A Galaxy-Sized Observatory For Gravitational Waves 190
KentuckyFC writes "Gravitational waves squash and stretch space as they travel through the universe. Current attempts to spot them involve monitoring a region of space several kilometers across on Earth for the telltale signs of this squeezing. These experiments have so far seen nothing. But by monitoring an array of pulsars throughout the galaxy, astronomers should be able to see the effects of gravitational waves passing by. They say such an array of pulsars should effectively shimmer as the gravitational waves wash over it, like a grid of buoys bobbing on the ocean. That'll create an observatory that is effectively the size of the entire galaxy. These observations should be capable of monitoring how galaxies and supermassive black holes evolve together, and shed light on the physics of the early universe. Best of all, the next generation of radio-telescope arrays should be capable of making these observations at a cost of around $66 million over ten years. That's a small fraction of the hundreds of millions that Earth-based observatories have already cost."
Re:Guess LIGO failed too many times (Score:4, Informative)
Re:Guess LIGO failed too many times (Score:5, Informative)
That's correct. Lack of evidence isn't enough to disprove a theory; what you need is evidence that directly contradicts the theory. In the case of gravity waves, it might be observation of an event that should produce detectable gravity waves, combined with our not detecting them.
And, while I'm at it, I'd like to point out that what Popper taught us was that a theory was useless unless there's a way to falsify it, at least in theory. If you can find a way to show that any conceivable experimental results can be viewed as confirming the theory, it's useless because it can't be tested. In the case of gravity waves, they're but one of many things predicted by General Relativity, and one of the few that's not been observed as yet.
Re:Guess LIGO failed too many times (Score:4, Informative)
LIGO and Pulsars set limits (or could detect) gravitational waves in very different parts of their frequency spectra - periods of milli seconds versus periods of months. The sources are different, the detection physics is different, etc. It's certainly worth trying both.
Also, none of the existing detectors are good enough that you can say for certain that there are known or likely astrophysical sources bright enough that they should see them. You can't talk about falsifiability until you cross that threshold, which I would expect to see happen in a decade or so.
Re:Guess LIGO failed too many times (Score:3, Informative)
> They just ruled out the higher end of the spectrum for gravitational waves.
No. They failed to detect high-frequency gravitational radiation above a certain level. Conventional theory predicts that the radiation they failed to detect should be fairly rare, so the result tends to confirm the established theory while leaving the proponents of some alternative theories with some explaining to do.
Re:This would be wonderful (Score:5, Informative)
I seem to recall an experimental observation in the last few years involving Jupiter, through which they verified with about 90% certainty that the speed at which gravity propagates through space/time is equal to the speed of light.
A little googling turned this up:
http://www.nrao.edu/pr/2003/gravity/index-p.shtml [nrao.edu]
Re:Any armchair physicists here? (Score:5, Informative)
A gravity wave, as derived from the theory of relativity, doesn't specify that the gravitational constant would oscillate - simply that the shifting of large masses, like co-orbital black holes and such, will distort spacetime in wavelike manner. Those perturbations of spacetime would travel from their origin outward at the speed of light.
It's best to think of it in terms of the bowling-ball-on-a-rubber-sheet analogy of space-time. If you take a large mass like a bowling ball and set it in the middle of a large rubber sheet, it will depress deeply nearby and taper off the further away from it you go on that sheet. If that bowling ball magically disappeared, there would be a wave that travelled across that sheet as well as if you had 2 bowling balls spinning around each other.
The way we've been trying to detect gravity waves so far (LIGO) uses lasers set up at right angles so if space were to compress or stretch in one dimension, the beams the were previously in phase would shift apart. This can detect a stretching of spacetime equal to a fraction of the wavelength of light used in the lasers.
In actuality, it is the change in the behavior of spacetime that lets us measure in that manner, but if the wave were to stretch spacetime in all dimensions, LIGO couldn't work. Hope that explains it.
Re:Any armchair physicists here? (Score:3, Informative)
Imagine a stretchy, rubber fabric that you pull/push or move upward/downward from one side such that a wave propagates through. Then two masses lying on this fabric, link ping pong balls that you would stick on, would move closer/further apart. That's basically the effect that people are trying to measure. Of course, if these "test" objects are perfect in such that they're infinitely small, everything behaves in a trivial way. The catch is when your object is not "perfect" anymore and possesses some finite size. This seems to be concept that you worry about and you are right. Because of it's finite size, the object itself would change size. However, it does not matter at all because this change is not significant. Here's why:
The amplitude of a gravity wave is express in a weird unit expressing the ratio of the spatial compression in one direction to the stretching in the orthogonal direction (see the nice animation here [wikipedia.org]). A typical gravity wave would have an amplitude of 10^-20., which basically mean that any object would change size by this fraction. So this is practically undetectable unless you consider something really big like the "arms" of the LIGO gravity wave detectors [caltech.edu] or this pulsar timing array. The other thing to take into account is the fact that what you are trying to detect acts like a wave. Waves that this pulsar array is after have frequencies of nanohertz, or wavelengths of 3*10^17 meters (this is about 32 light-year!). For LIGO, frequencies are the order of 1 hertz, so 300 000 km. Hence if your object, the pulsar for the pulsar array, or the mirror/detector for LIGO is much smaller that the wavelength that you attempt to detect, it really doesn't have any effect on what you are trying to measure.
Re:That's dumb (Score:3, Informative)
The guy you're responding to, tjstork, is an idiot, not worth your time. He's also a conservative, but I repeate myself. The only reason it's relevant is that his opinions come from his ideology. In his mind, you are already wrong because you like science, and science is paid for in large part by public dollars. This makes science the enemy to him.
He'll stick to his scientifically ignorant position, and you will fail to educate him.
Just a heads up.
Re:A complementary approach (Score:5, Informative)
The good thing is that the pulsars which glitch are the young ones (hundreds to millions of years old). The pulsars that we are using for NANOGrav are millisecond pulsars which are hundreds of millions or billions of years old, have much smaller magnetic fields than young pulsars, and basically never glitch. They are extremely stable rotators -- much better than normal pulsars.
Re:Any armchair physicists here? (Score:3, Informative)
Like I mentioned in the last sentence, it relies on the expectation that a gravity wave passing through an area would stretch one dimension of space while contracting another perpendicular to it.
If it causes all dimensions (including time) to expand and contract simultaneously, it can't work.
Of course, I have to defer my understanding of gravity waves to those who study this stuff for a living and have experimentally verified a large body of the predictions made by general relativity.