Talk:Gravitational-wave observatory

Basic physics of the LIGO family

"A passing gravitational wave will slightly stretch one arm as it shortens the other."

This is what I believed for a decade or so. However most of 'Those who should know' have, over the last seven (or more) years, stated instead that it is motion of the test masses which is sought. Their argument is that any change in the geodesic length of an arm is precisely countered by the gravitic change in proper time along the arm. ie: c remains constant regardless of gravity waves. I do not cite sources here simply because there are so many.

I am not qualified to modify the article at this level; but would welcome feedback from those who are. Pawprintoz (talk) 13:15, 18 June 2013 (UTC)

It may be difficult to distinguish a warping of spacetime from a modulation of c. Einstein's GR view is that c is fixed and spacetime warps. Dicklyon (talk) 05:37, 17 February 2016 (UTC)

Also, wouldn't the stretching of one arm or another depend on the direction the wave is coming from? My intuitive guess is that:

• A wave coming from the west would wrinkle the east-west arm.
• A wave coming from the north would wrinkle the north-south arm.
• A wave coming from the northeast would wrinkle both equally (if the arms are like the letter L)
• A wave coming from the southeast would wrinkle them differently (or out of phase?)

Since you're interfering the two beams against each other, some of the info above cancels out and you can really only detect waves with a southeast-northwest component to their travel, right? Therefore, to figure out the direction to the source, you need three Ls at right angles in space? Again, these are just educated guesses. Someone who knows should straighten out the reader on this topic. OsamaBinLogin (talk) 01:29, 15 February 2016 (UTC)

Yes, but two orthogonal beams on the ground is enough to build, and enough to detect a signal. Getting a good direction from it is much harder. With two detectors a long way apart, you get one time difference of arrival to constrain the direction. Another, making a triangle, would pin down the direction better. Several more would be even better. It's easier to build more on the ground than to make a vertical arm, I expect. Dicklyon (talk) 05:34, 17 February 2016 (UTC)

None of the above is relevant to my point that it is the motion of the test masses which provides the interferometry signal. Pawprintoz (talk) 06:33, 18 February 2023 (UTC)

Furthermore it's now 10 years since I raised this issue, and the position I took then is now the dominant one.

I plead for someone more authoritative to make the required changes. Otherwise I will have to do the rewrite myself. I'll look for some good sources in the meantime. Pawprintoz (talk) 07:07, 18 February 2023 (UTC)

Merger proposal: AGIS

The following discussion is closed. Please do not modify it. Subsequent comments should be made in a new section. A summary of the conclusions reached follows.

The result was Merge. RockMagnetist (talk) 15:36, 22 May 2014 (UTC)

---

The article Atomic gravitational wave interferometric sensor just went through an AfD, and it was established to everyone's satisfaction that there are enough sources for the subject to be notable. However, it has been a one-liner since 2011, and the sensor itself is still in the development stages. The subject might be better served as part of Gravitational-wave detector#Interferometers, which already provides much of the context for understanding it. Of course, a merger does not preclude re-creating the article in the future. RockMagnetist (talk) 16:07, 11 May 2014 (UTC)

This seems like an excellent idea to me. This page should include all detector types, so there should be a section on it. There can be spin-off pages for detector types with really in-depth information, e.g. pulsar timing arrays, but since atom interferometry is still in the very early stages it makes sense to leave it as a section. — BobQQ (talk) 21:27, 11 May 2014 (UTC)

• Support. The one sentence there should be part of this article anyway, and then it's best to make the AGIS page a redirect until someone comes up with more content. — HHHIPPO 16:00, 16 May 2014 (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.

