B How Does LIGO Detect Gravitational Waves Despite Changes in Spacetime?

[asca]

TL;DR Summary

How can an interferometer detect Gravitation waves, if the change in space time due to gravity affects all the rulers (and clocks) in that spot?

Sorry if the question has been already answered, but I didn't manage to find it. Let's go back to ligo detection of gravitational waves, my question is the following: if space time changes its texture due to a gravitational effect, all the rulers (and clocks) in that spot will be affected, so also the electromagnetic wave will have its own ruler and clock changing the same way they change for the LIGO arm, and so there should not be any change in the number of wawelenghts and no interference pattern changes, so no detection. What am I missing?

 

[phinds]

Summary: How can an interferometer detect Gravitation waves, if the change in space time due to gravity affects all the rulers (and clocks) in that spot?

Sorry if the question has been already answered, but I didn't manage to find it. Let's go back to ligo detection of gravitational waves, my question is the following: if space time changes its texture due to a gravitational effect, all the rulers (and clocks) in that spot will be affected, so also the electromagnetic wave will have its own ruler and clock changing the same way they change for the LIGO arm, and so there should not be any change in the number of wawelenghts and no interference pattern changes, so no detection. What am I missing?

You are missing that the change is directional, oriented towards the source of the gravity waves, and the two arms of LIGO get affected differently and THAT is what the "detection" really is.

 

[Michael Price]

Summary: How can an interferometer detect Gravitation waves, if the change in space time due to gravity affects all the rulers (and clocks) in that spot?

Sorry if the question has been already answered, but I didn't manage to find it. Let's go back to ligo detection of gravitational waves, my question is the following: if space time changes its texture due to a gravitational effect, all the rulers (and clocks) in that spot will be affected, so also the electromagnetic wave will have its own ruler and clock changing the same way they change for the LIGO arm, and so there should not be any change in the number of wawelenghts and no interference pattern changes, so no detection. What am I missing?

Objects held together only by gravity cannot detect the gravity waves, but objects held together by non-gravitational forces - which is most objects - will sense the purely gravitational (spacetime) distortions.

 

[PeterDonis]

Objects held together only by gravity cannot detect the gravity waves

They can if they exchange light signals. Which is basically what LIGO does. The actual LIGO sensors and mirrors are only "held together by gravity" in the plane of the LIGO apparatus (they do of course have proper acceleration perpendicular to that plane, but that can be ignored when modeling LIGO's workings--and there are plans for a space-based detector that will be entirely in free fall).

 

[Orodruin]

Curvature of spacetime essentially manifests itself as tidal gravity, leading to nearby objects not having the same acceleration unless acted upon by a force. This is true also for a gravitational wave. The mirrors in a gravitational wave detector are suspended in a pendulum contraption that in essence means that they move freely in the relevant direction. Since tidal gravity in essence is nearby things moving with different acceleration, the distance between the mirrors does change. The meter is still a meter, leading to a change in travel time for the light and therefore also a changed interference pattern.

Note that the frequency of the gravitational wave is typically lower than the time taken for light to travel back and forth between the mirrors. Thus, there is no relevant stretching of the light wavelength.

 

[PeterDonis]

if space time changes its texture due to a gravitational effect, all the rulers (and clocks) in that spot will be affected

No, they won't. You are misunderstanding what "changing spacetime geometry" means. It means that the physical distance, for example, between the ends of the arms of a GW detector changes, even though the arms feel no force (because they are only affected by gravity and so they are in free fall). The change in physical distance can be detected because it is a change in how many rulers (or wavelengths of light) will fit in each arm.

 

[Ibix]

The thing to note about the expansion and contraction is that it is slow compared to the flight time of light in the interferometer. So, although "in flight" light is stretched a bit, new "unstretched" light is being continually injected, so the ratio between wavelength and arm length varies.

 

[PeterDonis]

although "in flight" light is stretched a bit

This is a coordinate-dependent statement, and should probably not be used as a description. The invariant is the variation of the physical distance between the ends of the arms, as shown by interference fringes in a detector like the one in LIGO.

 

[pervect]

Summary: How can an interferometer detect Gravitation waves, if the change in space time due to gravity affects all the rulers (and clocks) in that spot?

Sorry if the question has been already answered, but I didn't manage to find it. Let's go back to ligo detection of gravitational waves, my question is the following: if space time changes its texture due to a gravitational effect, all the rulers (and clocks) in that spot will be affected, so also the electromagnetic wave will have its own ruler and clock changing the same way they change for the LIGO arm, and so there should not be any change in the number of wawelenghts and no interference pattern changes, so no detection. What am I missing?

While it's possible to view gravity as "rulers and clocks changing", that's not really the most popular analogy to use to describe gravitational waves.

In spite of the fact that it's not popular, let's look at an oversimplified example of how it might work.

