Measuring gravitational waves

[DParlevliet]

On the radio I heard the explanation of a scientific writer about measuring gravity waves (or better: space waves). He told that in principle one could not measure these with a ruler, because this would follow the wave (therefore it is measured with light). However I also read that while the universe (space) is expandig, that matter does not expand in the same way because the forces between atoms is much larger then the expension forces of space. But to my understanding a gravitation wave is an expension/shrinking of space in sinusform. So why would a mechanical ruler follow these waves?

[lekh2003]

I don't think I understand your question very well, could you explain what you mean by the ruler moving with the wave?

[DParlevliet]

A gravity-wave expands/schrinks space. If a mechanical ruler would expand/schrink in the same way, it is not possible to measure the gravity wave with a mechanical ruler (in theory, in practice it is not possible anyway of course). Or simple: if space expands, does a ruler expands with it?

[interested]

30 minutes ago, DParlevliet said:

A gravity-wave expands/schrinks space. If a mechanical ruler would expand/schrink in the same way, it is not possible to measure the gravity wave with a mechanical ruler (in theory, in practice it is not possible anyway of course). Or simple: if space expands, does a ruler expands with it?

Ligo is a bigger version of the michelkson morley experiment. It works by detecting a change in length of its arms which are about 4km long. The ruler analogy you are using is equivalent to the arms and it will expand and contract as the gravitational wave passes by as you say. 

Here is a link that attempts to explain it, but if you google how LIGO works, you will lots of hits https://www.quora.com/How-does-the-ligo-works

[Strange]

It is an interesting question. There are a couple of answers which, I think, are equivalent.

One is that the expansion is a really tiny effect so even on the scale of the solar system it is too small to affect the gravity holding planets in their orbits. Certainly not enough to affect electromagnetic bonds in material bodies.

The other thing is that the expansion only happens when the Einstein Field Equations are solved for a homogeneous distribution of mass. That is approximately true on cosmological scales but not when there're large lumps of matter around. So we shouldn't expect to see expansion locally.

Gravitational waves are independent of, and unaffected by, the presence of matter and so the stretching and compression effects can be measured. (As you say, not by using a ruler which would stretch and shrink by the same amount, but by using the speed of light which is invariant.)

[DParlevliet]

But what about the size of a planet. I have read that a planet does not expand, because the force of the expanding universe is very small compared to the atomic forces. Then I would expect that a ruler also does not expand.

Or an other example: at the time the univers was half its present size, would the earth (and distance between atoms) also be half the size?

[Strange]

14 minutes ago, DParlevliet said:

But what about the size of a planet. I have read that a planet does not expand, because the force of the expanding universe is very small compared to the atomic forces. Then I would expect that a ruler also does not expand.

It is very small on the scale of galaxies or clusters of galaxies (which don't expand) so it is definitely too small on the scale of planets and rulers. (I'm not sure it is accurate to regard it as a force anyway.)

14 minutes ago, DParlevliet said:

Or an other example: at the time the univers was half its present size, would the earth (and distance between atoms) also be half the size?

No. Early stars, for example, were actually bigger because they were purely hydrogen with no heavier elements.

Edited December 11, 2017 by Strange

[StringJunky]

6 minutes ago, DParlevliet said:

But what about the size of a planet. I have read that a planet does not expand, because the force of the expanding universe is very small compared to the atomic forces. Then I would expect that a ruler also does not expand.

Or an other example: at the time the univers was half its present size, would the earth (and distance between atoms) also be half the size?

Think of the  two arms of LIGO as two different frames. Each time one arm is affected by one part of a gravitational wave, the arm of the other is affected by a different part of the gravitational wave, so, one is shrinking and the other is expanding but only from the perspective of the other arm i.e. from a different frame of reference. The universal expansion is entirely different and is resisted by sufficient gravity and intramolecular forces. If there is a distance of about 200 mlyrs between galaxies they will separate with the expansion.