Observatory vs Detector

I am not sure if it right to equation GW observatory with GW detector. An observatory is not the same as a telescope. GW observatories are the places where GW detectors operate. It seems to me like a page on GW observatories should discuss the various collaborations (the infrastructure of GW science), and a page on detectors should talk about the actual physics and science of detecting gravitational waves. Seeing as the page is not in a terribly good state at the moment, this might be a moot point, but it might be worth considering for the future. — BobQQ (talk) 11:03, 26 October 2014 (UTC)

In the (emerging)field of gravitational wave astronomy the instruments used to observe gravitational waves are referred to as "observatories". See for example "Laser Interferometer Gravitational wave Observatory". The reason is that the ambition of these instruments is much larger than mere detection.TR 17:53, 26 October 2014 (UTC)

LIGO has three detectors: two at Hanford and one at Livingston, much like the Keck Observatory has two telescopes. Is there not a distinction between LIGO and the LIGO detectors (H1, H2 and L1)? Similarly, Virgo is the detector of EGO. I understand that the goal is to do more than just detect, but the analysis is done by computers, not the instruments themselves, so I do not think it is unfair to refer to the interferometers as detectors. — BobQQ (talk) 10:03, 28 October 2014 (UTC)

Maybe, but that is not how the terms are currently used. (I agree that "observatory" may linguistically not be the best description. I would generally prefer "GW antenna", as the more accurate description of what these instruments are.) See for example the eLisa website:

eLISA will be the first observatory in space to explore the Gravitational Universe.

TR 11:04, 28 October 2014 (UTC)

I guess that does make sense in context of space observatory. Antenna is the A in LISA, so there is some pedigree to that terminology. I still think a better name for the current article would be detector, but if observatory is in common usage... — BobQQ (talk) 19:29, 28 October 2014 (UTC)

I'm glad that I'm not totally alone in imagining the distinction, although it seems to be non-rigidly enforced: Characterization of the LIGO detectors during their sixth science run — BobQQ (talk) 09:42, 30 October 2014 (UTC)

...the extraordinarily small effect the waves would produce on a detector...

The article says:

The direct detection of gravitational waves is complicated by the extraordinarily small effect the waves would produce on a detector. The amplitude of a spherical wave will fall off as the inverse of the distance from the source. But this is no different to light, which is easily detected from remote stars William M. Connolley (talk) 23:50, 13 February 2016 (UTC)

First of all, shouldn't this be "inverse square"? It's a law of geometry, a spherical wave from a point falls off as 1/r^2, whether light or gravity waves or flying particles of sand, because the sphere increases in area by 1/r^2, and whatever is radiated spreads to fill. (And, light from an infinite line would fall off as 1/r and from an infinite plane as 1 - the light is the same intensity no matter your distance - you're always looking at 180° of a wall of light.) OsamaBinLogin (talk) 01:05, 15 February 2016 (UTC)

The intensity (physics) would be inverse square, which makes the amplitude just inverse, if it's true that the intensity is proportional to square of strain (as seems likely to me). Dicklyon (talk) 01:13, 15 February 2016 (UTC)

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How does this all work?

I know this is not a discussion forum, but I feel the article (as much as I've been able to digest it) does not address an obvious question about how these detectors work. And it seems like the article should, and should do so in layman's terms, in order to lay down a basic understanding of what is going on here.

To oversimplify, if my detector was a steel bar a mile long, and to measure changes in its length I laid down a steel ruler next to it and had both steel bar and steel ruler anchored at the "zero" end and I went out to the "mile" end and waited with a microscope for a change in length of the steel bar relative to the ruler, I'm not likely to see any, right? Any passing gravity wave would affect both steel bar and steel ruler exactly the same.

So in real life we are using this light wave interferometer setup to detect the changes in length or distance. But since (if I understand correctly) the gravity wave distorts the fabric of space/time itself, how is this actually any different from the steel ruler? In what manner is it exempt from the changes? I'm not a scientist (surprise!) but after all, gravity pulls light into black holes, right? Light bends observably when it goes past a massive star. Light does not seem exempt from the effects of gravity... how can you measure the difference in distance when every measuring device you have changes right along with the distance? — Preceding unsigned comment added by 2600:1700:B930:7B90:84B4:59C3:31C3:9042 (talk) 20:59, 10 September 2020 (UTC)

Because the detection is based on the test masses, not the geodesic within the arms. I'm trying to find a physicist to rewrite the offending sentences, without luck so far. Pawprintoz (talk) 02:24, 20 February 2023 (UTC)

Sorry- I should have said "MOTION of the test masses." Pawprintoz (talk) 02:26, 20 February 2023 (UTC)