 on clock B is simultaneous with tick 164 on clock A as judged by B and tick 120 on B is simultaneous with tick 200 on A. Again, A has ticked "more slowly". The resolution of the paradox of course is that the "relativity of simultaneity" effect takes the A clock from 36 to 164 on the change of B's frame. This is illustrated in the diagram in the article and accompanying text. Adding that 128s to the 72s computed using time dilation alone resolves the paradox. The bulk of the article is perfectly correct, it is only the single sentence in the summary that, IMHO, is misleading and not representative of the sources.

You said at the end "but B does not find A to have aged more slowly" but that is incorrect, B does find that A aged more slowly both on the outward leg and on the return leg, there is no finite period when A wasn't ageing more slowly according to B, but on completion of the trip, B finds that A has aged by a greater amount overall, that is the paradox, hence "has aged more slowly" is not the same as "has aged less".

If you consider removing references due to a typo ("older" instead of "younger" which I should admit I hadn't spotted, I even had one like that in my initial submission), I would suggest removing reference [3] which IMHO is more seriously wrong, it states that "This asymmetry, which involves accelerated motion, cannot receive a complete analysis within special relativity since special relativity applies only to uniform motion. It requires the tools of general relativity which is introduced below.". Obviously including relativity of simultaneity in SR is sufficient to resolve the paradox without resorting to GR. It is also incorrect to say that SR only applies to uniform motion, see Rindler Coordinates for example. Reference [5] also uses GR by the way.

George Dishman (talk) 13:29, 8 March 2017 (UTC)

Ok, I have removed source [3] and replaced the somewhat awkward phrase "has aged more slowly" with "has aged less": [1]. That's clearly better. Thanks for having spotted this. - DVdm (talk) 16:33, 8 March 2017 (UTC)

Excellent, thank you. George Dishman (talk) 18:23, 8 March 2017 (UTC)

Parodying "Molière" in the "femmes savantes" , your discussion reminds me of two sligthly pedantic poets having an argument over two versions : "how beautiful is that blue bouquet ... " or "how blue is that beautiful bouquet ..." ! Once you have admitted that simultaneity is relative , that fact plays no further role. A and B have the same clocks , playing the same rhythm , unchanged on the inward trip and the outward trip. Indeed we can say that the switch between the simultaneity planes at the turnaround point has mostly a negligible numerical impact , and no theoretical importance for the resolutions of the "paradox". Cordially ; I apologise if necessary ...Chessfan (talk) 19:54, 10 March 2017 (UTC)

Perhaps I should explain the context, I've been trying discuss the thought experiment with someone who is a newcomer to SR, he understands the basics but has been arguing that the paradox can be eliminated by taking length contraction into account. However, his "solution" only resolved the difference in clock rates which he insisted was the paradox, it couldn't consider what would happen if the twins were reunited. The error on this page made it impossible to explain that the "Twins Paradox" requires one to turn round and return to the other. Changes the rate of ageing to the total amount resolves the problem. George Dishman (talk) 16:26, 17 March 2017 (UTC)

REQUEST FOR COMMENT: Should “naive” statement be deleted?

The following discussion is an archived record of a request for comment. Please do not modify it. No further edits should be made to this discussion.

A summary of the debate may be found at the bottom of the discussion.

---

ISSUE

The current version of the article (as of this time) states as follows in the first paragraph (with the citations links inactivated):

This result appears puzzling because each twin sees the other twin as moving, and so, according to an incorrect^[1][2] and naive^[3][4] application of time dilation and the principle of relativity, each should paradoxically find the other to have aged less.

As the host of this RFC, I propose that the “naive” be deleted as insulting and un-encyclopedic.

Let the community discuss this for a while (as long as editors are weighing in) to see if we can arrive at a consensus.

RFC Protocol:

• Wikipedians who weigh in should feel free to update their support/oppose (*vote*) statements as they see fit so long as no one has commented on it in the Rebuttal and discussion; merely update your autosignature with a fresh quadruple-tilde (~~~~) when finished.
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• In the Rebuttal and discussion, to begin a thread when responding to another editor, use an “@” reference, e.g. @User:So-n-so: You wrote “blah blah blah” but I have a different take.
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• The duration of this RFC will remain flexible. The general objective will be to achieve a balance amongst three factors. One factor is to run this RFC long enough to give an opportunity to the widest segment of the community—uninvolved wikipedians and I.P. editors included—to weigh in. A second factor is whether—after providing opportunity for ample participation—a consensus one way or another has become apparent. And a third factor is whether there is a protracted period of participation having dropped off due to lack of interest.
• The procedure for calling this RFC will also be kept flexible. If the consensus is clear based upon a grin test, it's a simple call. If a consensus is anything short of crystal clear, we'll call in an uninvolved admin to bear judgement.

