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Author Topic: How does the "Laser Interferometer Gravitational wave Observatory" (LIGO) work?  (Read 11669 times)

Offline teragram

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As a complete idiot, I could do with some help:-

For some years now, the project known as LIGO has been in operation in the U.S.A.
This is Laser Interferometer Gravitational wave Observatory. In simple terms this consists of two tubes, a few centimetres in diameter and (I think) 4Kilometres long, arranged in an L shape. A laser beam is split into two, each traversing a tube from the corner to a mirror in the remote end many times, before reaching a phase detector at the corner.
The whole arrangement is set up so that each returning beam returns to the origin exactly in phase with its source.
The theory is that a passing gravitational wave (resulting from a distant supernova for instance) will alter the length of the tubes, thereby changing the transit path of the laser, with the result that the returning beam will be out of phase, this being detected as proof of the gravitational wave event.

I think there are three of these observatories in operation in different parts of the world.
As one with a poor understanding of relativity, my thoughts are as follows:-
I understand that gravity distorts space. This means to me that the gravitational wave changes the length of a tube by distorting the space which it occupies. If this is true, the distorting of space will result in the laser beam being red - or blue - shifted, with no change in its phase on return to the phase detector, i.e. nothing to be detected. I hope my description makes sense.
 
In simple terms, it seems similar to measuring the change in length of a steel bar under thermal expansion with a steel rule that suffers the same expansion. Perhaps someone could tell me where I am going wrong, or is this why no gravitational waves have yet been observed? 
« Last Edit: 15/04/2009 17:41:43 by chris »


 

Offline Vern

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If the reflected light is red or blue shifted this will result in a phase shift. The in-phase condition means that the peaks and troughs of the waves happen at the same instant. A red shift would result in the peaks and troughs being more distant than before, both in time and in space. This should show up as a phase shift and so should be observable IMHO.
 

Offline teragram

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A red shift would result in the peaks and troughs being more distant than before, both in time and in space. This should show up as a phase shift and so should be observable IMHO.

Vern, thanks for your prompt reply. However my simple brain imagines the tube (therefore the distance between the reflecting mirror and the phase detector) changing by the same amount that the beam length changes. In other words, the phase detector will move by the same amount that the phase has shifted, cancelling the effect.

 

Offline Vern

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Okay; I get your meaning. I don't see a flaw in your reasoning, given the assumptions you propose. I suspect the designers of the gravity wave detectors assumed that light would not follow the distortions in tube length.
 

Offline Soul Surfer

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The device is not connected to the tube.  It is like a seismometer decoupled as effectively as possible from any structure on the earth also the device effectively measures the difference in length between two axes at right angles to each other.  Gravitational waves cause a squashing in one direction and a simultaneous stretching in a direction at right angles so the change is theoretically detectable.  however the expected change is a good deal less than the size of a proton over the length used and so requires very extreme sensitivity and the reduction and cancellation of all noise sources.
 

Offline LeeE

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I too have problems understanding how these schemes are supposed to work, and teragram's example of measuring the expansion a steel rod using a steel rule that expands with the rod is a good illustration.

It seems to me that the effect of a gravity wave passing through a region of spacetime will be to change the reference frame of that region, but within that reference frame everything will remain consistent.  Where the gravity wave is travelling against or obliquely to the direction of the light it may lengthen or shorten the wavelength as it passes, but once it has passed the wavelength will return to 'normal'.  For light that is travelling in exactly the same direction as the gravity wave, both the light and the gravity wave will arrive at the detector simultaneously, and the detector will be effected just as much as the light.
 

Offline Bored chemist

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They have two arms at right angles. They bounce light up and down the 2 arms then they compare the phase of the returning light. Unless the garvity wave affects the 2 arms in exactly the same way then the 2 beams will be streched or shrunk to different extents and it is this difference they measure.
 

Offline LeeE

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Yeah, I know the basic design of these systems, but doesn't the precept that they're based on still require that just one thing is affected while the other is not?  i.e. either the light or the space that it travels through can be affected and show a change, but if both are affected then both will be consistent.

Ok, let's imagine that the apparatus is aligned so that one arm points directly towards an approaching gravity wave while the other is parallel to it.  The gravity wave will therefore reach the end of the arm pointing towards it, and then have to travel down that arm, before it reaches the other arm and the detector.  However, no indication of the gravity wave can be detected in the arm that the wave is travelling down until the wave reaches the detector at the junction of the arms, because the effects of the gravity wave cannot travel faster than the wave itself.

The effect of the wave is transient too, while it will change the shape of space over it's wavelength, the space will return to its original shape once the wave has passed, and within the region of the wave itself, everything should still seem to be consistent anyway.

It seems to me that only a distant observer could actually see the effects of gravity waves, precisely because they're in a different frame of reference; the instrument and its detector though, by definition, must be in the affected frame of reference.

Heh  :)  discussing this issue is something I have no confidence in at all.  People have clearly been given lots of money to perform these experiments, so I'm sure the experiments must have a solid basis, even if I just can't quite see them myself.
 

