LIGO Gravity Waves: Questions and Answers

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LIGO Gravity Waves: Questions and Answers

Postby khakipuce » Fri Feb 12, 2016 11:40 am UTC

I've merged the two LIGO threads, and further questions about it should go here. Apologies if it makes any of the responses unclear as to what they're replying to. - gmalivuk

Yesterday's announcement on the detection of gravity waves had a lot of information, age, distance, masses of the two black holes that merged, etc. But I am struggling to see how, given the available data, this could be solved. How did they determine that it was two objects of mass X and Y at 1.3 billion light years and not two objects of mass X/2 and Y/2 at 0.65 billion light years (OK, I know that is probably an incorrect assumption but you get the idea).

I know they have modelled various possible interactions but I am looking for a bit more than "because of modelling". Can anyone shed a bit more light on this?

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Re: LIGO Gravity Waves; how did they determine distance

Postby Xenomortis » Fri Feb 12, 2016 11:49 am UTC

I suspect different masses would have resulted in different frequencies.
I would guess the distance was calculated based on the amplitude of the signal, and some trickery for redshift correction.
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Re: LIGO Gravity Waves; how did they determine distance

Postby Flumble » Fri Feb 12, 2016 12:14 pm UTC

According to this SE answer, it is exactly what you assumed: "because of modelling". Run some simulations (using known characteristics of the black holes*), include redshift, compare to the observation, retrieve confidence interval, done. :P

*perhaps distance is one of those characteristics, so I don't know if you're going in circles with this

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Re: LIGO Gravity Waves; how did they determine distance

Postby doogly » Fri Feb 12, 2016 1:01 pm UTC

And it's important to note that there are many different modes being detected, in two detectors. There's a lot less degeneracy in what the sort could be for a gravitational wave like than for something like a constant m_1 m_2 / r^2, where you could really trivially shuffle some factors around.
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Re: LIGO Gravity Waves; how did they determine distance

Postby khakipuce » Fri Feb 12, 2016 1:18 pm UTC

Xenomortis wrote:I suspect different masses would have resulted in different frequencies.
I would guess the distance was calculated based on the amplitude of the signal, and some trickery for redshift correction.


It seems you have hit the nail on the head there (presumably causing your own gravity wave) - the link that @Flumble provided suggests that frequency (factored for red shift) allows mass to be determined and from that amplitude.

Thanks for all the replies.

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Re: LIGO Gravity Waves; how did they determine distance

Postby tomandlu » Fri Feb 12, 2016 1:34 pm UTC

My (embarrasing) question - what makes gravity waves special? If this is a wave in the medium (space-time?) that transmits gravity, then isn't all gravity a wave in this medium?
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Re: LIGO Gravity Waves; how did they determine distance

Postby doogly » Fri Feb 12, 2016 1:45 pm UTC

All gravity is a curving of spacetime. Gravity waves transmit energy though, whereas the earth doesn't lose any energy when it sends out its GM/r^2 at you.
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Re: LIGO Gravity Waves; how did they determine distance

Postby speising » Fri Feb 12, 2016 1:58 pm UTC

this event was pretty far away. what i'd like to know is how it would have felt from a closer distance, or, lets say, what is the minimum safe distance? would those waves rip our solar system to shreds if the collision happened in our galaxy?

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Re: LIGO Gravity Waves; how did they determine distance

Postby Izawwlgood » Fri Feb 12, 2016 2:07 pm UTC

doogly wrote:All gravity is a curving of spacetime. Gravity waves transmit energy though, whereas the earth doesn't lose any energy when it sends out its GM/r^2 at you.

The energy imparted in gravity waves - how is it lost? Does it diminish over distance?

What is the space-time 'rigidity'? Is it constant throughout the universe?
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Re: LIGO Gravity Waves; how did they determine distance

Postby sevenperforce » Fri Feb 12, 2016 2:12 pm UTC

LIGO has two different detectors hundreds of miles apart. Each detector received the same signal, but at slightly different times and from slightly different directions, so triangulation could be used to find the origin.

I don't know whether they immediately pointed Hubble in that direction to try and see if they could spot anything. I would have.

My back-of-the-envelope calculation suggests that the gravitational wave was strong enough to flex the Earth by about five times the width of a DNA strand. I doubt that gravitational waves would be strong enough to cause damage outside the general catastrophic destruction radius of whatever actually was creating them.