In advance, thanks to everyone for participating. Greg L (talk) 22:35, 30 March 2017 (UTC)

Support deleting “naive”

[Just single-paragraph posts here, numbered with "#"; discussion and replies below, not here]

1. Support deletion This isn't a huge deal. The issue is simple: Whether it is any way encyclopedic to state in the lede of an article that people are “naive” if they think the paradox exists. This wording has clearly been a source of controversy and much debate has occurred on this talk page on the subject; it is time to put this one to bed. As RFC proposer, it doesn't matter if certain cited experts believe it is naive to think a paradox exists; they are obviously referring to any other “expert” in the field to make the point that if one has a full understanding of relativity, one would perceive no paradox. Alas, Wikipedia is a general-interest encyclopedia directed to a non-expert readership. The simple fact is this article has 6000 words and lots of explanatory formulas, all of which also showed that the experts—including Einstein—discussed the apparent paradox at length and labored to explain it using various thought experiments and different points of view. For us to say it is “naive” for anyone to think there is paradox and come to Wikipedia to learn about it is insulting to our readership and is non-encyclopedic. Greg L (talk) 15:55, 31 March 2017 (UTC)

Oppose deleting “naive”

[Just single-paragraph posts here, numbered with "#"; discussion and replies to other editors in the below section, not here]

1. Oppose deletion. See all over the place on this talk page—including the previous RFC. There is nothing insulting about this well sourced qualifier. It holds no judgment about people and just accurately describes a mistaken (aka incorrect, as sourced) and uninformed (aka naive, as sourced) application of a theory. The qualifier is encyclopedic per the provided sources. The 6000 words of the article also solidly support both these qualifiers, so they are well-placed in the lead. - DVdm (talk) 20:57, 30 March 2017 (UTC)
2. Oppose deletion. See sources and previous RFC. --D.H (talk) 15:57, 31 March 2017 (UTC)
3. Oppose deletion It is a paradox, as it is easy to believe that they should age the same. Most people don't find special relativity easy to understand, but the twin paradox requires general relativity. Gah4 (talk) 19:06, 31 March 2017 (UTC)
4. Oppose deletion. It does not say that anyone is naive. It does not say that it is naive to think that the paradox exists. It is not insulting. This RFC does not make any sense. Roger (talk) 06:31, 1 April 2017 (UTC)

Section for rebuttal and discussion of other editors’ above position statements

[Unrestricted area for back & forth discussion]

The term non-relativistic is commonly used to describe physics that is correct in Newton's sense, but not in either special or general relativity. I don't know that there is a term for physics that is incorrect with special relativity, but correct with general relativity. The paradox comes when applying special relativity to a case that needs general relativity. Gah4 (talk) 19:08, 31 March 2017 (UTC)

Gah4: Are you clear what the issue is here? You wrote “The paradox comes when applying…” My issue is over how the article currently says you are naive for believing that. This RFC is whether it is appropriate to say that it is “naive” to suspect a paradox exists. That's rather rather insulting language for an encyclopedia directed to a general-interest readership trying to grasp the nuances of the Twin paradox. Greg L (talk) 19:14, 31 March 2017 (UTC)

Note that the article does not say that "you are naive for believing that", or that "it is naive to suspect a paradox exists". It says that an incorrect and naive application of some principles leads to a paradox. You make the same allusion on this other forum. Please don't misrepresent what the article says. And please have a look at WP:FORUMSHOP. - DVdm (talk) 19:27, 31 March 2017 (UTC)

Yes, but I didn't make it so obvious. Since general relativity isn't especially easy, it shouldn't be an insult to not understand it. Or, for the specific case, to not understand when it is needed and how to apply it. Telling someone that they don't completely understand general relativity should not be considered an insult. Gah4 (talk) 20:39, 31 March 2017 (UTC) There is a story, which I don't have a reference for, that the designers of GPS didn't know if the general relativity correction was needed. They included it with a switch to turn it on or off. Soon after it started working, it was found that it was needed. I suspect that they were all pretty smart. Gah4 (talk) 20:39, 31 March 2017 (UTC)

Oh… relax, DVdm. Too much drama—perhaps driven by some WP:OWN. There is a atomic-thin distinction between saying someone is naive and saying the perception that there is any paradox at all is founded upon a naive understanding of science. The text reads This result appears puzzling because each twin sees the other twin as moving, and so, according to an incorrect[1][2] and naive[3][4] application of time dilation and the principle of relativity, each should paradoxically find the other to have aged less. Greg L (talk) 20:19, 31 March 2017 (UTC)

The first time you alluded to intransigence and wp:OWNership was here. I chose to ignore that. Now this is the second time. Please stop resorting to personal comments. Thank you. - DVdm (talk) 21:10, 31 March 2017 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Error in the article?

the History section says, In his famous paper on special relativity in 1905, Albert Einstein deduced that when two clocks were brought together and synchronized, and then one was moved away and brought back, the clock which had undergone the traveling would be found to be lagging behind the clock which had stayed put.[A 4]

I in fact do not see that in Einstein's 1905 paper ?

