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Imagine yourself and anything you're able to perceive to be drawn onto a rubber band. Any means of determining distances would be affected by the stretching/contracting of the rubber band. The relation between any two lengths would remain exactly the same because every length is increased/decreased by the same factor as the band stretches/contracts. According to my understanding, that is what the dilation/compression of space is like for us and why it should not lead to any change in distance.

This also applies to relational measurements of length as they are performed, for example, by devices like LIGO. Although the deformation (e.g. gravitational waves) propagates only in one of two directions of measurement, the relation between both lengths should not be affected because none of the individual lengths should be affected.

Accordingly, anything that travels the four kilometers in LIGO should take the exact same amount of time, no matter whether space is currently stretched or compressed. The speed of light being constant should be irrelevant in this this case.

If gravitational waves stretch space itself, how can they have an effect on the interference of two laser beams? According to my understanding, compression or dilation of space itself (as opposed to something in space) should not affect the distance between two points.

Disclaimer: Obviously, the above is not true. It's just how I thought is was. Hopefully, it helps to see where my understanding went awry.

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In this question ( If space is "expanding" in itself - why then is there redshift? ), the problem I had has actually been solved. It comes down to the fact that atoms (because of the non-gravitational forces holding them together) are in fact not affected by spacetime deformations. This makes these deformations detectable.

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wehnsdaefflae
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  • Yes, indeed. Thanks for pointing to the other question! Although the accepted answer does not provide much insight to me... – wehnsdaefflae Feb 11 '16 at 21:24
  • I modified the question to reflect the particular question I have, independent from gravitational waves and LIGO. – wehnsdaefflae Feb 12 '16 at 09:04
  • the same question has also been asked in the first part of What makes us think we can actually detect gravitational waves? the answers to this question, however, are again limited to trampoline and rubber band analogies that are, as i see it, not sufficient in this case. – wehnsdaefflae Feb 12 '16 at 10:41
  • @wehnsdaefflae: I've updated my answer in Can LIGO measure anything. If that's not enough details for you, please let me know (preferably on that page & not here). – Kyle Kanos Feb 12 '16 at 11:09
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    Hi @wehnsdaefflae - in the first instance: regarding JR's answer below:note that the first half of the answer, is just explaining the trivial grade-school issue of how an interferometer works. I assume, wehnsdaefflae, that this is surely nothing to do with what you are asking Am I correct? ie the entirety of your question here is about why photon travel is not affected by the "metric of spacetime". Am I right? – Fattie Feb 12 '16 at 18:50
  • @joeblow Yes. I think that is an appropriate way of putting it. In case photon travel is an exception to other ways, which I think it is because of the speed of light. – wehnsdaefflae Feb 13 '16 at 08:32
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    Veritasium provides the answer in one of his videos. – wehnsdaefflae Aug 28 '19 at 19:12
  • When you stretch a rubber band, and measure the stretch, you are appealing to the existence of a stable reference-metric system outside the rubber band. I interpret the OP ? as: How can you measure the stretching (shrinking) of space itself (the reference system)? The reference has no super-reference in this case. – pbierre Dec 16 '22 at 22:30
  • @pbierre exactly. that's how i meant it. – wehnsdaefflae Dec 18 '22 at 01:25

1 Answers1

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Assuming I understand your question correctly, the reason for your confusion is that gravitational waves do not cause a uniform compression and expansion of spacetime. They compress it in one direction and stretch it in another.

Suppose you are looking at the Earth while a gravitational wave is passing through it. In this diagram the gravitational wave is coming out of the screen towards you:

Gravitational wave

As the wave passes through the Earth it alterately stretches it in the NS direction and compresses it in the EW direction, then compresses it in the NS direction and stretches it in the EW direction. (I've grossly exaggerated the deformation for clarity).

So if the two arms of LIGO are arranged NS and EW one arm would get longer while the other got shorter. It's this difference in the length of the arms that LIGO detects.

The magnitude of the difference depends on what direction the wave passes through LIGO, but for any direction there is always some difference.

The next question is how/why the gravitational wave changes the measured length of the arms. If you imagine spacetime like a rubber sheet with LIGO painted onto it then deforming the sheet should equally deform the length of the arms and the light waves inside them, so you might think there wouldn't be any observable change. However this is not what a gravitational wave does.

You'll often see gravitational waves described as a ripple in spacetime but this is a highly misleading description. Spacetime is a combination of a manifold and a metric. The manifold is what gives spacetime its dimensionality, and the metric is what determines lengths between points on the manifold. Gravitational waves are oscillations of the metric so they change the measured distance between two points on the manifold.