Izawwlgood wrote:
doogly wrote:All gravity is a curving of spacetime. Gravity waves transmit energy though, whereas the earth doesn't lose any energy when it sends out its GM/r^2 at you.

The energy imparted in gravity waves - how is it lost? Does it diminish over distance?

It follows an inverse-squared law just like light does.

The gravitational field of Earth is effectively static; it's just sitting here and we're balanced in it (either in orbit, with our speed balancing its pull, or on the surface, with the ground balancing its pull). If the Earth suddenly tripled in mass, the change in its gravitational potential would propagate out in a gravitational ripple at the speed of light, and Einstein's equations predict that this ripple would carry energy.

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Re: LIGO Gravity Waves; how did they determine distance

Postby doogly » Fri Feb 12, 2016 2:26 pm UTC

Or it diffuses like 1/r^2, and is lost when it transfers into things like setting a bead in motion or deforming a rod or what have you. Does that add a little something to the explanation?

What do you mean by rigidity in this context?
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Re: LIGO Gravity Waves; how did they determine distance

Postby tomandlu » Fri Feb 12, 2016 2:40 pm UTC

doogly wrote:All gravity is a curving of spacetime. Gravity waves transmit energy though, whereas the earth doesn't lose any energy when it sends out its GM/r^2 at you.


So, if we go with the (outdated?) rubber-sheet analogy, normally, mass just distorts the sheet statically, but gravity waves are moving ripples, as though you'd dropped something onto the sheet?

Is that a reasonable approximation?
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Re: LIGO Gravity Waves; how did they determine distance

Postby sevenperforce » Fri Feb 12, 2016 2:46 pm UTC

tomandlu wrote:
doogly wrote:All gravity is a curving of spacetime. Gravity waves transmit energy though, whereas the earth doesn't lose any energy when it sends out its GM/r^2 at you.


So, if we go with the (outdated?) rubber-sheet analogy, normally, mass just distorts the sheet statically, but gravity waves are moving ripples, as though you'd dropped something onto the sheet?

Is that a reasonable approximation?

Exactly. Of course, in the rubber-sheet visualization, the ripple is going to have a 1/r dependence rather than a 1/r2 dependence, but that's beside the point.

As would be expected, it takes a very, very large change in the sheet's distortion to produce a ripple that is detectable at all.

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Re: LIGO Gravity Waves; how did they determine distance

Postby eSOANEM » Fri Feb 12, 2016 2:46 pm UTC

My university astronomy department livestreamed the press conference and held questions with some of the members of the collaboration who're based here.

Two different detectors actually doesn't let you triangulate very well at all; it only lets you localise the signal onto a cone. The fact that the two detectors aren't aligned gives you polarisation information and lets you localise a bit better to a region on that cone. In this case, that region's about as big as a medium-sized constellation (in terms of solid angle from earth).

As for the distance, the answer is indeed modelling. More massive bodies lead to higher frequencies but, because of redshift, this alone isn't enough information to give you the mass or distance; for that you need the amplitude (which decays with distance). The 1.3 billion light years they've been talking about is what's called a "luminosity distance" because it's been determined by the decay in the intensity of a signal and it's not the same as the "radar distance" (based on bouncing a light ray off it and timing it) or the various cosmologically nicer co-ordinates.

I believe there have been ~25 subsequent EM searches trying to find a signal from this event but I don't think anyone's expecting to see anything, the localisation isn't really good enough. Also, there's no indication of any significant accretion disks or anything else that could give off any EM radiation.
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Re: LIGO Gravity Waves; how did they determine distance

Postby Copper Bezel » Fri Feb 12, 2016 3:06 pm UTC

sevenperforce wrote:
tomandlu wrote:
doogly wrote:All gravity is a curving of spacetime. Gravity waves transmit energy though, whereas the earth doesn't lose any energy when it sends out its GM/r^2 at you.


So, if we go with the (outdated?) rubber-sheet analogy, normally, mass just distorts the sheet statically, but gravity waves are moving ripples, as though you'd dropped something onto the sheet?

Is that a reasonable approximation?

Exactly. Of course, in the rubber-sheet visualization, the ripple is going to have a 1/r dependence rather than a 1/r2 dependence, but that's beside the point.

As would be expected, it takes a very, very large change in the sheet's distortion to produce a ripple that is detectable at all.