It should be taken out of Wikipedia. — Preceding unsigned comment added by 47.201.179.7 (talk) 14:02, 17 April 2017 (UTC)

@47.201.179.7:

Can you provide source link to this paper. Then use the find and replace tool to search for the text. Once you have determined whether the information is valid, make an edit to this page, and include your source in your edit summary. If the page is semi-protected create an account. Good Luck, CopernicusAD ^(u) (t) :) 14:08, 17 April 2017 (UTC)

It's near the end of paragraph 4 of https://www.fourmilab.ch/etexts/einstein/specrel/www/

From this there ensues the following peculiar consequence. If at the points A and B of K there are stationary clocks which, viewed in the stationary system, are synchronous; and if the clock at A is moved with the velocity v along the line AB to B, then on its arrival at B the two clocks no longer synchronize, but the clock moved from A to B lags behind the other which has remained at B by ${\frac {1}{2}}tv^{2}/c^{2}$ (up to magnitudes of fourth and higher order), t being the time occupied in the journey from A to B.

- DVdm (talk) 14:56, 17 April 2017 (UTC)

That is not the twin paradox, that is only one way, with no return trip mentioned. It should be removed from Wikipedia, Einstein did NOT address the twin paradox in 1905.47.201.179.7 (talk) 15:20, 17 April 2017 (UTC)

Indeed, it is not the twin paradox. It is about time dilation, which plays an essential role in the twin paradox, just like the sentence form the article does, that you have quoted in your message above. The referenced journal supports it, so it should not be removed from the article, as it introduces the concept of time dilation in its historical context.

Oh, by the way, I now see that I had copy/pasted the wrong sentence. That should be:

It is at once apparent that this result still holds good if the clock moves from A to B in any polygonal line, and also when the points A and B coincide. If we assume that the result proved for a polygonal line is also valid for a continuously curved line, we arrive at this result: If one of two synchronous clocks at A is moved in a closed curve with constant velocity until it returns to A, the journey lasting $t$ seconds, then by the clock which has remained at rest the travelled clock on its arrival at A will be ${\frac {1}{2}}tv^{2}/c^{2}$ second slow.

Sorry for the confusion. My bad. - DVdm (talk) 16:06, 17 April 2017 (UTC)

@DVdm: So are you saying the content should be removed from en Wikipedia? If so I will correctly source and remove the content. If it belongs let me know, or either way let me know too. From what I take away, that content should be removed. Good LuckCopernicusAD ^(u) (t) :) And PS, ping or leave me a talkback 17:53, 17 April 2017 (UTC)

Oh! Stupid me! Duh, keep content, case closed. — Preceding unsigned comment added by CopernicusAD (talk • contribs) 17:54, 17 April 2017 (UTC)

This is a false statement and must be removed from Wikipedia: In his famous paper on special relativity in 1905, Albert Einstein deduced that when two clocks were brought together... Einstein never said that. 47.201.179.7 (talk) 02:28, 18 April 2017 (UTC)

As you can see, the content is properly sourced and formally attributed in the article. - DVdm (talk) 07:19, 18 April 2017 (UTC)

The statement is FALSE. Einstein never talked about bringing the clocks together in 1905. It must be removed from Wikipedia. Einstein never said that in 1905. He said it years later, in 1911. So it must be removed from Wikipedia about 1905. He did NOT address the paradox in 1905. So remove it.47.201.179.7 (talk) 14:24, 18 April 2017 (UTC)

Indeed, as I said before, he did not address the paradox. He addressed time dilation, which is what is addressed here in the article too. I have put the relevant sentence in boldface. If that is not clear, perhaps the passage in the original language of the 1905 paper (http://onlinelibrary.wiley.com/doi/10.1002/andp.19053221004/pdf) is helpful:

Man sieht sofort, daB dies Resultat auch dann noch gilt, wenn die Uhr in einer beliebigen polygonalen Linie sich von A nach B bewegt, und zwar auch dann, wenn die Punkte A und B zusammenfallen. Nimmt man an, daB das für eine polygonale Linie bewiesene Resultat auch fur eine stetig gekrümmte Kurve gelte, so erhalt man den Satz: Befinden sich in A zwei synchron gehende Uhren und bewegt man die eine derselben auf einer geschlossenen Kurve mit konstanter Geschwindigkeit, bis sie wieder nach A zurückkommt, was $t$ Sek. dauern möge, so geht die letztere Uhr bei ihrer Ankunft in A gegenüber der unbewegt gebliebenen um ${\frac {1}{2}}tv^{2}/V^{2}$ Sek. nach.

I see no difference between this and the sentence in the article:

In his famous paper on special relativity in 1905, Albert Einstein deduced that when two clocks were brought together and synchronized, and then one was moved away and brought back, the clock which had undergone the traveling would be found to be lagging behind the clock which had stayed put.

Hope this helps. - DVdm (talk) 15:42, 18 April 2017 (UTC)

You are dead wrong. Time dilation is one thing, explaining the twin paradox is quite another. Nowhere in 1905 did he talk about bringing the clocks back together. This Wikipedia article is about the twin paradox, not time dilation. It must be removed about bringing the clocks back together, he NEVER said that in 1905 as Wikipedia incorrectly asserts. Remove it. 47.201.179.7 (talk) 16:44, 18 April 2017 (UTC)

I'm sure that most people will agree that the phrase "It is at once apparent that this result still holds good if the clock moves from A to B in any polygonal line, and also when the points A and B coincide" is precisely about keeping one clock at A, while taking another clock away from A and then bringing it back to A.

