# Use time-warp to detect gravity waves?

serengeti
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Topic 192242

Since there are so many things that can disrupt the lasers like earthquakes, why not use time to detect them? If you had a clock on the moon synchronized with a clock on the earth, then the gravity wave came along and hit the moon first, the clocks would go out of synch until it reached the earth, then they would end up back in synch after it passed. Nothing else could cause it to happen. I just haven't figured out how you would know they were out of synch if the gravity wave is traveling at the speed of light. You couldn't get the information from the clock on the moon fast enough to know they were out of synch.

Maybe the wouldn't stay in synch anyway, because of the motion of the earth and the moon, but you could adjust for that in your calculation.

Gerry Rough
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### Use time-warp to detect gravity waves?

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Since there are so many things that can disrupt the lasers like earthquakes, why not use time to detect them? If you had a clock on the moon synchronized with a clock on the earth, then the gravity wave came along and hit the moon first, the clocks would go out of synch until it reached the earth, then they would end up back in synch after it passed. Nothing else could cause it to happen. I just haven't figured out how you would know they were out of synch if the gravity wave is traveling at the speed of light. You couldn't get the information from the clock on the moon fast enough to know they were out of synch.

Maybe the wouldn't stay in synch anyway, because of the motion of the earth and the moon, but you could adjust for that in your calculation.

Actually, the gravity wave would not have the same effect on both equally because of the mass difference, so the time thing would not have to come into play. I would think that finding a gravity wave here, and then measuring the effect on the moon would suffice. Measure the difference in effect and you might get there. Sounds good to me, but someone who knows this issue better could give some better insight.

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serengeti
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### If the gravity wave effected

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If the gravity wave effected the moon differently than the earth because of their mass difference, then the gravity wave would be even easier to detect by the clocks going out of synch, because they would never go back in synch after the wave passed. You could calculate how much they would be out of synch and see if it happens.

barkster
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### At the risk of hi-jacking the

At the risk of hi-jacking the thread.... This made me think of two questions.

1. I understand why having the LIGO arms at 90 degrees is ideal. But is it necessary? Could two arms that are at a more acute angle, but long enough, still act as an interferometer and provide the needed sensitivity? (i.e. two beams pointed at/reflected from opposite edges of the moon?)

2. Could any of the light speed "slowing" techniques be applied to LIGO?

Glenn

"No, I'm not a scientist... but I did stay at a Holiday Inn Express."

Mike Hewson
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### RE: 1. I understand why

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1. I understand why having the LIGO arms at 90 degrees is ideal. But is it necessary? Could two arms that are at a more acute angle, but long enough, still act as an interferometer and provide the needed sensitivity? (i.e. two beams pointed at/reflected from opposite edges of the moon?)

90 degrees isn't required. The space based LISA will have 60 degrees between arms as it is an equilateral triangle. I think 90 degrees makes it simpler though and minimises any preferential behaviour to cosmic origin/direction.

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2. Could any of the light speed "slowing" techniques be applied to LIGO?

I suppose you could but why? We're effectively doing that anyway with examining the differing phase of the returned beams from the two arms. You would like it to assist the precision of that phase comparison. I think that's optimised with some 'chopping'/heterodyning of the laser at the moment ( complex, arcane, not sure... ).

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

barkster
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### Thanks, Mike. RE: 90

Message 58387 in response to message 58386

Thanks, Mike.

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90 degrees isn't required. The space based LISA will have 60 degrees between arms as it is an equilateral triangle. I think 90 degrees makes it simpler though and minimises any preferential behaviour to cosmic origin/direction.

I was just pondering (with no significant level of intelligence) if greatly increasing the length of the arms would make up for highly acute angles, or make any real difference at all.

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I suppose you could but why? We're effectively doing that anyway with examining the differing phase of the returned beams from the two arms. You would like it to assist the precision of that phase comparison. I think that's optimised with some 'chopping'/heterodyning of the laser at the moment ( complex, arcane, not sure... ).

Complex for sure.... Was also just pondering if any of the light slowing (or faster-ing) techniques would in any way allow the virtual "lengthing" of the arms without too much adverse effect.

Eh... chalk it up as another Glenlivet moment.

Glenn

"No, I'm not a scientist... but I did stay at a Holiday Inn Express."

Mike Hewson
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### RE: I was just pondering

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I was just pondering (with no significant level of intelligence) if greatly increasing the length of the arms would make up for highly acute angles, or make any real difference at all.

For sure longer is better. That gives longer distances over which the phase of the photons can diverge from the ones in the other arm, if a gravity wave is scooting by.

The idea of the resonant cavity section of the arms is to 'fold' the path many times. Resonant simply means bounce it back and forth, back and forth, back and forth etc ...... like playing ping-pong with the other half table propped up vertically. I think the average number of circuits per photon for the LIGOs is several hundred. This way a 4km beam tube can yield a path length of hundreds of km. It is still only re-sampling 4km though. The photons accumulate their phase, which is then compared back at the corner station.

One issue is whether the gravity wave is going to change 'sign' while the photons are traversing the interferometer. If that were so then for some parts of the trip the phase accumulates in one fashion, and conversely for other parts. Hence that reduces the total. So if the arm is too long with respect to the length of the gravity wave, we lose sensitivity. A small boat on a big wave responds really well to wave heights, whereas a big boat on small ripples is fairly undisturbed.

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Complex for sure.... Was also just pondering if any of the light slowing (or faster-ing) techniques would in any way allow the virtual "lengthing" of the arms without too much adverse effect.

Aaahhh .... but what doesn't seem as well advertised about the light-speed breakers is that they haven't done it in a vacuum. They are really only studying material properties, not light per se.

