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."
In a perverse sort of way, I'm hoping that this experiment generates all the wrong data. Data that is completely the opposite of what people expect.
Think of all the fun that would be! Think of the chaos, the pontificating, the explanations, the TV specials! Think of all the dissertations that would generate! Yes sir, that would be wonderful.
If tits were wings it'd be flying around.
"Researchers WILL detect..."
On the whole, i think that's not necessarily true. There are several mathematically consistent fringe quantum-physical theorys (usually something akin to higher-order-symmetry electrodynamics) in whcih gravity waves are indistinguishable from e.m. waves, or are longitudinal-time e.m. waves.
You can also read the story here.
The LISA experiment, which gets mentioned in passing, is really quite audacious - three spaceships orbiting the sun in a clever rotating triangle pattern, 5 million miles apart from each other, and detecting changes in distance between each other to an accuracy of 20 picometers!
In essence, it's just a really, really big version of the Michelson interferometer we all played with in 1st year physics - I remember the thrill back then of realising what tiny changes in distance you could discern with just a couple of mirrors, a lamp and something to measure the recieved intensity.
It's exciting to witness the nascence of an entirely new form of astronomy.
Scientists are extremely uptight about exact numerical analysis. We get the data and compare them to a tightly parameterized model which includes everything we know about our detector response as well as the probable sources of the events we are detecting. Good models have small numbers of parameters and many constraints compared to the richness of the data. "With enough free parameters you can fit an elephant", the saying goes, which indicates how important it is to scientists to keep the number of parameters small--no one wants to see a comment like that on a referee's report!
With regard to gravitational observatories, the data are very rich: polarization, amplitude, phase and frequency spectra will be available, possibly from several detectors with different orientations. Detector response is also extremely well understood. The theoretical physics of the sources--general relativity--is also very well understood, and models of stellar collapse, neutron star collisions, etc, contain few parameters (masses, angular momenta, impact parameter...that's about it.)
As such, we can compare model to reality and produce a statistically valid likelihood that the model is false. The Baysians in the audience will point out that relative to our prior knowledge we can also produce the probability that the model is true.
So it isn't a matter of getting something that "roughly fits"--the analysis either produces a fit within error or it does not. If it does not, we dig more deeply into the possible sources of disagreement. The data are sufficienty rich that many, many types of cross-checking and internal consistency checking will be available.
To a hardened skeptic, this of course will not do. But hardened skepticism is an anti-scientific attitude. Scientists are open-minded skeptics, who are able to keep the contingent nature of their beliefs in mind while at the same time maintaining a commitment to distinguish clearly between probable truth and probable falsehood.
--Tom
Blasphemy is a human right. Blasphemophobia kills.
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.)