And yes, this article is about the twin paradox, which is based on an incorrect application of time dilation, which is introduced in the history section. No problem there. - DVdm (talk) 17:32, 18 April 2017 (UTC)

OK I agree, Einstein did have the twin paradox in his 1905 paper, not much, but it is there. 47.201.179.7 (talk) 19:18, 18 April 2017 (UTC)

All right everybody, were all done here. No more debating, I give to @DVdm:. Are we clear. Bye CopernicusAD ^(u) (t) :) 01:12, 19 April 2017 (UTC)

Incorrect, naive application?

I edited this to indicate that the application of time dilation referred to cannot be unequivocally ruled as 'incorrect naive', purely on the strength of two citations. The edit I made did not, as has been suggested, introduce incorrect information - on the contrary, it eliminated an expression which was not legitimate, for the reason just given. It's not acceptable for the article to be as dogmatic as it was. My edit simply indicated that the assessment of this application as 'incorrect naive' is a matter of the opinion of the authors of the cited references, or of the author of this article, on the strength of the content of those references. Please restore my edit. Ed Addis (talk) 14:33, 22 September 2016 (UTC)

Ha, sorry for having hit the wrong button - I should have hit the wp:NPOV in stead of the error-button. My mistake.

The content is properly sourced with two relevant sources, and I'm sure that we can find more sources. So there is nothing dogmatic about it. Your change ("... held by some to be incorrect and naive ...") used wp:weasel words, and more or less induced your personal opinion —doubts perhaps— about this. If sufficient relevant authors say that something is naive, then Wikipedia can (and should) say that it is naive. - DVdm (talk) 14:58, 22 September 2016 (UTC)

Nevertheless, the reality is that there are many, myself included, who do not agree with this interpretation. It's therefore not acceptable to just label it as 'incorrect'. It would be OK to say that there is a strong (or even a majority) body of opinion that favours a different interpretation, but you can't escape the accusation of dogmatic I'm afraid. I'd be happy to accept a different wording from the one I used, but it must allow for the existence of differing opinions on the key point of whether the paradox as simply stated here is valid. Or is this going to be another case of the WP police overriding valid objections?Ed Addis (talk) 17:06, 22 September 2016 (UTC)

Wikipedia reports on and reflects the scientific consensus. The overwhelming academic scientific consensus is that incorrect, naive application of the principles is right at the heart of the paradox. Pretending (by using wp:WEASEL words) that this is just a matter of different interpretations would be de-facto wrong, and, creating a false balance, would give undue weight to fringe theories. Please have a careful look at wp:UNDUE, wp:FALSEBALANCE and wp:PROFRINGE. - DVdm (talk) 17:32, 22 September 2016 (UTC)

It is always wrong to describe variant viewpoints as incorrect, when any differences of opinion exist. It is just academic imperialism, and sadly there's far too much of it on this platform. Unfortunately, it undermines the credibility of Wikipedia, and is contrary to its spirit. If you refuse to soften this dogmatic statement, I will take the matter to a higher authority.Ed Addis (talk) 19:20, 22 September 2016 (UTC)

Please indent talk page message as outlined in wp:THREAD and wp:INDENT. Thanks.

You will find that indeed Wikipedia is designed this way. Have you taken a look at the spirit of Wikipedia as it is described in wp:UNDUE, wp:FALSEBALANCE and wp:PROFRINGE? - DVdm (talk) 20:13, 22 September 2016 (UTC)

FWIW, I have added 3 more sources. There's a lot more, but that should be sufficient.- DVdm (talk) 22:11, 22 September 2016 (UTC)

@ DVdm. It doesn't matter if there are sources from experts who believe it is naive to believe there is a paradox and whether you've cited it to those experts. The simple fact is this article is full of 6000 words and lots of explanatory formulas, all of which also showed that the experts, including Einstein, wrestled with the paradox and labored to explain it in different ways. For an an encyclopedia directed to a general interest readership to say it is “naive” to believe there is any paradox is insulting, non-encyclopedic, and incorrect given the intended readership.

I deleted it yesterday and you put it back (∆ edit here) with the edit summary of This properly sourced qualifier was extensively discussed on talk page and there was no consensus to remove it.

Well, “well debated” it wasn't; it was just you and one other editor. Now I'm wading in and I feel precisely as user:Ed Addis does: your insulting declaration has no place. There is now a consensus to remove it so…

I’ve deleted it again. Please stop cybersquatting and reverting editors on this issue. It looks like you're more anxious to show off how insightful you are on the subject. We're not here to demonstrate that as wikipedians, we are terribly smart-smart by being insulting to our readership. Greg L (talk) 17:05, 30 March 2017 (UTC)

There was no consensus to make the change to delete it, so per our policy wp:NOCONSENSUS, we can keep it:

• In discussions of proposals to add, modify or remove material in articles, a lack of consensus commonly results in retaining the version of the article as it was prior to the proposal or bold edit. However, for contentious matters related to living people, a lack of consensus often results in the removal of the contentious matter, regardless of whether the proposal was to add, modify or remove it.