Consider Einstein's description of a curved spatial geometry via a heated marble slab, as described in chapter 24 of Einstein's book "Relativity: The Special and General Theory". https://www.bartleby.com/173/24.html.

The basic idea is that you have a heated marble slab, the rods expand with heat and contract with cold, and that you find the geometry of the slab as measured by these rulers isn't Euclidean.

Our heated marble slab represents only spatial geometry, and not space-time geometry. This is oversimplified, but it's the best I can do to remain comprehensible.

One can definitely detect the change in geometry caused by the heat. A simple example illustrates this. Suppose you make a quadrilateral figure with four equal sides, and two equal diagonals, a square.

Then, in an Euclidean geometry, the ratio of the diagonals to the sides will always be $\sqrt{2}$.

Now, let's represent a non-Euclidean geometry, following Einstein's suggestion, by heating the center of the marble slab, but not the outer perimeter. Unheated, the outer rods do not expand, but sections of the diagonal rods do. This causes the ratio of the diagonal length to the outer side length to change from it's Euclidean value.

Then the ratio of the diagonal to the side is not $\sqrt{2}$ anymore, and we see a measurable result from the geometry changing.

Most descriptions of Ligo do not involve the idea of rulers expanding, but it's a possible interpretation of the metric changes, which is what the mathematics actually models.

Ligo does not do the equivalent of taking the ratio of the outside of the square to the diagonal - it's got a different setup. I imagine it might be possible to make a more detailed setup that operates in the same way as the Ligo detector does to detect the changes in geometry, but this short post is not that detailed, it just answers the question of how one could detect the changes induced in the geometry.

 

[PeterDonis]

Then the ratio of the diagonal to the side is not $\sqrt{2}$ anymore, and we see a measurable result from the geometry changing.

Yes, but this measurable result is a result of the geometry changing--from Euclidean to non-Euclidean--not of the rulers changing. If the rulers changed along with the geometry, you would not be able to measure the change in the geometry, because it would change the rulers right along with it. That is basically the misconception that the OP gave.

Most descriptions of Ligo do not involve the idea of rulers expanding, but it's a possible interpretation of the metric changes

No, it isn't. All of the different ways of modeling LIGO (different coordinate charts) agree that the physical lengths of the arms change--more precisely, the physical distances between the sensor and the mirrors changes. This cannot be interpreted as "rulers expanding" for the same reason as given above.

The best way, IMO, to think about a gravitational wave is that it is a wave of changing tidal gravity. Tidal gravity doesn't change the lengths of rulers*. It changes the physical distance between objects following nearby worldlines.

* - Strictly speaking, any real ruler will have a finite tensile strength, so the effects of tidal gravity on it will not be zero, because tidal gravity will induce some finite strain in the material of which the ruler is made. But, first, we can imagine an idealized ruler, the limit of real rulers as the induced strain from tidal gravity goes to zero, and those idealized rulers, strictly speaking, are what I have been talking about. And, second, for a case like LIGO, the induced strain in a real ruler (or in, for example, the substance of the Earth, the tubes enclosing the paths of the laser light in the arms, etc.) is so much smaller than the induced change in arm length between the sensor and the mirrors, that it can be considered zero for analyzing the results.

 

[A.T.]

if space time changes its texture due to a gravitational effect, all the rulers (and clocks) in that spot will be affected, so also the electromagnetic wave will have its own ruler and clock changing the same way they change for the LIGO arm, and so there should not be any change in the number of wawelenghts and no interference pattern changes, so no detection.g?

This might help:

 

[Ibix]

This is a coordinate-dependent statement, and should probably not be used as a description. The invariant is the variation of the physical distance between the ends of the arms, as shown by interference fringes in a detector like the one in LIGO.

If the physical distance between the ends of the arms changes then the wavelength of light in flight must also change, surely? To illustrate the point, imagine using light with a wavelength equal to the arm length. If the physical distance to the far mirror changes (on a time scale much less than the flight time of light through the apparatus) then the physical distance to the far end of the wave must also change.

In any case, isn't the invariant that is directly measured the flight time? Or the variation in the difference in flight times for the two arms, more precisely.

 

[pervect]

Yes, but this measurable result is a result of the geometry changing--from Euclidean to non-Euclidean--not of the rulers changing. If the rulers changed along with the geometry, you would not be able to measure the change in the geometry, because it would change the rulers right along with it. That is basically the misconception that the OP gave.

The whole "shrinking and expanding" rulers idea, to my way of thinking, can be useful as a tool for someone who is reluctant to abandon Euclidean geometry.

One basically imagines that the real, physical, rulers, which we can actually make, are "distorted". Then one can envision a familiar, underlying, Euclidean geometry when one "undistorts" these distorted rulers. But one has to keep in mind that this "undistortion" is basically a mathematical trick - the physical measurments we make are all made with "distorted" rulers.

With this approach, one has to keep track of two sets of rulers then - the actual, physical rulers, and the unphysical, unmeasurable, "undistorted" rulers. One typically winds up with a philosophy that the non-physical , non-measurable rulers are more real than the physical ones.