[DParlevliet]

49 minutes ago, Strange said:

It is very small on the scale of galaxies or clusters of galaxies (which don't expand) so it is definitely too small on the scale of planets and rulers. (I'm not sure it is accurate to regard it as a force anyway.)

No. Early stars, for example, were actually bigger because they were purely hydrogen with no heavier elements.

Brian Greene mentioned it as being a force.

So a ruler will not expand with space and so will not follow a gavity wave, which essentially expands/schrinks in sinus form.

 

 

[StringJunky]

3 minutes ago, DParlevliet said:

Brian Greene mentioned it as being a force.

So a ruler will not expand with space and so will not follow a gavity wave, which essentially expands/schrinks in sinus form.

 

 

A ruler will not expand from universal expansion but will shrink/expand in a gravitational wave.

[DParlevliet]

Why? What is the diference between expanding universe, and expanding during a gravity wave. Both are expanding space.

1 hour ago, StringJunky said:

Think of the  two arms of LIGO as two different frames. Each time one arm is affected by one part of a gravitational wave, the arm of the other is affected by a different part of the gravitational wave, so, one is shrinking and the other is expanding but only from the perspective of the other arm i.e. from a different frame of reference. The universal expansion is entirely different and is resisted by sufficient gravity and intramolecular forces. If there is a distance of about 200 mlyrs between galaxies they will separate with the expansion.

I think the arms are not expanding/shrinking, but the space in between. Only light is effected by that. Now I understand why one cannot measure a gravity wave with a ruler: it would measure no expanding/shrinking of the arms because there is none. It's the space between the detectors which expands/shrinks.

[StringJunky]

12 minutes ago, DParlevliet said:

Why? What is the diference between expanding universe, and expanding during a gravity wave. Both are expanding space.

I think the arms are not expanding/shrinking, but the space in between. Only light is effected by that. Now I understand why one cannot measure a gravity wave with a ruler: it would measure no expanding/shrinking of the arms because there is none. It's the space between the detectors which expands/shrinks.

Yes.  The LIGO experiment relies on the invariance of the speed of light, which stays the same regardless of which arm (frame of reference) is moving : the time component has to change which is what is measured in the LIGO experiment.

[swansont]

18 minutes ago, DParlevliet said:

 I think the arms are not expanding/shrinking, but the space in between.  

What is the practical difference between the two cases?

[DParlevliet]

27 minutes ago, swansont said:

What is the practical difference between the two cases?

Nothing, but the difference of theory is important. As a result of StringJunky's answer: LIGO measures an invariance in licht speed during a gravity wave, so not a constant light speed. I have never realised that. So when we were there when the universe was half the present size, we would measur a different light speed: twice faster or slower (not sure yet what).

[Strange]

No. Light speed is both constant and invariant.

1 hour ago, DParlevliet said:

Why? What is the diference between expanding universe, and expanding during a gravity wave. Both are expanding space.

They happen in different places for different reasons. 

The expansion of space only happens where there is an even (homogeneous) distribution of mass. Approximately true at cosmological scales. Not true around the LIGO instrument (or in the solar system, or our galaxy, or a cluster of galaxies). So nothing to measure.

Gravitational waves travel through space, independently of the presence of mass. So they travel through LIGO and cause the stretching and compression we measure.

Edited December 11, 2017 by Strange

[DParlevliet]

So when the universe would be fully (homogeneous) massive mass, this mass would expand with space and the distance between atoms would be expand too? When this type of universe would be half the present size, the distance between atoms would also be half the present distance?

[Strange]

38 minutes ago, DParlevliet said:

So when the universe would be fully (homogeneous) massive mass, this mass would expand with space and the distance between atoms would be expand too? When this type of universe would be half the present size, the distance between atoms would also be half the present distance?

In the very early universe, the high temperature means there were no atoms, just a dense quark-gluon plasma.

Once the temperature and density had dropped sufficiently for atoms (and molecules) to form, then the distances between them would be determined by the usual physics of gases (the universe was 90-something% hydrogen). But, yes, the hydrogen would have been denser than the average density of the universe now, so the atoms would be closer together.