Also directly equivalent to the difference between a static magnetic field (refrigerator magnets don't take batteries) and a dynamic one (iPod speakers do), right?
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LIGO - don't understand how detection worked

Postby p1t1o » Fri Feb 12, 2016 3:23 pm UTC

So, I familiarised myself with the instrumentation setup and how it is supposed to work, and it all seemed to make sense [at least to the level at which I understand the underlying science anyway].

Except for one thing.

The detection was based on measuring the change in length of two "arms" caused by the passage of gravity waves.
As the wave passes, the difference in length between the two arms causes the two laser beams to traverse different distances, meaning, with sensitive equipment, we register the passage of the wave.

If spacetime itself is being distorted, how can "distance" or "pathlength" change?

The passage of a gravity wave is supposed to change the pathlength of the arms, detected by a change in the relative phase of a laser beam that travels up and down each arm.
What I don't get is if the laser is travelling through the same distorted spacetime that the physical arm resides in, how can the pathlength change?

Is EM radiation affected differently by the distortion than the phyical distance being measured?

At the moment I see it as trying to measure a changing distance with a ruler that is also changing in length by the same amount, no matter how much space distorts, I always measure the same distance. I know that in the experimental setup we have two perpendicular arms, and we are not directly measuring the length of a single arms but the change in the difference between the two (right?), but I still can't grasp how spacetiem distortions cause a detectable signal at the interferometer, unless EM radiation is somehow less affected by the physical matter it is passing by.


What am I missing? Is it something I'd need a doctorate to grasp?



**edit**

Is it the *frequency* that is changing rather than the *path-length*?

Because that would tally with what I know about gravitational red/blueshift and does not require a measured *distance*.

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Re: LIGO Gravity Waves; how did they determine distance

Postby sevenperforce » Fri Feb 12, 2016 3:23 pm UTC

Copper Bezel wrote:
sevenperforce wrote:As would be expected, it takes a very, very large change in the sheet's distortion to produce a ripple that is detectable at all.

Also directly equivalent to the difference between a static magnetic field (refrigerator magnets don't take batteries) and a dynamic one (iPod speakers do), right?

Exactly.

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Re: LIGO - don't understand how detection worked

Postby sevenperforce » Fri Feb 12, 2016 3:46 pm UTC

p1t1o wrote:The passage of a gravity wave is supposed to change the pathlength of the arms, detected by a change in the relative phase of a laser beam that travels up and down each arm.
What I don't get is if the laser is travelling through the same distorted spacetime that the physical arm resides in, how can the pathlength change?

Is EM radiation affected differently by the distortion than the phyical distance being measured?

At the moment I see it as trying to measure a changing distance with a ruler that is also changing in length by the same amount, no matter how much space distorts, I always measure the same distance. I know that in the experimental setup we have two perpendicular arms, and we are not directly measuring the length of a single arms but the change in the difference between the two (right?), but I still can't grasp how spacetiem distortions cause a detectable signal at the interferometer, unless EM radiation is somehow less affected by the physical matter it is passing by.

Hah, we both have KSP forum accounts.

You have two rulers, but the rulers are changing in length by different percentages, causing the misalignment.

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Re: LIGO - don't understand how detection worked

Postby p1t1o » Fri Feb 12, 2016 4:00 pm UTC

sevenperforce wrote:
p1t1o wrote:The passage of a gravity wave is supposed to change the pathlength of the arms, detected by a change in the relative phase of a laser beam that travels up and down each arm.
What I don't get is if the laser is travelling through the same distorted spacetime that the physical arm resides in, how can the pathlength change?

Is EM radiation affected differently by the distortion than the phyical distance being measured?

At the moment I see it as trying to measure a changing distance with a ruler that is also changing in length by the same amount, no matter how much space distorts, I always measure the same distance. I know that in the experimental setup we have two perpendicular arms, and we are not directly measuring the length of a single arms but the change in the difference between the two (right?), but I still can't grasp how spacetiem distortions cause a detectable signal at the interferometer, unless EM radiation is somehow less affected by the physical matter it is passing by.

Hah, we both have KSP forum accounts.

You have two rulers, but the rulers are changing in length by different percentages, causing the misalignment.


Hey Seven, small world!

This is really getting me!

Lets say we have a ruler, 1km long.
Lets say we distort space, smashing it down to 1% of its original length.
My ruler still says 1km at the end, so when I check, I still measure 1km.