The philosophical view is not the one I personally prefer. In fact, it's a bit odd, in which one imagines that non-existent, non-observable things are "more real" than the ones we can actually make. But, it's possible to use it to get correct results.

In any case, I do agree that the actual geometry of General relativity (and of gravitational waves), as measured with the physical rulers, the ones we can actually create, is non-Euclidean.

 

[Tomas Vencl]

Suppose, that I can freeze the gravitation waves while they are passing through the detector. So there will be some static changes in spacetime (or suppose, that frequency of the wave is much smaller than actually is). Can I still see the interference pattern change ?

 

[PeterDonis]

If the physical distance between the ends of the arms changes then the wavelength of light in flight must also change, surely?

There isn't a single thing called "the wavelength of the light". To construct an invariant related to the wavelength of the light, you would have to specify which observer is observing it and how.

isn't the invariant that is directly measured the flight time? Or the variation in the difference in flight times for the two arms, more precisely

Yes.

 

[PeterDonis]

One basically imagines that the real, physical, rulers, which we can actually make, are "distorted". Then one can envision a familiar, underlying, Euclidean geometry when one "undistorts" these distorted rulers. But one has to keep in mind that this "undistortion" is basically a mathematical trick - the physical measurments we make are all made with "distorted" rulers.

This approach has a good track record of causing confusion--I can think of plenty of past PF threads where that has happened. The problem is that anyone who is capable of really understanding and adhering to your caveat that the "undistorted ruler" thing is a mathematical trick, doesn't need the trick because they have already grasped that the only real geometry of spacetime in the presence of gravity is the non-Euclidean one. The reason people are attracted to the "undistorted ruler" trick is that it allows them to retain their pre-relativistic intuition that those undistorted rulers, or the undistorted Euclidean geometry, are somehow still "real", and that's precisely the intuition they need to let go of.

 

[PeterDonis]

Suppose, that I can freeze the gravitation waves while they are passing through the detector. So there will be some static changes in spacetime (or suppose, that frequency of the wave is much smaller than actually is). Can I still see the interference pattern change?

This doesn't make sense for two reasons.

First, spacetime doesn't "change". It is a 4-dimensional geometry that already contains the entire history of whatever is being modeled. All the "changes" that take place because of the gravitational wave, or anything else, are already contained in that 4-dimensional geometry. So it makes no sense to talk about "changes in spacetime".

Second, "freeze" would mean "look at some particular spacelike slice cut out of the spacetime". But in any such slice, the interference pattern can't change, because the change we see in the pattern is a change from one spacelike slice to another.

 

[Ibix]

There isn't a single thing called "the wavelength of the light". To construct an invariant related to the wavelength of the light, you would have to specify which observer is observing it and how.

Ah - I see what you are getting at. I need to measure the positions of the endpoints of the wave simultaneously, and in a dynamic spacetime there isn't a unique way to define simultaneity. So I need to specify time in terms of free-floating clocks or whatever.

 

[Ibix]

Suppose, that I can freeze the gravitation waves while they are passing through the detector. So there will be some static changes in spacetime (or suppose, that frequency of the wave is much smaller than actually is). Can I still see the interference pattern change ?

As Peter says, this doesn't really make sense. However you can definitely consider very low frequency gravitational waves. In principle a LIGO-like detector could detect an arbitrarily low frequency wave. However, waves from cosmological sources will usually have lower powers associated with lower frequency waves and there may well be low-frequency cutoffs in the signal processing, so it would not surprise me if the answer is no in practice.

 

[asca]

This might help:

I'd like to thank everybody for your attempts at giving an answer. I feel that A.T. provided the most convincing one.
Thank you all again

 

[Tomas Vencl]

……... However you can definitely consider very low frequency gravitational waves. In principle a LIGO-like detector could detect an arbitrarily low frequency wave. However, waves from cosmological sources will usually have lower powers associated with lower frequency waves and there may well be low-frequency cutoffs in the signal processing, so it would not surprise me if the answer is no in practice.

Yes, this was exactly point of the question. Thank you.
I know, that LIGO has some cutoffs in sensitivity at low and high frequencies, I did not know if this is caused by setting in principle or technical reasons (yes, the boundary is very weak).

 

[sandy stone]

First, spacetime doesn't "change". It is a 4-dimensional geometry that already contains the entire history of whatever is being modeled. All the "changes" that take place because of the gravitational wave, or anything else, are already contained in that 4-dimensional geometry. So it makes no sense to talk about "changes in spacetime".

So, do researchers treat the emission and detection of gravitational waves as a 4-D solution of the EFE's? Or is there another approach?

(Hopefully this is not too much of a thread hijack.)