 

[DParlevliet]

But if the universe would be solid iron.

[Strange]

7 minutes ago, DParlevliet said:

But if the universe would be solid iron.

How would that happen?

But if it did, then the interatomic forces would prevent the natural expansion or contraction. In the same way that standing on the surface of the Earth stops you falling to the centre of the Earth, which is the natural consequence of the curvature of space-time around the Earth. (Although you will be a few centimetres shorter than you would be in zero gravity!)

[swansont]

3 hours ago, DParlevliet said:

Nothing, but the difference of theory is important. As a result of StringJunky's answer: LIGO measures an invariance in licht speed during a gravity wave, so not a constant light speed. I have never realised that. So when we were there when the universe was half the present size, we would measur a different light speed: twice faster or slower (not sure yet what).

No, it would be the same. It's invariant.

[DParlevliet]

1 hour ago, Strange said:

But if it did, then the interatomic forces would prevent the natural expansion or contraction.

Is a gravity wave also a natural expansion/contraction, but now in ~~sinus schape?

Or: what is the difference?

[DParlevliet]

2 hours ago, Strange said:

But if it did, then the interatomic forces would prevent the natural expansion or contraction.

Or a better question: suppose the arms of the LIGO are made of solid iron on with de detectors/mirrots etc. are build. If a gravitywave passes this iron, why would the interatomic forces not prevent the space expansion or contraction of the wave?

[StringJunky]

13 minutes ago, DParlevliet said:

Or a better question: suppose the arms of the LIGO are made of solid iron on with de detectors/mirrots etc. are build. If a gravitywave passes this iron, why would the interatomic forces not prevent the space expansion or contraction of the wave?

 Spacetime, which changes in the LIGO experiment, and space, which expands in the universal expansion are two different phenomena. 

Edited December 11, 2017 by StringJunky

[DParlevliet]

I have been looking for a scientific description of LIGO and gravitywave detection (difficult to find) and found a probably misunderstandig from me. According Wikipedia: "measure gravitational-wave induced motion between separated 'free' masses" and another website: "uses widely-separated test masses freely suspended as pendulums". So the mirrors are not fixed on a solid base as I expected, but can freely move and therefore follow the gravity wave, without interatomic forces preventing it.

But there is still the question of a solid metal ruler and:

18 hours ago, Strange said:

interatomic forces would prevent the natural expansion or contraction.

Simple answers "no" and "different phenomena" are no science. Explain what is the physical effect on space of space expansion and gravitywave and differs in what. I think only in the shape. Expansion/scrinking of space results in large/smaller distances in space and a gravitywave does the same. So why would in one case interatomic forces prevent this in a solid mass and in the other case not?

[StringJunky]

2 hours ago, DParlevliet said:

I have been looking for a scientific description of LIGO and gravitywave detection (difficult to find) and found a probably misunderstandig from me. According Wikipedia: "measure gravitational-wave induced motion between separated 'free' masses" and another website: "uses widely-separated test masses freely suspended as pendulums". So the mirrors are not fixed on a solid base as I expected, but can freely move and therefore follow the gravity wave, without interatomic forces preventing it.

But there is still the question of a solid metal ruler and:

Simple answers "no" and "different phenomena" are no science. Explain what is the physical effect on space of space expansion and gravitywave and differs in what. I think only in the shape. Expansion/scrinking of space results in large/smaller distances in space and a gravitywave does the same. So why would in one case interatomic forces prevent this in a solid mass and in the other case not?

You can measure universal expansion from the redshift of an astronomical body, which is a single parameter i.e. a single frame,  whereas you need two frames, represented in the two arms, to measure GW's. When a GW passes through the arms, the relative time between the arms is not in sync because spacetime is being perturbed differently in the locality of each arm by the GW and this difference creates a sinusoidal wave when the measurements are combined as they happen..

Edited to add.

Edited December 12, 2017 by StringJunky