Lets say we have another ruler, 1km long.
Lets say we distort space, this time stretching it to 1000% of its original length.
This ruler still says 1km at the end.

Comparing the two, both say 1km, I cannot tell any difference.

Now lets say that both rulers have a mirror next to the 1km mark, part of a LIGO-style interferometer.
Lets say I use the rulers in place of a laser beam [the rulers are like, SUPER accurate].
Lets say a gravity wave passes by and causes the distance to each mirror to fluctuate between 1% and 1000% of the original distance rapidly.
I look at my rulers and both mirors seem to be static at the 1km mark.

Ergo, I dont detect the passing wave.

I think that about sums up my un-understanding :)

There appears to be some property, either of EM waves or of spacetime, or the relationship between the two, that I am not aware of.

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Re: LIGO Gravity Waves; how did they determine distance

Postby Izawwlgood » Fri Feb 12, 2016 4:14 pm UTC

doogly wrote:What do you mean by rigidity in this context?
Light is bent by a prism. Is there an equivalent for gravitational waves? I meant 'rigidity' insofar as 'space that resists or encourages propagation of gravitational waves', which I understand is probably best answered with 'we literally just confirmed they exist'.

Would areas of high deformation (dense objects) bend gravitational waves? Does highly deformed space resist further deformation?
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Re: LIGO - don't understand how detection worked

Postby sevenperforce » Fri Feb 12, 2016 4:17 pm UTC

Well, I suppose in a way it does have something to do with the properties of electromagnetic waves.

If you had two rulers, you wouldn't be able to tell the difference, because they are stationary. But a laser beam is traveling at the speed of light, just the same as the gravitational wave. And so the phase shift is different for one laser beam than it is for the other, relative to the angle between the propagation direction of the wave and the direction of each beam.

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Re: LIGO - don't understand how detection worked

Postby p1t1o » Fri Feb 12, 2016 4:44 pm UTC

I'm aaaaaaaaaaalmost getting it, I feel like there's a connection or a leap I am not making.

I read something about rubber sheets and rulers not being great visualisations, I also read something which mentioned metrics and manifolds, and also something else which related the effects within LIGO to temporal displacement rather than a physical one.

But its not coalesced into a solid...anything, in my head yet.

I feel like if I was capable at all of making the leap, I would probably be in a different profession :)

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Re: LIGO Gravity Waves; how did they determine distance

Postby Flumble » Fri Feb 12, 2016 5:28 pm UTC

Izawwlgood wrote:
doogly wrote:What do you mean by rigidity in this context?
Light is bent by a prism. Is there an equivalent for gravitational waves? I meant 'rigidity' insofar as 'space that resists or encourages propagation of gravitational waves', which I understand is probably best answered with 'we literally just confirmed they exist'.

Would areas of high deformation (dense objects) bend gravitational waves? Does highly deformed space resist further deformation?

AFAIK there's nothing except black holes that can "absorb" gravity waves (I don't know if two opposing waves will cancel out; probably not). But they should be "bent" by the proximitity to mass in the same way light gets "bent" in those situations (unlike the refraction in a prism).

So you could gravitationally lens gravitational waves with e.g. black holes. And even gravitationally lens gravitational waves with other gravitational waves. Yo dawg...

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Re: LIGO Gravity Waves: Questions and Answers

Postby doogly » Fri Feb 12, 2016 6:53 pm UTC

You get the kind of superposition you're thinking of what linear systems, and GR is not like that. Gravitational Waves are generally studied as a linearization of full GR though, and in that context (small perturbations of flat space) you get cancellation if they happen to be set up just right to do it.
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Re: LIGO Gravity Waves: Questions and Answers

Postby Sizik » Fri Feb 12, 2016 7:20 pm UTC

What would be needed to perform a gravitational double-slit experiment (aside from a lot of detectors)?
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Re: LIGO Gravity Waves: Questions and Answers

Postby doogly » Fri Feb 12, 2016 8:25 pm UTC

Gravity doesn't diffract around edges because nothing counts as an edge for gravity.
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Re: LIGO Gravity Waves: Questions and Answers

Postby PM 2Ring » Sat Feb 13, 2016 6:28 am UTC

@p1t1o:

I Am Not a Real Physicist, so I might be totally wrong, but this is my take on how LIGO works.