 

[vanhees71]

Well, in theory the gravitational-wave signals of black-hole or neutron-star mergers (which are the ones which are observed by LIGO/VIRGO) are solved by solutions of the EFE's numerically (employing GR hydro or transport for the matter part). For details, see the nice web sites of my colleague Luciano Rezzolla:

https://relastro.uni-frankfurt.de/research/

 

[PeterDonis]

do researchers treat the emission and detection of gravitational waves as a 4-D solution of the EFE's?

Yes.

 

[Ibix]

do researchers treat the emission and detection of gravitational waves as a 4-D solution of the EFE's?

Yes. At a great distance when the waves are weak enough, you can treat them as a small perturbation to flat spacetime (or other analytically described spacetimes such as FLRW, I presume) and the maths is fairly tractable. Studying the emission is purely numerical work.

 

[asca]

Here I am again. Sorry If I'm bugging you all, but I have the impression that we get carried away by the math or by some fancy concepts or special cases. The question here is pretty simple, and, I must say, after reconsidering the answer which I thought was convincing, I got back to my initial doubt. Let me try to rephrase it:
General Relativity says the presence of mass creates a distortion of space time, in ANY point of space time. The first prove of this conclusion was provided by Eddington's experience. Light rays emerging form the two star behind the sun were convinced they were traveling in straight line, but near the sun, "straight line", is actually a curved line (if seen from some other reference frame which is less affected by this distortion), so those rays reached the Earth and "we" saw the "ghost" image of the two star. Very well, this tells us the light too is sensible to space time distortion, exactly what GR states. Now let's go back to a generic LIGO like experiment. For the moment let's forget about the source of gravitational wave, its frequency and so on. Light is running through the two arms of the interferometer, and for some reason space time in one arm gets distorted. Well, the light ray that is running through that arm will be affected by such change in the very same way the light ray in Eddington's experiment were affected, so form the light rays perspective the arm's length is not changed at all. Here we are not dealing with a signal the gives a message that space time has changed (that signal was the gravitational wave, and it already took the proper amount of time for the signal to get form the source to all the points of the interferometer), so we cannot say "it takes more (or less) time for the light ray to stretch than the time it takes for the arm's length to stretch", or we cannot say "a new light ray traveling the same path would not be stretched, of would be stretched differently". It is clear to me that "at any point in time" (whatever that means) the distortion can be different in any point of the two arms (like the temperature effect some of you guys mentioned in a previous example) , but any "portion" (again whatever that means) of light ray will be stretched the exact same amount, so overall no change of the interference pattern should emerge, UNLESS the gravitation wave effect that we detect is a gravitation effect of the mirrors ONLY (not the effect on space time), meaning the the mirrors (and only them) will may be feel a different force toward the source of the wave so their distances change accordingly to two different forces, so the interference pattern gets affected. I really cannot see how an experiment the is "immersed" into a stretched space time could detect such stretch.

Thank you again for reading all this.

A

 

[PeterDonis]

Light rays emerging form the two star behind the sun were convinced they were traveling in straight line, but near the sun, "straight line", is actually a curved line (if seen from some other reference frame which is less affected by this distortion)

No. Both light paths are straight lines--geodesics. But in a curved spacetime, geodesics that are initially parallel don't stay parallel. (Think of the meridians of longitude on the Earth--they are parallel at the equator but intersect at the poles. They are all geodesics--straight lines.) There is no "reference frame which is less affected by this distortion".

Also, you are confusing the path in spacetime taken by the light rays with the number of wavelengths of the light that fit along the path. That number changes due to the curvature of spacetime. For example, consider the following thought experiment: I place a light source on a rocket that uses its engines to maintain a position one Earth orbit diameter from Earth in a fixed direction in space, i.e., in the direction of some particular star. I pick a time when that star is overhead at midnight and have the light source emit laser light of a known wavelength which is detected on Earth. In principle I can measure the time it takes for the light to get from the source to the Earth, and the exact number of wavelengths of that light that make up the length of the path the light takes from the source to the Earth.

Then I wait six months, so that the Sun is now between the light source and the Earth. I do the same experiment again, but now the laser beam from the light source just grazes the limb of the Sun and is bent by the Sun and comes to Earth. Again I can measure the time it takes for the beam to get from the source to Earth, and the exact number of wavelengths of light that make up the length of the path. I will find that the time is longer than it was six months before, and the number of wavelengths is larger. The path got "stretched" because of the bending of the light by the Sun, but the light itself did not.

(Btw, although I described this as a thought experiment and the exact setup I described has not been realized, similar experiments with radar transmitters and reflectors on space probes have actually been done, and the time delay and length increase have been verified.)

the light ray that is running through that arm will be affected by such change in the very same way the light ray in Eddington's experiment were affected, so form the light rays perspective the arm's length is not changed at all.

No. First, there is no "from the light rays perspective". There is no "perspective" for light rays; you can't construct a reference frame in which a light ray is at rest. So there is no "arm's length" from the light ray's perspective; the concept doesn't make sense.

