Slashdot Mirror
Examining Gravity Waves
Joseph "JoeDaMac" Haake writes "Sometime within the next two years, researchers will detect the first signals of gravity waves -- those weak blips from the far edges of the universe passing through our bodies every second. Predicted by Einstein's theory of general relativity, gravity waves are expected to reveal, ultimately, previously unattainable mysteries of the universe."
You can also read the story here.
It is only mentioned briefly in the article, but I'll try to elaborate.
Basically gravity waves will stretch space in one direction (say x) and contract space in a perpendicular direction (y). Given this, the "easiest" way to detect gravity waves is to build a very large interferometer. LIGO is the current ongoing gravity wave interferometer, which splits one laser beam into two lasers beams, sending each perpendicularly down a vacuum "hallway" four kilometers long. At the end, the beams are reflected by mirrors. The two lasers meet again after another 4km.
The two beams are recombined afterwards. If the distances the two travel are exactly equal, then the two beams will interfere constructively. But if the lengths which the two beams are stretched/contracted by a passing gravity wave, the beams will interfere since one will be "shifted" (it had to travel a longer/shorter distance. By measuring the interference pattern between theses two beams, and hopefully physicists will be able to detect a gravity wave.
The amount that a gravity wave will shrink/extent one of the beam lines is amazingly small. Each 4km beam line will have it's distance changed by 10^-18 meters, or on the scale of attometers! Because of this, any vibration or local variation will affect the beam length. So the physics who are part of the LIGO collaboration built two such laser devices, one in Livingston, Louisiana and the other in Hanford, Washington. When a gravity wave (from outer space) travels through the earth, hopefully both sites will measure the same small variation, which will correspond to a passing gravity wave.
You can get more information about LIGO at:
LIGO's Home Page
LIGO collaboration page.
Slashdot recently had a science story about LIGO.
Would this help unify quantum gravity and GR?
No. The waves we're going to see are a prediction of the classical theory of gravitation, general relativity. This is, of course, only an approximation to some "quantum" theory, but on this level of accuracy we're going to see only classical effects.
Compare this with classical electrodynamics (which predicts electromagnetic waves, ie. light): merely detecting gravitational radiation is going to tell you just as much about quantum gravity as seeing sunlight tells you about quantum electrodynamics.
Could it give evidence to bolster string theory?
No.
The results of this experiment should be very interesting.
Yes, but not in the way you seem to be expecting.
No "new" physics is likely to come out of these experiments (at least not directly). The exciting part is, like the article says, that this is going to give us a whole new way of doing astronomy: remember that a century ago the only way to get any information from distant objects was to look at them, but there's a whole lot of objects that are sending stuff at us on wavelengths not visible to the human eye. So, the early astronomers missed many very important things of what we're now able to see.
Being able to observe the whole electromagnetic spectrum has completely revolutionized astronomy in the past 100 years. Just think of cosmic background radiation: for a long time, it was completely missed since nobody was doing astronomy with microwaves. Similarly, there are many interesting things out there that could be sending us a signal through gravitational waves (like, for example, merging black holes) - and soon we'll be able to see that signal and whatever it's telling about these events.
Of course, the resolution will really be of the sort "an event lasting t seconds was recorded...", but we can extract useful information from even this kind of observations, especially if we can combine them with others (like optical telescopes). (This way we may even indirectly discover something totally new.)