Read the remainder of this talk page. I have restored the properly sourced qualifier, as the text was there since a long time, and there was no consensus to remove it, so please don't try to turn this around as you did in this edit summary. You can always apply for other types of dispute resolution. DVdm (talk) 17:32, 30 March 2017 (UTC)

Well, I see that of the last 500 edits, 42 of them are nothing more than you pressing the “undo” button on other editors. I have better things to do in my life than mess around on Wikipedia with intransigent editors who think they own an article and quickly invite others to delve into “other types of dispute resolution.” Perhaps you thrive on all that drama. I don't. Greg L (talk) 19:48, 30 March 2017 (UTC)

I think there is a clear and (to me) obvious compromise here, and that is to just remove the naive. Being incorrect is a factual statement, and while it might not be appropriate to say that somebody is incorrect if they phrase the statement as a question i.e. "Wouldn't the time dilation from traveling at the speed of light violate the principals of relativity?", ultimately what the article is saying is that "Traveling at the speed of light violates the principals of relativity." is incorrect. However, adding that it was "naive" was redundant, opinionated and insulting. When I came to this article earlier today trying to understand what I saw as a clear violation, I honestly was a little insulted by how I was labeled naive just because I did not fully understand special relativity. Granted the article does not say "Incorrect, naive, stupid, foolish application that only a complete idiot could ever start to reason." However, the presence of "naive" slightly undermines the objectivity of the whole statement, and Wikipedia's standards are pretty close to perfection. As such I am going to just remove the one redundant word.50.53.86.168 (talk) 23:32, 29 October 2017 (UTC)

This is not the compromise here. There are two RFC's about this elsewhere on this talk page. There is no wp:consensus to remove the well-sourced term. On the contrary, there is a clear consensus to keep it. - DVdm (talk) 07:39, 30 October 2017 (UTC)

#oly crap you're right there are TWO other RFCs about this! And they are enormous. Forget it I am getting out of this before it's too late. I don't know why I ever decided to go to this talk page and then erase just two words. But now I see that they are cursed. The entire page should be erased before it consumes all of our lives. Just rewrite it from the ground up, then perhaps we can all move on. — Preceding unsigned comment added by 50.53.86.168 (talk • contribs) 09:00, 30 October 2017 (UTC)

Relax, there's no reason to take the word personally. I do at least two naive things each day—and that's even before breakfast . - DVdm (talk) 09:02, 30 October 2017 (UTC)

Take a stand on acceleration, and talk about length contraction more

The following discussion is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

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This article seems to want to take an even hand in evaluating the role of acceleration and gravitational time dilation. This includes all four sources that note how the paradox arises from "naive" and "incorrect" applications of SR. They all invoke GR, and it's pretty well established that you don't need GR to explain it (e.g. Wheeler and Taylor, Introduction to Special Relativity Second Edition), so bringing it up again and again just clouds understanding. Anything that mentions the acceleration should be moved to a section called "Why Acceleration Doesn't Matter" with explanations why.

Also, as the section "Specific Example" shows, the concept that is necessary to clearly unravel the "paradox" is length contraction. Conceptually, the Earth observer is standing on the end of a 4 light year long ruler. The observer watching a ship whizzing over the ruler at 0.8c to the end and back observes the journey to take 5+5 = 10 years (even though the ship is length contracted). The traveling observer also sees the ruler whizzing by underneath at .8c, but it is length contracted to a mere 2.4 light years. The moving traveler therefore observes his journey to take 3+3 = 6 years. The key is simply that both observers are watching the same journey. That's it! There is no need to invoke different inbound/outbound frames, or talk about slow moving clocks, or draw planes of simultaneity to explain "missing time".

Bkennedy99 (talk) 02:27, 7 May 2018 (UTC)

There are many explanations possible, and the one you show here is one of them. Noting that the travelling twin needs to accelerate, whereas the home stayer needs not, is another way. Referring to the equivalence principle (and thus to GR) is yet another, which is mentioned at the end of the lead. Note that it's pretty well established (—see Taylor and Wheeler—) that invoking acceleration is not equivalent with invoking GR: it is well known that SR can perfectly handle acceleration. See also the third last paraphraph of the lead of article Special relativity. So the four sources don't all invoke GR. Note that the total elapsed time on Earth can be calculated as a function of (1) the elapsed time on the ship, combined with (2) the history of the proper accelaration only—see section Twin paradox#Difference in elapsed times: how to calculate it from the ship. - DVdm (talk) 09:02, 7 May 2018 (UTC)