Second, the change in the arm length is a physical change in distance; it is tidal gravity. Detecting a change in tidal gravity by using light rays in an interferometer is perfectly possible, and that is all that LIGO is doing. For example, in principle I could dig a very long, narrow tunnel through the Earth and put a mirror at one end and a sensor at the other, and bounce a laser beam back and forth through the tunnel, and thereby detect the changing tidal distortion of the Earth due to the Moon along that axis over the course of a day, because that tidal distortion physically changes the shape of the Earth. LIGO is doing the same thing for a gravitational wave, which is just a propagating wave of changing tidal gravity.

 

[A.T.]

I really cannot see how an experiment the is "immersed" into a stretched space time could detect such stretch.

If the effect wasn't detectable in principle it would not be physics. As @PeterDonis suggests, you should try to understand how tidal gravity is related to space-time curvature.

 

[Ibix]

but any "portion" (again whatever that means) of light ray will be stretched the exact same amount,

I said that (Peter pointed out that it's a coordinate dependent description). I also said why it wasn't important, as did the video A.T. linked. The point is that (assuming you adopt coordinates where this makes sense) light exits the interferometer in much less time than the gravitational wave period - so "unstretched" light is continually fed into the system and it is this unstretched light that is compared.

 

[pervect]

Here I am again. Sorry If I'm bugging you all, but I have the impression that we get carried away by the math or by some fancy concepts or special cases.

To really understand gravitational waves, there isn't any substitute for the math. My impression of what's going on is that you are attempting to leverage your intuition about Euclidean geometries to more general geometries, and your intuition is leading you astray. Unfortunately, without the math, it's very hard to tell you exactly where your intuition is leading you wrong. We can try, but you dismiss the issues we raise as "not being significant". The fact that gravitational waves are detectable and has been detected, though, should be a "heads up" that something is "off" with your approach, even if you don't know what it is, yet.

Well, the light ray that is running through that arm will be affected by such change in the very same way the light ray in Eddington's experiment were affected, so form the light rays perspective the arm's length is not changed at all.

"Light rays" do not, unfortunately, have a perspective. You need something more / different than a single light ray to create or have a perspective. You apparently dismiss this as a "fine point" or "special case", but it's important to your argument, and I believe it underlies much of your confusion.

Without a shared notion of "perspective", it's difficult to communicate anything useful. The notions of "perspective" in differential geometry would be the most useful, but those are too hard to talk about, being a graduate level topic.

Really, the only thing I can say at this point is that you're stuck with some intuitions that are steering you astray. Realistically, I can only hope that you realize that something must be wrong, somewhere, because your predictions of what happens are not the same as that of the professional literature.

 

[Heikki Tuuri]

As a gravitational wave passes an observer, that observer will find spacetime distorted by the effects of strain. Distances between objects increase and decrease rhythmically as the wave passes, at a frequency equal to that of the wave. This occurs despite such free objects never being subjected to an unbalanced force.

https://en.wikipedia.org/wiki/Gravitational_wave

asca, the frequency of a gravitational wave in LIGO is around 300 Hz. The speed of sound in rock is roughly 5 km/s. During one cycle of the wave, sound only propagates 17 meters, which is a small fraction of the LIGO arm length of 4 km.

That is, the rock does not have time to adjust to the changed distances.

The light which is used to measure the distance, on the other hand, does adjust very quickly. We are able to measure the new distance with the light.

If the frequency of the gravitational wave would be very slow, then the stress in the rock would eventually bring the ends of the arm to the same distance as they were before the gravitational wave. Then you would no longer notice any changed spatial geometry.

A source of confusion may be that an arm of the LIGO itself is a "ruler". If that "ruler" would immediately adjust to the new geometry, so that it would no longer be under a stress, then we would not be able to observe the changes in the spatial metric.

 

[asca]

Once again, thank you all for the help you are providing. I want to ensure you the I am pretty sure I am missing something, as Pervect says, but I still didn't manage to see what.
If I go thorough all your replies, I still feel something is missing in finding the proper answer. For example, I see that Peter Donis says that I am wrong when I say the the path is curved in Eddington experiment, but then he says that it is a straight lines--geodesics (and he also uses as an example the Earth meridian), which is exactly what I said. However I think I find a common pattern in all your replies, which probably could lead to the point I am missing: it seems to me that you all say that the light wavelength does NOT get "stretched" while passing through a stretched portion of space-time. Ok, assuming this is true (I confess I do not see where Relativity states this fact but I'll try to double check), let's think about the following scenario : we setup a LIGO like detector, we run it, and it generates an interference pattern. We keep it running for months and the pattern does not change. Three months later we manage to put a mass somewhere so that one and only one of the two arms "pierces" a portion of a stretched space-time. Let's say that that arm is made up of three segments, the first one lies in the non-stretched space time, the middle one lies in the stretched space time, and the last one again in the non stretched. Now the interference pattern has changed its shape and it will stay like that until we remove the distorting mass. This change, according to what I guess you are all saying, is due to the fact that the light wavelength does NON get stretched, so if I put a detector in the middle segment, that detector should measure a shorter wavelength than the wavelength measured by two other identical detectors placed in the first and last segment of this arm. Is that what you guys are trying to tell me (please do not reply that if I put a detector I change the experiment, I really think this "quantum" effect would be negligible)?
Let me assure you I believe the signal was detected, I believe GR works, I am not one of those "flat Earth" guys , or "no moon landing" guy. I just do not manage to get around this "flaw" in my own reasoning, but if someone shows me where relativity says that wavelengths do not get affected by space-time distortion, I believe we are done. Thank you again.