The LIGO interferometer has two "rulers" that are perpendicular to one another. Let's call the direction of one path X and the other path Y, with Z the direction perpendicular to both X & Y. Let W be the normal vector of a gravitational wave that's traveling through the detector. If W is aligned with Z then the distortions on the X & Y paths will be in sync, and the detector will measure nothing. But if W has any other alignment, the distortions on the X & Y paths will be out of sync. If W is perfectly aligned with X or with Y then we get maximum detection: when the length of the X path is reduced the length of the Y path is increased, and vice versa. So the travel times of photons on those paths get out of sync, upsetting the destructive interference that normally happens when no (measurable) gravitational wave is passing through the system.

Hopefully, our resident expert on spacetime curvature will correct any errors I've made here. :)

- - - - - - - - - - -

Wikipedia says that the two black holes that collided in the GW150914 event had masses around 36 and 29 solar masses, with the combined black hole having a mass of around 62 solar masses, so around 3 solar masses worth of gravitational energy were liberated. How does that energy get out of the event horizon(s)? Or was the liberated energy never actually inside the progenitor black holes' event horizons, and was "merely" the gravitational potential energy of the system?

A quick Newtonian PE calculation for a pair of objects with those masses and a separation of 350 km (the orbital separation of the black holes before the merger) gives roughly 4.4 solar masses worth of energy, unless I've done something stupid. :)

Using 1.98855E30 kg as the solar mass, and plugging this into Google Calculator:

(G*(36*1.98855E30 kg)*(29*1.98855E30 kg))/(350 km)/(c^2)) / (1.98855E30 kg)

gives 4.40472675

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Re: LIGO Gravity Waves: Questions and Answers

Postby elasto » Sat Feb 13, 2016 7:47 am UTC

PM 2Ring wrote: How does that energy get out of the event horizon(s)? Or was the liberated energy never actually inside the progenitor black holes' event horizons, and was "merely" the gravitational potential energy of the system?

IANAP but my understanding is that gravity is the distortion of space-time, and gravity waves are when space-time oscillates around the mean distortion - like the ripples you'd get when dropping a heavy weight into a rubber sheet. So gravity waves no more 'escape' the event horizon than gravity itself does: If they couldn't 'escape' then we'd not feel the effect of gravity either...

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Re: LIGO Gravity Waves: Questions and Answers

Postby ijuin » Sat Feb 13, 2016 9:15 am UTC

The inteferometer works, despite the Equivalence Principle telling us that we should not be able to detect changes in spacetime, because the gravity wave is passing through the detector. In other words, it does not hit every part of the detector at once. This means that one arm of the inteferometer will be distorted before the other one, rather than simultaneously with it. The result is that the two lasers end up slightly out of phase with each other.

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Re: LIGO Gravity Waves: Questions and Answers

Postby Goemon » Sat Feb 13, 2016 8:01 pm UTC

If you have two neutrons 1km apart, then a passing gravitational wave will physically change the distance between them: the actual measured distance will oscillate, becoming momentarily and alternately slightly less than 1km and slightly more than 1km.

If you have a physical ruler stuffed in between the neutrons, then the gravitational wave will push on the ends of the ruler, but the electromagnetic forces of the particles making up the ruler will resist the compression. The ruler experiences a gravitational force, just like it would if it were standing on end in Earth's gravitational field. It will get slightly shorter and longer, but the change would be a lot less than the distance change between the unconstrained free particles. Edit: The free neutrons initialy located at each end of the ruler would move relative to the ends of the ruler, since there's nothing "pushing back" against the tidal forces exerted on them.

Lasers bouncing between mirrors attached to the end of fixed rods might be able to detect a passing wave, but the signal would be weaker than if the mirrors are not constrained.

Also: with detector arms in the X and y directions, a wave passing through in the z direction gives the largest signal - take a look at the GIFs on the "gravitational wave" wiki page. A wave travelling in the z direction that passes through the xy plane causes a ring of free particles in the xy plane to oscillate with x distance shrinking at the time the y distance is increasing and vice versa. (The acceleration of the particles is perpendicular to the direction the wave is travelling). You'd get a weaker signal if the wave was travelling in the x direction; in this case you'd detect the particles along the y axis oscillating in and out but no effect on the particles on the x axis.
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Re: LIGO Gravity Waves: Questions and Answers

Postby PM 2Ring » Sun Feb 14, 2016 4:26 am UTC

Goemon wrote:Also: with detector arms in the X and y directions, a wave passing through in the z direction gives the largest signal - take a look at the GIFs on the "gravitational wave" wiki page. A wave travelling in the z direction that passes through the xy plane causes a ring of free particles in the xy plane to oscillate with x distance shrinking at the time the y distance is increasing and vice versa. (The acceleration of the particles is perpendicular to the direction the wave is travelling). You'd get a weaker signal if the wave was travelling in the x direction; in this case you'd detect the particles along the y axis oscillating in and out but no effect on the particles on the x axis.