The effect of gravitational waves can be compared to the expanding universe. When we say the universe is expanding we really mean the metric is changing with time so the distances we measure (e.g. to distant galaxies) increase smoothly with time. A gravitational wave is also a change in the metric, but an oscillating one rather than a smooth continuous one. And just as the expansion of the universe causes real measurable changes so do the oscillations due to gravitational waves.

A final note/clarification:

I've seen several questions asking wouldn't the gravitational wave stretch the light just like it stretches the length of the arm. If the light and arm are equally stretched than no change would be observed.

It's quite true that a change in the metric does stretch light (i.e. red or blue shift it) but remember that the lasers are continually shining new light into the arms. The length of the arms is 4 km so the light makes the round trip in about 27 microseconds. However the highest frequency of the detected wave was 250 hz, making the shortest period 4 milliseconds. So the length of the arms is changing more than a factor of a hundred times more slowly than the light is measuring that length. The end result is that the frequency of the light is not affected to any significant extent by the gravitational wave, and that's why it can detect the change in the length of the arms.

John Rennie
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  • Thanks a lot, this explains at least part of my confusion. The orthogonal configuration of the LIGO devices makes distortions in dirfferent directions comparable.Still, however, I cannot wrap – wehnsdaefflae Feb 12 '16 at 08:45
  • ...my head around the fact, that deformations of space reflect in distances. To be able to measure the deformation of space by measuring within the deformed space appears to me like like measuring temporal effects of light speed with a stop watch on board the space craft. The instrument of measure is under the complete influence of what it is supposed to measure in the first place. – wehnsdaefflae Feb 12 '16 at 08:52
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    @wehnsdaefflae: I've extended my answer to address your comment – John Rennie Feb 12 '16 at 09:11
  • i feel like this answer is the only one offering a new perspective by differentiating manifold and metric. it enables to understand why the worn out analogies with trampolines and rubber bands are not sufficient for an explanation. – wehnsdaefflae Feb 12 '16 at 10:45
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    I am confused by the mention here of "expansion of the universe" issues. In fact .. http://physics.stackexchange.com/questions/223610/if-space-is-expanding-in-itself-why-then-is-there-redshift .. the consensus there seems to be that the changing metric of the universe is indeed only detectable because atomic forces "work against it", making our instruments meaningful. Here measuring gravity waves (also it turns out, relating only to the "metric"), it seems we have to say photons DON'T change with the metric of the universe. – Fattie Feb 12 '16 at 19:03
  • @JohnRennie: "Gravitational waves are oscillations of the metric so they change the measured distance between two points on the manifold." How about the rulers and the distances on them? Do they change? And the light? Does the change of measured distance on the manifold mean the light travels above or below c? – bright magus Feb 13 '16 at 11:14
  • @JoeBlow: I've extended my answer to address your and related comments. Light is stretched by changes in the metric, but in the LIGO experiment the timescale for this is much slower than new unstretched light is shone into the arms. – John Rennie Feb 13 '16 at 12:49
  • @JohnRennie: the real question is whether the light is stretching/contracting along the oscilations of the metric it is propagating through. Whether it follows the changes in "real time". – bright magus Feb 13 '16 at 13:35
  • @JohnRennie: "So the length of the arms is changing more than a factor of a hundred times more slowly than the light is measuring that length." Which means that at any given time, we can assume the proportions of the arms is constant. However, if the metric in one direction is stretched (as compared to the other arm), then the time flow in this direction is also stretched so that $c$ is maintained. But this means that the interferometer should show no (significant) difference. – bright magus Feb 13 '16 at 18:03
  • @brightmagus: No. In the rest frame of the interferometer the changes are purely spatial, just as in our rest frame the change in the geometry of the universe is purely spatial. – John Rennie Feb 13 '16 at 20:37
  • @JohnRennie; The bigger the curvature, the more time slows down. That's called gravitational time dilation. And that's basic GR. – bright magus Feb 13 '16 at 21:11
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    The arms do not change as a physical body. There is not such an arm. Earth is bound while the target mirror is not. It is like the ring around a rod famous example. This is how I see it – Alchimista Dec 25 '18 at 15:08
  • Alternatively as per the answer, interference is what makes the trick. If there is interference either the path were different or the frequency @wehnsdefflae – Alchimista Dec 25 '18 at 15:16