 

[Heikki Tuuri]

Right, the wavelength of the laser in LIGO is constant.

It is the distance between the ends of the LIGO arm which changes.

The light which goes to the "stretched" arm does not know anything about the stretching. Its wavelength is not stretched. The light simply will think that the arm is now longer/shorter than it used to be.

Thus, the "stretching" of space is really that the distance between inertial masses (like the ends of the detector) changes. There is no "ether" which would stretch.

 

[Ibix]

I'm not sure the experiment you are describing really makes sense - you wouldn't use an interferometer to detect the presence of a static mass. You also seem to be talking about "stretched spacetime", which I suspect is the root cause of your problems.

Imagine using a radar set that emits discrete pulses of radio waves to measure the distance to a target. The radar emits a 1ns long pulse, which bounces off a target 150,000m away and returns 1ms later. Then a gravitational wave comes in. In the 1ms a radar pulse is in flight, the gravitational wave increases the distance to the target by 0.1% (a ridiculously large number, but I can't be bothered typing all the zeroes). The radar pulse gets stretched by the same fraction. So the echo returns 1.001ms later and the pulse duration is increased to 1.001ns.

But the gravitational wave is still coming in and, while the next pulse is in flight, again grows the distance by 0.1%. So the echo return time of this pulse is 1.002ms, while the pulse duration (again initially 1ns) is stretched to 1.001ns.

The gravitational wave is still coming in, and the distance grows by another 0.1%. The echo return time for the third pulse is 1.003ms and the pulse duration is 1.001ns.

Now the gravitational wave crest has passed, so the distance to the target starts to shrink. The next radar pulse then takes 1.002ms to return and has a pulse duration of 0.999ns. The next one takes 1.001ms with a pulse duration of 0.999ns, and the next one 1.000ms with a pulse duration of 0.999ns. Etcetera.

Do you see? The effect of the wave on the cavity length is cumulative because the cavity is a persistent object. But the light is moving through the cavity and isn't affected the same way.

Of course, we use an interferometer not a radar set. But this is because we only need to measure changes in flight time, not directly measure the flight time, and an interferometer can do this much more precisely than a radar set. I think the way to think of the interferometer in this context is that the light in each arm is being used as a clock to measure the return time in the other arm, while the radar set compares the flight time to an electronic clock. But it's basically the same method.

 

[A.T.]

it seems to me that you all say that the light wavelength does NOT get "stretched" while passing through a stretched...

What we say is that this doesn't really matter. It's the changing timing that matters. Watch the video again starting at 5:23. It addresses exactly your misconception at 5.42.

 

[PeterDonis]

That is, the rock does not have time to adjust to the changed distances.

The light which is used to measure the distance, on the other hand, does adjust very quickly. We are able to measure the new distance with the light.

That doesn't mean the light "adjusts" to the changed distance. It means the distance changed, and the light, which didn't change, tells us the distance changed.

The rock also tells us the distance changed, but in a different way: by the change in its internal stresses. But that's much, much harder to measure given the tiny changes involved. That's why Weber-style bar detectors for gravitational waves, which operate on the same principle--sensing the changes in internal stresses in a large solid object caused by GWs--never got to the point that LIGO has reached.

If the frequency of the gravitational wave would be very slow, then the stress in the rock would eventually bring the ends of the arm to the same distance as they were before the gravitational wave.

Not if the rock remains solid; then its length would change much less than the length between the sensor and end-of-arm mirrors in LIGO, which can move independently of each other. The atoms of the rock can't because they are bound by internal forces; so the effect of the GW shows up in the rock mostly as a change in internal stresses, rather than a change in externally measured length. There will be some small change in overall length, but again, much less than the length changes in the arms that LIGO measures.

Then you would no longer notice any changed spatial geometry.

Yes, you would, because the internal stresses in the rock will have changed. See above.

 

[PeterDonis]

it seems to me that you all say that the light wavelength does NOT get "stretched" while passing through a stretched portion of space-time.

That's not quite what I said. See my exchange with @Ibix upthread, where he ended up saying (correctly) this:

I need to measure the positions of the endpoints of the wave simultaneously, and in a dynamic spacetime there isn't a unique way to define simultaneity. So I need to specify time in terms of free-floating clocks or whatever.