Ok. That makes sense. FWIW, I've been worried about that since I made my previous post. :)

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Re: LIGO Gravity Waves: Questions and Answers

Postby mfb » Sun Feb 14, 2016 12:15 pm UTC

Goemon wrote:Lasers bouncing between mirrors attached to the end of fixed rods might be able to detect a passing wave, but the signal would be weaker than if the mirrors are not constrained.
In LIGO, they are not constrained. The speed of sound is of the order of kilometers per second or less, with the ~100 Hz frequency sound has no way to propagate from one side to the other within an oscillation.

The different timescales are a crucial part in the experiment: compared to the speed of sound, the oscillation is fast - so the rigidity of Earth does not matter. Compared to the speed of light, the oscillation is slow - light needs about 25 microseconds to go through an interferometer arm and back, while the oscillation period is about 10000 microseconds. If those times would be similar, things would be a bit more messy.

doogly wrote:Gravity doesn't diffract around edges because nothing counts as an edge for gravity.
Matter (or energy in general) does influence the propagation of gravitational waves, but the influence is so tiny that we cannot measure it (yet?).

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Re: LIGO Gravity Waves: Questions and Answers

Postby Flumble » Sun Feb 14, 2016 8:20 pm UTC

doogly wrote:Gravity doesn't diffract around edges because nothing counts as an edge for gravity.

What if you have a massive emission of gravitational potential (like a thousand black holes evaporating at once for some reason) behind two large black holes? (Or, for a more complicated but more realistic scenario: a binary system behind two static black holes)

Would you get nice interference patterns?

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Re: LIGO Gravity Waves: Questions and Answers

Postby ijuin » Mon Feb 15, 2016 4:26 am UTC

Gravitational lenses differ from refractive lenses in an interesting way. While the angle by which light is bent in a refractive lens increases with distance away from the central optical axis, with a gravitational lens, the light is bent more the closer it is to the center.

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doogly
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Re: LIGO Gravity Waves: Questions and Answers

Postby doogly » Mon Feb 15, 2016 4:37 am UTC

Gravitational lenses are not things lensing gravity waves, they are gravity lensing the light from a distant object.

And yeah, they interact with matter, but that's at the next order. It's not nothing though, that's true.
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Re: LIGO Gravity Waves: Questions and Answers

Postby ijuin » Mon Feb 15, 2016 4:53 am UTC

Here's a thought: would intersecting gravitational waves form interference patterns with each other analogous to light waves forming interference patterns?

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doogly
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Re: LIGO Gravity Waves: Questions and Answers

Postby doogly » Mon Feb 15, 2016 1:29 pm UTC

If you're fully in the linear approximation then yeah, it'd be just like that, but eventually superposition stops working as simply.
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Re: LIGO Gravity Waves: Questions and Answers

Postby Trapperkeeper » Mon Feb 15, 2016 7:16 pm UTC

I read in a story (sorry, no link) that this technology would allow us to see what was happening in the big bang. Which didn't make any sense to me and wanted to get clarification from folk here. I understand basically what the gravitational waves are and that they are not inhibited by intervening matter. But what gets me is that my understanding says that once the wave goes by, its gone. We were looking at these two black holes at the time when their gravitational waves reached us. But once they merged the way I understand it is that the ship has sailed. Those waves went by and are gone.

How on earth would this allow us to see gravitational waves from the big bang?

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Re: LIGO Gravity Waves: Questions and Answers

Postby Sizik » Mon Feb 15, 2016 7:32 pm UTC

Because the gravitational waves emitted from the big bang 13.82 billion light-years away (post-inflation) are just now reaching us.
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Re: LIGO Gravity Waves: Questions and Answers

Postby thoughtfully » Mon Feb 15, 2016 11:54 pm UTC

It's the same idea as with the microwave background from the surface of last scattering, only 380k years or so earlier.

Incidentally, there's also expected to be a neutrino background signal.
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