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Regarding the wonderful 2016 news about gravitational waves.

Travel time in one arm of the LIGO is ~ 30μs.

A gravitational wave affects the arm for some few hundred of these laps.

Then for example as RobJ explains "the arm changes length ... subsequent wavecrests will have had successively further to travel and so there is a phase lag that builds up ..."

But we always use the language that gravitational waves are affecting the spacetime metric. I just don't see how the phase ("speed") can change if the spacetime metric is changing.

Let me put it this way:

Say I said (A) "gravitational waves stretch and squash objects - amazing!". Say a physicist says (B) "gravitational waves stretch and squash the spacetime metric - amazing!"

What really is the difference between saying "A" versus "B", and, what is it in the experiment that shows us "B" happened, not "A"?

In other words, say we deliberately merely did "A" - using say a really precise bulldozer chained to one end of the tube - what specifically would we look at in the signal and conclude "oh, that was merely 'A' - not actually 'B'".

Or indeed are "A" and "B" indistinguishable?

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Fattie
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  • You are asking why the wavelengths of light aren't affected in the same way as the arms of the interferometer? they are, but there are GR subtleties going on... – innisfree Feb 12 '16 at 14:23
  • Hi Innisfree (btw - Morar??!) - that sounds tantalizing, but deserves a very long answer! :O – Fattie Feb 12 '16 at 14:47
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    Related and/or dupe: http://physics.stackexchange.com/q/153657/ – Kyle Kanos Feb 12 '16 at 15:28
  • Hi @innisfree .. Hmm, you mention "GR subtleties". Can you elaborate? In the linked answer, JR I believe is essentially saying "photons don't change with the metric". Is that what you mean by "GR subtleties" or do you mean some further point? – Fattie Feb 12 '16 at 18:48
  • Also Innisfree (and indeed @JohnRennie), Innisfree you state that the wavelengths of light *are* affected in the same way the arms are - this would seem to be at odds with what JR is saying. – Fattie Feb 12 '16 at 18:53
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    "since it would be deforming the space and time of the photons equally." the photon is a point in space and time, it cannot be deformed. It can be red shifted or blue shifted , but the experiment is measuring phases , not frequencies, and phases enter in the way the photons build the electromagnetic wave. John is saying that the amplitude changes of the gravitational wave is much slower then the return track of the laser light, where the phases should cancel if spacetime is quiet. Did you see the webcast? – anna v Feb 13 '16 at 14:09

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I've extended my answer to the question I linked to address this, but for completeness I'll also post the answer here.

In the LIGO experiment light is continually being shone into the arms, and the travel time along an arm and back is about 27 $\mu$s. The maximum frequency of the gravitational wave is 250Hz making the period 4 ms. So the gravitational wave is changing the length of the arm more than a factor of a hundred times more slowly than the light is measuring that length change.

The changes in the metric caused by the gravitational wave do indeed affect light, but because they are so slow the light is (to a first approximation) unchanged by them. The light simply doesn't hang around for long enough to be stretched.

John Rennie
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  • Whoa. that is deep. – Fattie Feb 13 '16 at 14:41
  • You know those annoying points where experts have to explain it to non-experts over and over? I hate being on the non-expert side of that :) I guess I'm having trouble seeing how if the space*time* metric is altered, there can be a difference in the speed. – Fattie Feb 13 '16 at 14:57
  • I understood the opposite, that the paths of the light being lengthened or shortened, this introduced a phase diff between the arms which is measured by the interferometer –  Feb 13 '16 at 18:01
  • The gravitational wave changes the length of the arms. This is a real change that you could measure with a ruler if you could do it in 4 ms. The gravitational wave does not change the wavelength of the light. The end result is a change in the number of wavelengths along the arm, which is why there is a phase change. – John Rennie Feb 13 '16 at 20:36