In other words, as I said earlier in that subthread, there is no invariant "wavelength" of the light, so thinking in terms of whether or not this thing that isn't an invariant gets "stretched" or not is going down the wrong path.

The invariant is the presence of interference in the detector; that tells you that the physical distance between the arm ends changed. And that, as I said earlier, is the expected result of a change in tidal gravity.

Three months later we manage to put a mass somewhere so that one and only one of the two arms "pierces" a portion of a stretched space-time.

You can't. The "stretching" (not really a good term) can't be isolated like this.

 

[PeterDonis]

you wouldn't use an interferometer to detect the presence of a static mass

Actually, you could. For example, you could put it in a circular orbit around a planet like the Earth with one arm oriented radially and the other oriented tangentially. Then tidal distortion due to the Earth would make the arm lengths slightly different and a static interference pattern would show up in the detector.

 

[asca]

Ok, we probably made it. I say the latest example of RADAR pulse posted by IBIX really helps me in clarifying what was stated in the video mentioned by AT, which I liked at the beginning, but later on I was having trouble in picturing the scenario described starting minute 5.23. It also seems to me we all agree the wavelength get affected, although some of your posts seem to deny that. However the radar pulse example clears out all my troubles, Thank you all again for everything.

 

[Ibix]

It also seems to me we all agree the wavelength get affected, although some of your posts seem to deny that.

Not quite. There are two issues - one is that defining "wavelength" in non-static spacetime isn't trivial. The other is that the wavelength emitted by the laser doesn't change, and doesn't stretch by 1% just because it entered an interferometer arm that's been stretched by 1% (the idea that it does is a fairly common misconception, I think).

 

[A.T.]

... It also seems to me we all agree the wavelength get affected, although some of your posts seem to deny that. ...

As the video explains in simple terms: A wave that is present in the space while that space is being stretched will also be stretched with that space. But a wave that enters an already stretched space, will not be stretched upon entry, but will merely need longer for the passage and that's what causes the interference pattern shift.

 

[Ibix]

But a wave that enters an already stretched space,

An opinion: I'm wondering if we ought to avoid talking about "stretched space", since I think that's how we end up with the idea that things ought to stretch when they enter it. That's why I've been trying to talk always about the arm length in this thread.

 

[PeterDonis]

I'm wondering if we ought to avoid talking about "stretched space"

I think that's a good idea since it's a coordinate-dependent concept.

 

[pervect]

An opinion: I'm wondering if we ought to avoid talking about "stretched space", since I think that's how we end up with the idea that things ought to stretch when they enter it. That's why I've been trying to talk always about the arm length in this thread.

Stretched space is such a common idea, though, it's difficult to avoid. Even if it does tend to cause confusion.

The ideal way of talking about stretched space is to talk about coordinates as labels, and then introduce the metric tensor. Then we can identify the stretching of space with the metric tensor. But I don't think that's a B-level approach, it's I-level at best.

So we are left with saying that space stretches, but (physical) rulers don't. So "stretched space", whatever it may be, isn't something that's measured with physical rulers.

That may not be a good explanation of what stretched space is (the good explanations that I'm aware of are not B-level), but at least it tells us what it isn't.

The other simple point that I think needs to be made (and has been made, to some extent, but is mostly being ignored by the OP) is that it is important to consider the round trip time when light is being used as a ruler to measure distances.

The OP, though, wants to ignore the round-trip requirement, and seems to have the idea that we can talk about length from the perspective of a light beam. It seems to be leading them to incorrect conclusions about the round-trip time, so I assume they are doing something wrong. Exactly what they are doing wrong isn't entirely clear in detail.

We can certainly say that the notion that light has a wavelength (or frequency) that's independent of the observer is wrong. The OP seems to be assuming otherwise (as near as I can tell), and I'm guessing this may be the source of some of their confusion. But I could be wrong about the source of their confusion. I am sure they must be confused about something, though, because they're getting the wrong answer about the round-trip travel time.

The correct notion of wavelength (and frequencey) is that they are not the property of light, but a property of light as measured by some specific observer.

 

[PeterDonis]

The correct notion of wavelength (and frequencey) is that they are not the property of light, but a property of light as measured by some specific observer.

Strictly speaking, frequency is what a specific observer measures about a specific light ray. Wavelength is either (1) deduced from the frequency by assuming that the speed of light is $c$, or (2) deduced from measurements by multiple observers who adopt some common definition of simultaneity in order to compare their measurements "at the same time" (this is what @Ibix described in an earlier post). #1 is not a direct measurement of wavelength since it only occurs at one event in spacetime; #2 is not an invariant measurement of wavelength because it depends on the definition of simultaneity that is adopted.

