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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.)
Actually, Newton and Einstein did the same things. Newton combined the works of Kepler and Galileo into a theoretical framework that predicted helluva lot more than balls rolling on a slope (Galileo) or descriptive formulas for planet motion (Kepler). Newton generalized this into mechanics and especially a theory of gravity, that could predict the motion of the entire solar system (or more), minus "anomalies" such as retrograde motion of Mars.
Einstein in turn took Lorenz' equations and Maxwells theory of electromagnetism as a starting point. Remeber that c is defined as constant in electromagnetism, so what Einstein really did was just to combine this fact with the relativity equations. This is of course ingenious, and even more so to use Non-Euclidean geometry to extend SR to GR by curved spacetime.
Newton did away with absolute space and Einstein did away with absolute time, so their contributions are very similar in structure.
Newton _invented_ caclulus as a byproduct, though, while Einstein had to borrow extensively from recent mathematics (Minkowski space, tensors and all), all of which he had to have help with to fully understand in the context of relativity.
This fact justifies Newton being the greater of the two, because mechanics and calculus are fundamental in all of physics, whereas GR is a very specialized field. We went to the moon with the help of Newton, not Einstein.
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You can't buy t-shirts or posters with Newton.
t-shirt
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I wonder if they will be at all able to measure the speed of a graviton with this current setup.
No.
For instance, some spin-2 thermodynamics could be experimentally demonstrated if gravitrons could be isolated and easily detected.
No (see my earlier post on quantum gravity and gravitational waves).
According to current theory, there is absolutely no way we could even begin to dream about detecting individual gravitons, much less confine them. These experiments aren't a step towards this "goal" any more than any other experiment out there: according to current theories of physics (yes, the most fundamental ones), confinement of gravitons is an absolutely unimaginable task for all foreseeable technology.
Compare this with neutrinos. They only interact weakly, through the "weak force" (and, of course, they also interact gravitationally), as opposed to, say, protons and electrons, which interact through the strong force and the electromagnetic force. The important difference is that the weak force is, well, like the name says, weak: the probability of any interaction of neutrinos (with any known form of matter) is much, much lower than the probability of protons or electrons interacting with each other.
Remember that quantum mechanics only predicts probabilities. Imagine a neutrino traveling towards the Earth: it has a very low probability to "collide" with any particles of the Earth, so most likely it will just shoot right through the Earth like it's just empty space. A proton or an electron wouldn't do that (or, at least, this would be very, very improbable): they can also interact through the two stronger forces, which means that they would have a much higher probability to collide with some particle of the Earth. For this reason it takes some incredibly complicated arrangements to detect neutrinos (you need a huge detector to get even a single neutrino collision per day).
Now, while neutrinos can interact through the weak force (and gravitationally), the problem with gravitons is that they can only interact through gravitation. And gravitation is much, much weaker than even the weak force! The difference is actually many, many orders of magnitude larger than the difference between the electromagnetic force and the weak force. So we're not goint to see any gravitons for a very, very long time!
Maybe some time in the future we'll be able to build some galaxy-sized detector in intergalactic space and finally see some gravitons... but, unless the "coming" theory of gravity predicts some totally new effects (and it might), it's really that far off.
If you didn't see it, crosscheck this article with the experiment to measure the speed of gravity that took place last month. There they observed the effect of Jupiter's gravitational field on incoming radio waves.
Does anyone know if results have been published yet?
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In case anyone is interested, "gravity waves" also refer to the buoyantly driven waves in the atmosphere and ocean.
This is an interesting thing I had not heard before. Everything I have read on GR that talks about "speed of gravity" and speed of propagation of gravity waves mentions c. I guess I thought someone had done the integration and showed that the observed effects on orbits (like perihelion advance) are either not affected by gravity speed or show that this speed is c.
If what you say is correct, you could in theory send a signal by gravity wave much faster than c. This of course would revolutionize the communications industry when we have sufficiently sensitive gravity wave detectors.
Also, the speed of any wave is the product of its wavelength and frequency (c=lambda*nu). The generator of the wave can only affect the frequency, since the speed is dependent on the medium and the wavelength is therefore determined. So, if the speed of gravity wave propagation is dramatically higher than predicted, the wavelength must be dramatically longer, not shorter. This won't help LIGO out any, since it is sensitive to wave amplitude, not wavelength.
Can you provide any details or links about this effect?
Lots of technical and environmental problems are solved by the application of vast amounts of nuclear power