The very first sentence is "the twin paradox is a thought experiment in special relativity". There are enough easy to understand, acceleration-free formulations of the "paradox" such that talking about acceleration is not necessary to understanding it in SR. The article should be about this simpler formulation. Otherwise, the risk is that the reader goes "oh, acceleration" and continues to not have even a basic grasp of length contraction, which is a core SR concept. Now if you could solve the Twin Paradox using only acceleration (SR or GR) and *not* invoke length contraction that would be interesting, but I have never seen that claim. It sounds dubious anyway as identical accelerations cab be invoked at ends of different differences, so what would the different observed time be proportional to? Bkennedy99 (talk) 14:08, 7 May 2018 (UTC)

Indeed, talking about acceleration is not necessary to understanding it in SR, which is what the subsection Twin paradox#Role of acceleration carefully explains to the reader (with 3 sources), even if 4 sources use acceleration. In the subsection Twin paradox#Relativity of simultaneity we have an explanation without acceleration and without length contraction, with 5 sources. The subsection Twin paradox#Travellers' perspective uses the length contraction explanation. So I think the article is fairly complete and balanced, and that balance is well reflected in the lead, and i.m.o. we don't need to worry about readers going "oh, acceleration". - DVdm (talk) 16:43, 7 May 2018 (UTC)

Length contraction is casually mentioned in the "Specific Example" section that describes the math of the distance, but its explicit role in gaining an intuitive understanding is not mentioned - namely, that the spaceship is simply traveling through less space. A reader should be able understand that a round trip from New York to Pittsburgh takes less time than a round trip from New York to Los Angeles. The only remaining conceptual component is that the traveler and the home observer are viewing the same ruler and same spaceship, but the apparent lengths are different - that is, plain-old length contraction. This completely accounts for the difference in observed times, and should be spelled out under "Resolution of the paradox in special relativity", with an example of a giant 4 light year long ruler. As it stands, the first paragraph under the section says things like "This acceleration, measurable with an accelerometer, makes his rest frame temporarily non-inertial. This reveals a crucial asymmetry between the twins's perspectives: although we can predict the aging difference from both perspectives, we need to use different methods to obtain correct results.". This, by the way, is before the section called "Role of acceleration". It's also just irrelevant (explained below) because even without this "crucial asymmetry", you still get the "paradox". It's not crucial at all, as it can be completely removed.

The next section, if there must be one, could be on the "Relativity of simultaneity". And while this doesn't use acceleration, it assumes length contraction, which *must be understood to make sense of this section*. This is while the red and blue lines are sloping. It's nice to be able to account for the "missing seconds" in some orderly way, but this doesn't resolve the paradox. You still need to explain why the spaceship got to the turnaround point in less proper time in their own frame.

Finally, I would advocate that anything mentioning acceleration just be thrown out - it's muddled thinking, and the sources that the articles link to are perpetuating it. In science, if we want to know whether experimental effect A (constant relative motion) or effect B (initial acceleration + turnaround acceleration) cause something (the apparent paradox), we simply run the experiment without one of the variables. And, lo and behold, we discover that removing acceleration through a slightly modified experiment gives the same result (the apparent paradox). If there is story that uses *only* acceleration and not length contraction, that would be very interesting - but it probably wouldn't be SR. Once the story includes length contraction, the role of acceleration should be dropped as unnecessary clutter as length contraction is sufficient.

I know you disagree, which is fine. But there is a ton of bad information out there on the Twin Paradox, and Wikipedia was not helpful in sorting through it. Maybe my thoughts on this talk page will be helpful to the next person who really wants to get their head around the issue. Honestly the whole article is all over the map, it should probably be rewritten by a single subject matter expert with a cohesive take on the issue. Bkennedy99 (talk) 18:56, 7 May 2018 (UTC)

Regarding "... anything mentioning acceleration just be thrown out ...": we tend not to throw out properly sourced stuff ^{(Ohanian, Harris, Rindler, Weidner)} in Wikipedia. DVdm (talk) 19:44, 7 May 2018 (UTC)

You can obtain correct results by fully exploring the Earth twin's perspective. This does not fully solve the paradox. As explained very effectively in this article, the apparent paradox arises from this doubt: "what about the traveling twin's perspective? Doesn't he observe space contraction and time dilation as well?"... In the Specific example section, for instance, this question is not answered, as only the Earth twin's perspective is fully explored. In that poerspective, the acceleration phase between the two legs of the trip plays no role. However, in the space twin's perspective, the acceleration phase explains everything (see Relativity of simultaneity), even if you imagine infinite acceleration. And by the principle of relativity, this perspective is as legitimate as the Earth twin's perspective.

Also, if you use the modified thought experiment in which the space twin is represented by two different inertial observers B and C (outgoing and ingoing), you need to explain why you cannot use the "opposite" or "reciprocal" experiment, in which the Earth twin is represented by B and C and the space twin is represented by only one, motionless, inertial observer. And you need "experienced acceleration" to explain that this is impossible ("experienced acceleration" makes the space twin non-inertial during the acceleration phase).