Of all materials, vacuum is the emptiest and light travels the fastest there. Anything denser always involves a slower propagation speed compared to vacuum. So when you hear of light speed being exceeded in substance X: it is comparing to the usual case of X, not to vacuum. Light propagation in a material can be viewed as photon capture by the electrons and then re-radiation - so the energy gets passed from atom to atom like a bucket chain fire-brigade. A photon might go at top speed between captures, but is then delayed by the handling so to speak. It goes in an indirect zig-zag path, and there's effects related to interference between it and other photons.

Now what often happens is the phenomenon of dispersion, which means that the speed of light at a given frequency now depends on that frequency. Speed depends on color, and we can see this everyday by looking at or through jewellery stones. Ahhh ..... diamonds! So one can fire a bunch of photons, of various colors/frequencies, up a glass fibre - and then fiddle actively with the fibre properties in some way that makes the exact manner of dispersion vary. Imagine a horse race, where we could get the lead runners to slow down and await the slower part of the field catch up. Or the reverse. This can thus manipulate the average speed of the runners in the race either way.

[aside] There is an interesting effect that can occur when a particle exceeds the speed of light for that substance. A spray of photons is produced in a conical fashion forwards from the line of the particle's travel. This is called Cherenkov radiation and is a very useful effect in cosmic ray studies and those big tanks that count and analyse neutrinos.[/aside]

It's often not highlighted that when we say the speed of light is constant, we are referring to the phase velocity. You could define this as the speed of a single unmolested photon in free space. Group velocity is a figure derived from statistics on photon groups with a range of frequencies. It is that speed which is being made slower and faster.

To involve those speed varying effects we'd have to go off vacuum. That then introduces some bucket of stuff to manage and it's material properties to account for and control. Vacuum can be made clean, empty and simple.

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Eh... chalk it up as another Glenlivet moment.

Hey, nice stuff!! May you have many more. :-)

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

barkster
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### Photons, cavities,

Photons, cavities, dispersion, radiation, big boats, small boats.... AHHH!

I'm about to have another "moment".

Greatly appreaciate the explanation, Mike. So... longer is better (she said), to a point, for sensitivity at any given frequency. And vacuum is cheap/easy. Knew the beam was bouncing, but didn't know it was that many times.

1. If I understand this slightly better, now... the physical distance the beam travels is what the measurement from each arm depends on (for phase sampling)... not the length of time the beam takes to travel that distance.
2. Hypothetically speaking... since GWs are ripples in space/time... with a "faster-ized" beam of light in substance X, would any measured deltas (in distance, phase, etc?) between that beam and a slower parallel beam also in X be amplified (and measurable) as the wave passed through it?

Glenn

"No, I'm not a scientist... but I did stay at a Holiday Inn Express."

Mike Hewson
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### RE: I'm about to have

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I'm about to have another "moment".

May I suggest a Glenlivet to match then.... :-)

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Greatly appreaciate the explanation, Mike. So... longer is better (she said), to a point, for sensitivity at any given frequency. And vacuum is cheap/easy. Knew the beam was bouncing, but didn't know it was that many times.

Yup it's a cluey design.

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1. If I understand this slightly better, now... the physical distance the beam travels is what the measurement from each arm depends on (for phase sampling)... not the length of time the beam takes to travel that distance.

Both time and distance really. This is the hard bit to wrap one's head around, and I may well have this dead wrong. Beware.

As far as I know the full General Relativity treatment of the interferometers depends on the mirrors/masses being 'free'. Well, free-ish as we are certainly affecting them in certain ways, but the dangling of the mirrors is important here. If we say the masses are in free-fall, or if you like not affected by any other force but gravity, then certain things follow:

- we can view the masses as being of fixed position, but with spacetime between them warping etc. So the masses don't 'move' but the intervening universe 'flexes'. Hmmmmm you say, and so do I. Nonetheless this 'transverse traceless gauge' viewpoint as it is called ( and no, I don't know why it is called that ) when used to predict is quite adequate to predict results. Go figure... :-)

- alternatively you can say, I as an observer am not 'free', but I'm standing nearby looking at this contraption. So what should I see? I will note a phase difference between the light in the two arms. The photons entered the interferometer in phase, because they came from the same laser etc - we set it up that way and split the photon group, then flicked them up different arms. Now that they've returned to the corner station to compare, they now differ to the extent I can sense an interference between the two beams. This is deemed a 'fringe shift' for historical reasons but is actually a variation in light intensity at a photodetector of some sort. So I can attribute this phase disagreement to a length change ( shorter/longer ) in one arm compared to the other, or I can say that time is altered ( faster/slower ), or even blame a mix of the two effects. We never can actually measure phase as an absolute number, the best is to remark upon differences between cases. That's a quantum mechanical thing with photons and stuff.

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2. Hypothetically speaking... since GWs are ripples in space/time... with a "faster-ized" beam of light in substance X, would any measured deltas (in distance, phase, etc?) between that beam and a slower parallel beam also in X be amplified (and measurable) as the wave passed through it?

I'm not sure, but I don't see why not. The next question is whether there is any advantage to be gained should that be true. My gut feeling is that with this new field of endeavour the likelihood, at present, is to get it working in a basic sense. With experience and feedback/wisdom obtained that may come.

I seek forgiveness in advance if I've blown this explanation. :-)

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Lt. Cmdr. Daze
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### Hi Mike, Thanks for all

Hi Mike,

Thanks for all your explanations. It makes a lot more sense to me (well, I think so :) )

Do you know how much phase shift could be expected? If I recall well, the searched signal is smaller than the noise level. So, it must be quite small...

But if the phase shift is small, is it "easier" to detect the shift at higher frequencies of the laser beam (I'm thinking of a kind of strain analogy)?

Cheers,
Bert

Somnio ergo sum