Offline teragram

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LeeE and Vern understand my predicament. I too assume that better brains are in charge. None of the explanations so far deal with my original problem. I understand that two legs are used, each with different halves of the same laser beam, with a Michelson - Morley interferometer summing the two halves, searching for a phase change. I understand too that each leg will be deformed differently by a gravitational wave.
Perhaps I should re-state my problem in different terms:-
Assume for clarity that the wave arrives exactly parallel to one leg and perpendicular to the other.
The gravitational wave changes the length of one leg.
This results in the distance between reflecting mirrors in that leg changing.
This results in the laser beam in that leg traversing a different distance, with the returning beam arriving at the detector with a phase change, and the detector responding.
This is all common sense, but to me the whole thing relies on the laser beam itself remaining completely unaffected by the gravitational wave.
As stated previously, my (flawed, surely) reasoning suggests that the laser beam will be red/blue shifted, by I think the same amount that its length (due to change in distance between mirrors) changes, thus cancelling the effect of any phase shift.
The basic question seems to be, is the laser beam affected by the gravitational wave?
My apologies for being so pedantic.


 

Offline yor_on

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Interesting :)
A gravitational wave? described as a 'stretching' in spacetime. and all that is, 'stretches' too. But won't that light still register as being 'shifted' no matter the 'streching' of the matter consisting of that 'system/detector'. Light is light and matter matter not :)
Or?
 

Offline Vern

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Quote from: teragram
The basic question seems to be, is the laser beam affected by the gravitational wave?
My apologies for being so pedantic.
I understood your question; sorry for the incomplete answer. I am inclined to the notion that the light will be affected by the gravitational wave exactly as you suggest and so we should expect a null result.

Professor Cahill of Flinders university proposes that the gravitational wave detector might show positive results if it were filled with a gas rather than the vacuum.
« Last Edit: 15/04/2009 15:03:29 by Vern »
 

Offline latebind

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Offline Bored chemist

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I think I understand what the problem is, I'm just not sure I know the answer.
Imagine I set up an ordinary boring Michelson interferometer like this one.
http://en.wikipedia.org/wiki/Michelson_interferometer
Then I put a couple of pipes round the two beams.
Then, just to be awkward, I fill one of the pipes with a bit more air- which slows the waves down, but, at the same time, I squash that pipe a bit so the path length is slightly smaller.
If I'm careful, I can make the two effects cancel out.

Now imagine that a gravity wave passes by- it shrinks one of the pipes (it streches the other but that's not the point) but, at the same time, it also shrinks the light.
There's no aparent change of phase- so there's nothing to detect.

Since they spent a lot of $$$ building this I presume that, unlike everything else, the light from the laser is unaffected by the gravity wave and so it does not shrink, but the pipe does- that's how it can detect the gravity wave. Either that or they have screwed up big time and are hoping that
1 nobody notices and/or
2 they can sell it to the dept of homeland security so that "The DoHS will be the first to know if a terorist detonates a gravity-wave bomb.". for which sort of nonsense, they will, it seems, pay an indefinitely large amount of money.
 

Offline yor_on

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But the light isn't measured against what length that pipe has, is it? There will just be a 'detector' reacting on the changes of the lightwave. So even though both effects are created by the same 'gravity wave' the detection of what possible 'redshift' there might be shouldn't have anything to do with the materials that light pass through, well, as I see it?
 

Offline yor_on

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You made me very confused here :)
This is the best description I found of how Ligo is thought to work.
http://arxiv.org/pdf/gr-qc/0308090
 

Offline teragram

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Thanks for the links, BC and yor_on, I shall read them and try to understand.
 

Offline yor_on

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Thinking of it again, how is this 'gravity wave' thought to act?
If it is just a 'ripple' in spacetimes fabric :) will it have a depth?
Will it act as a 'vertical plane' passing the interferometer by if you see how I mean?
Or will it have a 'horizontal' saturation with a peak?

Can gravity have that?
If I think of it as a vibration propagating in tightly woven 'strings' there seems to be that possibility, but then I think we should noticed it macroscopically already?
 

Offline Vern

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Thinking of it again, how is this 'gravity wave' thought to act?
If it is just a 'ripple' in spacetimes fabric :) will it have a depth?
Will it act as a 'vertical plane' passing the interferometer by if you see how I mean?
Or will it have a 'horizontal' saturation with a peak?

Can gravity have that?
If I think of it as a vibration propagating in tightly woven 'strings' there seems to be that possibility, but then I think we should noticed it macroscopically already?

I guess after we have exhausted conventional theory and still don't have a satisfactory conclusion, we can speculate about the cause of gravity. My speculation is that it results from the quantum nature of electromagnetic fields. A quantum of electric and magnetic charge propagates through space as two saturated points of charge with fields extending outward diminishing in amplitude away from the saturated points. Fields of all photons contribute to the saturation amplitude of all other photons. This contribution causes the points to reach saturation at an offset toward increasing field strength.

So there is my own speculation about gravity. :)
 

Offline yor_on

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So are you saying that there is a saturation with a 'peak' somewhere in it?
 

Offline Vern

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So are you saying that there is a saturation with a 'peak' somewhere in it?
My speculation is that electromagnetic quanta exist as saturated points of electric and magnetic amplitude. These points move through space surrounded by fields of diminishing electric and magnetic amplitude. The dynamics of the surrounding fields is a sinusoidal action of increasing and decreasing field strength. So if you could measure the action of a passing photon, you would see one complete cycle of a sine wave as it increased to saturation in one polarity, then decreased through zero, then increased to saturation in the other polarity.

I have a little computer program that makes the schematic and puts it in motion, I'll try to get a screen shot of it.

« Last Edit: 20/04/2009 13:51:19 by Vern »
 

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