So, computing the correct results without considering acceleration is possible, but only if you start from the Earth twin's perspective (as in the Specific example section). However, in order to solve the above mentioned doubt, you need to fully explore both perspectives. Solving the paradox is not the same as computing correct results...

Paolo.dL (talk) 18:02, 8 May 2018 (UTC)

This is why acceleration needs to be removed from the "solutions". Imagine that at some point in the journey, the earthbound twin gets lonely decides he wants to catch up. So he gets in the same spaceship, accelerates to the same velocity, but changes his mind. He stops at the moon, and pulls the exact same U-Turn as his brother, experiences the exact same acceleration forces, returns to earth, and gets back to waiting. If acceleration alone explained age differences, both twins would have the same age because they had the same acceleration profile. But clearly they won't, due to... length contraction! *that explains the paradox*. If the star is 4 light years away in an inertial frame with earth, then the travelling twin is literally moving through less space than 8 light years of space to make the round trip. Thus the elapsed time is shorter. Full stop. The symmetry is that earth sees a squished ship, and the ship sees a squished distance between the stars. The "squishiness" is plain old constant velocity SR length contraction. Bkennedy99 (talk) 21:30, 8 May 2018 (UTC)

@Bkennedy99: they would definitely not have the same acceleration profile. For obvious reasons their maximum "halfway velocity" would be wildly different from the other traveller's. But this has become off-topic here. The article talk page is not the place where we discuss or explain the subject or aspects of it, in order to educate contributors. It is where we discuss the article — see wp:Talk page guidelines. Due to some serious misconceptions that you have about this, you proposed a change to the article, but that is not going to happen due to the proper sourcing of the content that you like to see removed. This is not the place where we can help you. If you like, you can go to our wp:reference desk/science, or read a good book, or find some online discussion board such as Usenet, but we have to cut this short here. I have put a little formal warning about this on your user talk page. Good luck. - DVdm (talk) 06:41, 9 May 2018 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Dennis Sciama

I had undone this edit by user Las1817 (talk · contribs) because it puts wp:UNDUE weight on an old Machian wp:FRINGE view from Dennis Sciama. It's also just a wp:primary source, and a rather old one: the book is from 1959. User Las1817 put it back. I think that this fringe view does not belong here. Comments welcome. - DVdm (talk) 09:01, 20 October 2018 (UTC)

Per wp:NOCONSENSUS and lack of comments, I have undone the edit again: "In discussions of proposals to add, modify or remove material in articles, a lack of consensus commonly results in retaining the version of the article as it was prior to the proposal or bold edit." Also note that the first ("Although...") part of the edit is even wp:unsourced and amounts to wp:original research. Without proper wp:secondary sources, this is just another wp:UNDUE opinion. Comments still welcome. - DVdm (talk) 09:58, 21 October 2018 (UTC)

Special Relativity alone can explain the paradox

General relativity is not necessary to explain the twin paradox; special relativity alone can explain the phenomenon. — Preceding unsigned comment added by 47.201.179.7 (talk) 14:10, 30 May 2017 (UTC)

References

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Yes, but I made a little tweak, replacing that pop-sci blog with a proper textbook source: [2]. - DVdm (talk) 16:02, 30 May 2017 (UTC)

I'm not really getting which side everyone is on here, but I diagree on getting general relativity into this. The problem still works be it that the "stationary" twin stays on a planet or in a theoretical point in empty space free from the influence of any gravitational force. --uKER (talk) 15:35, 21 October 2018 (UTC)

Clarification/confirmation about jump discontinuity please

In the section "Relativity of simultaneity" I think it would be worth confirming the nature and direction of the discontinuity. Am I correct in thinking it means that the stationary twin "suddenly" seems older (rather than younger) from the point of view of the traveling twin? Am I also correct in thinking that the the stationary twin does not have any discontinuity in their view of the other (the traveling twin is viewed as smoothly continuing to age more slowly - apart from during any acceleration phase - and does not have any "sudden" age change)? FrankSier (talk) 09:50, 2 March 2019 (UTC)

The stationary twin does not suddenly seem older or younger form the point of view of the traveling twin. When the travelling twin calculates the "current age" of the remote stationary twin immediately before and immediately after the turnaround, they will get a different result. The stationary twin's calculated "current age" will suddenly increase. I think that is clearly explained in the section. The only thing the traveller would see, is a sudden blue shift of the image (of the stationary twin's clock) that was sent out a long time ago, and from then on, the remote clock at the stationary twin would seem to run faster than it seemed to be running during the first phase of the trip.

