Gravity Waves: mp3 Files of the Universe
Well, this is an interesting way to look at gravity waves.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) has been looking for gravity waves for the past two years and scientists are beginning to pour over the data.
What they are looking for are gravity waves which manifest themselves when massive objects produce big deflections in the spacetime surrounding it, the disruptions are usually things like supernovae, rotating black holes, etc.
Gravitational waves are produced by violent events in the distant universe, such as the collision of black holes or explosions of supernovas. The waves radiate across the universe at the speed of light. (Press Release)
They haven’t found any yet but they’ve only just started looking. Einstein said they should be there back in 1916 but there hasn’t been any way to detect these things till now.
Well, umm, except they still haven’t detected them so maybe I’m being premature.
What I find interesting however, is the way they’re thinking about these waves. In addition to looking for them at all, astronomers are looking for characteristic patterns in the waves, which they are calling sound waves.
So, the characteristic gravity wave pattern of a supernova would correspond to its ’sound’. When a massive star blows up, it would emit a ’sound’ in the form of gravity waves that would traverse spacetime.
Looking for gravitational waves is like listening to the universe,” Brown says. “Different kinds of events produce different wave patterns. We want to try to extract a wave pattern — a special sound — that matches our model from all of the noise in the LIGO data. (Press Release)
Now anyone who knows anything about anything knows that sound waves as we think of them cannot travel in space, there are not enough molecules in the ‘vacuum’ of space for them to propagate through.
Gravity waves, however, propogate through spacetime just like sound propogates through our atmosphere, we just need special ears to hear the sound, that’s what LIGO is.
If they exist that is.
From my perspective of someone interested in dark energy, it would be great if they can pin down these waves, we’re starting to doubt our notions of gravity and this would be an excellent confirmation of what we think we know.
BTW, there is another mission, the Laser Interferometer Space Antenna (LISA) that is supposed to launch in 2015 (I have no news on whether it actually will though), and will be a better platform for finding these waves.
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POSTED IN: cosmology
11 opinions for Gravity Waves: mp3 Files of the Universe
R06u3AP
Feb 12, 2008 at 8:40 pm
One thing that I’ve always wondered about when it comes to “gravity waves” is that they should propagate at the speed of light. It is obvious that if there are to be “waves” then the speed of propagation must be finite.
However, by my admittedly fallible understanding, the speed of light itself derives from the very character of spacetime to begin with.
So, it has never been clear to me, and I don’t recall seeing a clear explanation, as to just why changes in spacetime itself should necessarily be expected travel at lightspeed also.
Tony
Feb 13, 2008 at 1:08 pm
Hi R06u3AP,
Actually, the speed of light limit is inferred from a postulate that Einstein used to develop relativity stating that the speed of light is constant in a vacuum for all observers.
As to why gravity wave propogate at that speed, it was always explained to me as part of the definition of a gravity wave: a perturbation of spacetime propagating at the speed of light.
I don’t suppose they HAVE to travel at c, but they certainly can’t go faster.
R06u3AP
Feb 13, 2008 at 6:32 pm
Hi, Tony,
I always thought that Einstein proposed the postulate that the speed of light is a constant regardless of frame of reference to account for the observed failure to detect a change in light speed in the Michelson-Morley interferometer experiment. In other words, the Theory of Special Relativity derives from the need to make space and time variable in order to always make the speed of light the same!
That LISA interferometer that they are proposing to detect gravity waves sounds an awful lot like that Michelson-Morley experiment! They’re hoping to detect changes in distance between the three instruments, but it seems that they’ve forgotten about the time part of spacetime. That changes, too, and it’s bound to affect the interference phase as well.
Wouldn’t it be hilarious, if the time variable cancelled out the space variable in just the same amount as Michelson-Morley such that you can never detect a gravity wave in an inertial frame, for the very same reason that the speed of light is always the same?
R06u3AP
Feb 15, 2008 at 6:59 pm
More thoughts on the aforementioned…
As far as I know, you can’t tell whether you’re in a gravitational field or in free fall by measuring the local speed of light, since that would violate Relativity. And yet, what LIGO and LISA does, in essence, is the equivalent of looking for changes in the speed of light when a gravity wave passes by. Again, reminiscent of Michelson-Morley.