 

[pervect]

Strictly speaking, frequency is what a specific observer measures about a specific light ray. Wavelength is either (1) deduced from the frequency by assuming that the speed of light is $c$, or (2) deduced from measurements by multiple observers who adopt some common definition of simultaneity in order to compare their measurements "at the same time" (this is what @Ibix described in an earlier post). #1 is not a direct measurement of wavelength since it only occurs at one event in spacetime; #2 is not an invariant measurement of wavelength because it depends on the definition of simultaneity that is adopted.

There's a third, very old, technique, with many variations. Basically, one does not have a single light beam going in one direction, but rather a pair of light beams moving in opposite directions, which generate interference fringes.

Typically, light on the return path is reflected from a mirror.

This technique requires a round trip though, so it's not really any different than the radar method in that requirement.

The interference fringes require light moving in both directions to exist, they don't exist for light moving only in one direction. One typically considers a frame where the interference fringes are stationary, and both the light source and the reflecting mirror are stationary as well. Then the distance between the fringes is said to be one wavelength in the specified frame where everything (the source, the mirror, the fringes) is stationary.

 

[Tomas Vencl]

Once again, thank you all for the help you are providing. I want to ensure you the I am pretty sure I am missing something, …..

Maybe can also help next idea (not sure if this is really the cause).
Distances in GR are physically measured by 2 ways.
By rulers (how many of them I can put along some space interval), and radiolocation (by time interval when I receive mirrored signal).
And in GR those two are not equal. The mirrored signal is also affected by time dilatation/contraction along its way. So even when I can imagine the number of rulers equal to number of wavelengths, there is still difference causing interference pattern shift.

 

[Ibix]

Stretched space is such a common idea, though, it's difficult to avoid. Even if it does tend to cause confusion.

I think a variant on Feynman's sticky beads might help. Instead of sticky beads I want frictionless beads, able to slide freely along a rod. You can then explain that the beads free-fall apart, and if the rod were sliced into a stack of thin discs then the discs would separate too. But in the actual rod, internal forces prevent that and the beads move relative to the rod. A LIGO arm isn't (conceptually) radically different from this - we've just mounted a beam splitter on one bead and a mirror on the other. The rest of the differences stem from engineering concerns driven by a desire to reduce noise, basically.

For the point about "no stretched space", imagine a really low frequency gravitational wave, with a period of minutes or hours. The rate of change of separation of the beads is undetectable on a timescale of seconds, but the cumulative change is detectable. The takeaway point would be that while the wave is coming through, there is no experiment other than "wait and see what happens" that will differentiate between the cases of the beads being distance $d+\delta$ apart because the gravitational wave has moved them $\delta$, and them having been built $d+\delta$ apart and always being that distance apart.

 

[pervect]

I think a variant on Feynman's sticky beads might help. Instead of sticky beads I want frictionless beads, able to slide freely along a rod. You can then explain that the beads free-fall apart, and if the rod were sliced into a stack of thin discs then the discs would separate too. But in the actual rod, internal forces prevent that and the beads move relative to the rod. A LIGO arm isn't (conceptually) radically different from this - we've just mounted a beam splitter on one bead and a mirror on the other. The rest of the differences stem from engineering concerns driven by a desire to reduce noise, basically.

This isn't too much different from my preference, which would basically to use Fermi-normal coordinates around a single point rather than the "stretched space" idea.

This mathematics of this approach are unwieldy, so it's not my preference for actual calculations, just my preference for visualizing what's going on.

The approach leads to the idea that gravitatioanl wave consists of tidal forces, which causes an array of test masses to actually move, as in the image from Wikipedia below. For small, planar cross sections, we can even get away with imagining that the space in which the test masses are moving is Euclidean.

I think it's less confusing than the "expanding space" idea, but it doesn't get a lot of discussion, except for the occasional diagram like the one shown above. WIthout more literature references to back it up, I'm a bit cautious about over-promoting the idea.

The idea does have some limitations that the expanding space idea does not. One of them is the issue of size. If we consider only a plane, such as in the diagram, the limitations are very modest. It's not until the diagram becomes so big that the relative velocities between the test masses start to become relativistic that we start to see the idea break down.

In three dimensions, the size limits are more severe. The basic idea of a unchanging, Euclidean space in which particles move due to "forces" breaks down when we consider a 3-d volume that's an appreciable fraction of one wavelength of the gravitational wave.

So if we had a 1khz gravitational wave , a tenth of a wavelength would be 30 kilometers, and we'd start to see some detectable issues in a volume of that size with careful enough measurements of distances between particles.

To understand most of the LIGO results, though, we don't need 3d, just 2d.

In the end, though, nothing can really replace "doing the math". But that takes more mathematics than is possible at the B level.

 

[PeterDonis]

To understand most of the LIGO results, though, we don't need 3d, just 2d.

I'm not aware of any LIGO results that cannot be understood this way.

In fact, to understand gravitational waves in general using this visualization method, 2d is sufficient, because gravitational waves are purely transverse, so all of the changing tidal effects are orthogonal to the direction of propagation.