And indeed the stationary twin does not find any discontinuity in their calculation (or even view) of the travelling twin. - DVdm (talk) 11:39, 2 March 2019 (UTC)

Clarification of "the noted differences are not symmetrical, and the asymmetry grows incrementally" please

This is in the section "A non space-time approach". I think it is clear what is meant by "...the noted difference is symmetrical between the two parties" earlier on in the explanation (both see the other's clock as running the slow by the same proportion). But I do not think it is clear what is meant by the asymmetry at this second stage. Does it mean that both no longer see the other's as running slow (which does not seem right to me)? Or is it to do with the clocks not having been set to the same zero at any time? Or something else? FrankSier (talk) 09:36, 2 March 2019 (UTC)

See next section. - DVdm (talk) 11:41, 2 March 2019 (UTC)

Clarify Assumptions

It should be noted that this solution is only valid if one of the twins remains stationary in the original reference frame. It is presumably the Earth-bound twin who does so, but the more general case is that both twins go walkabout in their own spaceships. In that case, the elapsed time in the initial reference frame is t0 = gamma1 * t1 = gamma2 * t2 and their relative velocity is given by the velocity addition formula as +/-( v2 - v1 ) / ( 1 - v1 * v2 / c^2 ). The time transform is then t2 = gamma1 * t1 / gamma2. --Relativity Guy (talk) 22:09, 26 May 2019 (UTC)

Is there a WP:reliable source to back this up? Otherwise it would be wp:original research. - DVdm (talk) 08:21, 27 May 2019 (UTC)

I cannot find any documentation on this, but according to our local relativity expert (http://physics.usask.ca/~dick/251.htm), it is a well known fact in GR theory, which is consistently ignored in SR textbooks. It follows from conservation of momentum, since the reference frame in which the net 3-momentum is zero (throughout the experiment) has the maximum possible lapse of proper time. --Relativity Guy (talk) 01:51, 28 May 2019 (UTC)

Some professor's personal webpage is not sufficient. Wikipedia needs reliable wp:secondary sources for all challenged new content. See wp:RS and wp:BURDEN. - DVdm (talk) 08:06, 28 May 2019 (UTC)

Contradiction

The article says:

However, this scenario can be resolved within the standard framework of special relativity: the travelling twin's trajectory involves two different inertial frames, one for the outbound journey and one for the inbound journey, and so there is no symmetry between the spacetime paths of the twins. [...] General relativity is not necessary to explain the twin paradox; special relativity alone can explain the phenomenon.

Britannica, however, seems to disagree:[3]

The answer is that the paradox is only apparent, for the situation is not appropriately treated by special relativity. [...] A full treatment requires general relativity, which shows that there would be an asymmetrical change in time between the two sisters.

The role of acceleration is also unclear: SR-only explanations tend to say acceleration has nothing to do with the resolution of the paradox, but if this is true, what makes GR relevant? GregorB (talk) 20:28, 16 July 2019 (UTC)

Britannica, which would merely be a wp:tertiary source is wrong. - DVdm (talk) 20:53, 16 July 2019 (UTC)

It appears so, and that's remarkable. To me, the article itself, as well as all the explanations I've seen so far, are really frustrating because they appear to be circular in nature. Again, the article says:

However, this scenario can be resolved within the standard framework of special relativity: the travelling twin's trajectory involves two different inertial frames, one for the outbound journey and one for the inbound journey, and so there is no symmetry between the spacetime paths of the twins.

But, as stated in the formulation of the paradox, both twins may see themselves as stationary, and the other twin as moving, which is why the above doesn't really explain anything. Another example is Carlo Rovelli's explanation:[4]

‘In motion’ in relation to what? How can we determine which of the two objects moves, if the motion is only relative? This is an issue that has confused many. The correct answer (rarely given) is this: in motion relative to the only reference in which the point in space where the two clocks separate is the same point in space where they get back together.

So "the only reference in which the point in space where the two clocks separate is the same point in space where they get back together" is the rocket then?

IMO, a natural resolution of the paradox would be the following sentence: "The rocket-twin is wrong when he considers himself stationary and the Earth-twin moving because {insert reasons}." It is quite incredible that, to my knowledge, nobody has been able to come up with such a sentence. GregorB (talk) 22:26, 16 July 2019 (UTC)

That Rovelli sentence is of course wrong because there's an absolutely essential qualifier missing. And you just debunked it, because both the Earth (which is what he had in mind) and indeed the rocket (as you have in mind) are such "references". He should at least have said "the only inertial reference frame in which the point in ("its") space..." And even then, the phrase "a reference in which a point in space" is pretty poor.

Your proposal "...when he considers himself stationary..." is indeed correct, and it is of course fully compatible with the sentence in the lead, which is more general, and at that point does not reveal which twin "is wrong" yet. Note that that sentence is explitised further down in the article body, with proper references.

Note that there are tons of incomplete, poor, bad and wrong explations out there, but this is not the place to discuss them, unless they are mentioned in the article itself, and based on reliable sources treating them. Here we can only discuss the content and format of the current article, not the subject itself or how the subject is (mis)treated on the internet and in some sources. See wp:talk page guidelines. - DVdm (talk) 09:22, 17 July 2019 (UTC)

Thanks for the comment, this is finally illuminating.

My point here was not merely to comment on the poor quality of various explanations, but primarily on the difficulty of understanding the resolution of the paradox as currently presented by the article. I can't help thinking the article could do better in this respect, although frankly I can't suggest anything concrete right now. GregorB (talk) 10:13, 17 July 2019 (UTC)