I’m beginning to suspect that there has been some major fog in the cockpit regarding detecting gravity waves…… Hmmmm……
R06u3AP
Feb 17, 2008 at 10:16 am
And then, I come across this:
http://www.einsteinathome.org/ask/archive/relativity-qa5.html
Suffice to say, this fellow is operating under the notion that all he has to do, in order to detect gravity waves, is to pick a frame of reference that distinguishes between the length of the interferometer arm and the number of light wavelengths within it.
Now, I don’t claim to be one of these degreed professional scientists who have been schooled, undoubtedly via an exquisitely expensive college education, to ‘pick the frame’ that most expediently solves a particular problem. But I don’t think that I’m quite the world’s biggest dumbass when it comes to comprehending the essence of Relativity, either. But maybe I’m missing something quite essential here, so let’s get a bit technical……
We’re not talking about picking a frame of reference to conveniently measure some phenomenon occurring within the arena of spacetime. We’re talking about detecting changes in spacetime itself. But when it comes to talking about spacetime itself, we already know that the spacetime interval is frame invariant. There are no frames of reference that you can choose in order to make the speed of light different, for example. There can be no such thing as a “frame where the arm length changes and the wavelength doesn’t” as claimed by the esteemed physicist in the above referenced article.
As I say, I may be missing an essential concept here, but I really, truly thought that I had a pretty darned good handle on the whole ‘nature of spacetime’ business. Could it possibly be that I’m right and that the literally hundreds of scientists working in these projects are all brain-dead?? Doesn’t seem too likely, does it?
But then, they haven’t actually detected, verifiably, any gravity waves, have they?
Tony
Feb 18, 2008 at 9:35 am
Hi R06u3AP,
I think I know what’s annoying you about the above referenced webpage about gravity waves, I think it seems to you like the physicist is changing frames of reference just to make his answers come out right.
That’s not what he’s doing though. You can do your calculations in any reference frame you want if it makes them easier. as he said, people do that all the time.
There is one crucial sentence in that webpage:
“Fundamental physical observables are the same in any frame of reference, however it’s moving.”
And with respect to gravity waves:
“What is really measured by the instrument is basically the number of wavelengths that fit into an arm or the number of wave periods it takes light to traverse an arm, and that number is the same in all frames.”
Now, having written that, I must say you raise an excellent point that I need to think on more. I think what you’re saying is that spacetime is some kind of ‘absolute’ frame of reference, and when making measurements of it (while you’re in it), one cannot arbitrarily choose a convenient reference frame.
So, what frame do we choose to measure gravity waves?
Interesting question.
r06u3AP
Feb 18, 2008 at 11:09 am
And I have an even more interesting question!
It occurs to me that we already have a wonderful ‘gravity wave’ detector! Works like a CHAMP!
It’s called ‘tides’.
I haven’t actually made any measurements myself, mind you, but I feel pretty safe in assuming that the H2O molecules in the oceans move quite a bit further than a nuclear width.
Hmmmmmmmmmmmmmmmmmmmmmmm………
A third of a billion dollars spent on LIGO, but it can’t even see the MOON???
:-D !!!!!
r06u3AP
Feb 20, 2008 at 9:55 am
Just in case I made a premature assumption, which I may have done, in my last snide remark, I did a little more research and came across this:
http://www.ligo-wa.caltech.edu/ligo_science/earth_tides.pdf
OK, so they have indeed considered the effects of lunar tides on the instruments. (You’ll have to forgive me if this is all quite new to me - I didn’t even know that this thing existed until Tony posted his article the other day.)
So, presumably they are compensating for up to 100 micrometers of physical deformation of the arms due to the presence of the sun and moon (in addition to microseismic and thermal effects).
So now, the question becomes, what is the light doing in response to the cyclic change in spacetime every 12 or more hours? If you mitigate the mechanical change in arm length due to the tides, but the light isn’t changing wavelength, doesn’t that mean that you’re not really seeing a change in spacetime due to gravity using light? Or put another way, is there a difference between the amount of spatial distortion due purely to spacetime and the mechanical displacement of matter?
I guess I need to study further….
r06u3AP
Feb 21, 2008 at 1:28 pm
I believe that I now understand what is going on with LIGO. And, as a result, I also have a strong suspicion that LISA isn’t going to work.
Actually, the answer is pretty much right there in the paper I last referenced, “The Effect of Earth Tides on LIGO Interferometers”. The relevant part:
“Thus the moon can displace equipotential surfaces by as much as 53.52 cm peak-to-peak (P-P). The tides due to the sun are weaker, causing maximum displacements of equipotential surfaces by 24.61 cm P-P, or 0.4599 of the lunar tide. . . . .
2.2. Elastic Deformations of the Earth
The shift in equipotential surfaces causes tidal forces in the earth that deform the earth’s surface. Since the earth is a solid, it resists this force and the corresponding deformation is much less than the shift in equipotential surfaces.”
So there you have it. LIGO isn’t so much seeing changes in spacetime as it is being affected by the elastic deformity of the ground it rests upon. LIGO is basically a very expensive seismometer. It is, unfortunately, not a “spacetime/gravity wave” detector.
Why? Because if it were, it would see the difference between the ground deformity and the change in spacetime equipotential surface caused by the tidal variations. But when you compensate for the lunar+solar tide and geologically based seismology, it sees nothing. The light wavelength changes right along with, and just as much as the equipotential surface.
So, why isn’t it seeing the higher-frequency gravity wave stuff, that should be stretching/compressing LIGO’s earth foundation
also? Most likely, simply because the frequency response of the ground isn’t too good - all else being equal, the power it generally takes to displace/oscillate a mass over a particular distance increases with frequency. Conversely, for the same power level, the higher the frequency, the smaller the displacement. Ultraweak cosmic gravity wave-induced vibrations are probably just buried in the random ground noise.
What does this imply for LISA, then? Obviously, LISA will free-float in outer space. It neither rests on a ground substrate nor are the three instruments mechanically connected. There is therefore no ponderable material between the instruments to change the distance between them. So, it won’t see anything, either.
At least, that’s what I’m predicting. LIGO fails because the ground isn’t being shaken by high frequency cosmic gravity waves enough to be detectable and you can’t see a gravity wave by measuring the local speed of light, and LISA will fail for the same reason.
Am I wrong? Comments, rebuttals?
R06u3AP
Feb 24, 2008 at 12:33 pm
I have continued my investigation of LIGO as time permits. The more I study LIGO, the more questions I have, no doubt due to my own vast ignorance. The latest paper I have come across, “LIGO: Status and Recent Results” by Raymond E. Frey, dated July 27, 2007, is the first paper that I’ve read that actually describes the apparatus in any technical detail. Here is what I’ve found out.
The whole idea behind the way this is supposed to work, involves the use of a mirror that is part of a test mass weighing about 23.6 lbs. at the end of each arm of the interferometer. These test masses are suspended by wires, making them pendulums with a resonant frequency of about 1 Hz. There are, of course, arrangements that serve to mechanically isolate them from the environment. As stated in the article, “for motion along the arms at frequencies well above 1 Hz, the suspended optics represent intertial, “freely falling” test masses. . . . a required strain sensitivity of 10^-21 corresponds to a phase shift of 10^-9 rad. . . ” (of laser wavelength).
He then goes on to describe the technical innovations that make this level of sensitivity possible, quite an impressive read in it’s own right.
Very well, then. The idea that they are depending upon is the use of a mass that inertially resists changes in spacetime, and they expect that, given the sensitivity of the interferometer, they should be able to dig a signal out of the numerous sources of noise that affect it. It should be possible to see spacetime shifting past the test mass by way of wavelength phase changes.
But, as usual, I have a number of stupid questions about all of this. For one thing, wouldn’t all parts of the interferometer involved in handling the light need to be isolated from the environment? So far, I have not read of any provisions for protecting the laser itself, the beam splitter, the photodiode, or the power recycling mirror.
Another thing that I wonder about is whether it wouldn’t be better to use a massive, rigid structure as the substrate of the entire apparatus, instead of a “free falling” test mass.
It would tend to render any environmental disturbances as common mode noise, while still resisting changes in spacetime just as the test mass does. And perhaps be able to do it with far less arm length.
Also, wouldn’t one of the first things you’d do with such an instrument, in order to simply check that it works, is optimize it to detect a known CW source? Take that binary star system that is radiating energy via gravity waves, PSR B1913+16. Couldn’t they enchance the gain of the interferometer by making the test masses resonant with a period of 7.75 hours in order to specifically detect that system?
Nobody is commenting on my entries thus far, is anyone else reading this?
R06u3AP
Feb 24, 2008 at 8:17 pm
In my last note, I stupidly proposed that it may be better to attach everything to a rigid structure instead of using free falling test masses. Please forgive that brain-fart - that won’t work for obvious reasons. Sorry.
Perhaps a free falling test mass suspended from the same massive structure that everything else is mounted to, all of which is isolated from the earth. Or just put the whole thing in outer space and be done with it. You could even call it “LISA” or something. . . .
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