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Lab Tuned to Gravity's 'Ripples'
Krishna Dagli writes "One of the great scientific experiments of our age is now fully underway. Success would confirm fundamental physical theories and open a new window on the Universe, enabling scientists to probe the moment of creation itself. The experiment is trying to detect ripples created in the fabric of space-time that sweep out from merging black holes or exploding stars and detection would be a final test of Albert Einstein's General Theory of Relativity. "
Right now we are uncertain of the exact speed of gravity.
We are always "uncertain" about the exact value of any physical quantity, because no quantity can be measured with infinite precision.
There is very little doubt that the speed of gravity is equal to the speed of light.
Some measurements resulted in speed between 0.8 and 1.2 times the speed of light
The Taylor-Hulse pulsar measurements have measured the accuracy of that speed to within a few percent, much better than the 20% figure you cite. Furthermore, most of the gravitational physics community is convinced that the experiment mentioned did not measure the speed of gravity (as the Wikipedia article alludes to).
If the speed of gravity is greater than the speed of light, does that violate the general relativity?
Yes. It also violates special relativity and the laws of cause and effect.
It is important that we find what gravity is, because if it is a wave of particles, then maybe there is a possibility to find a way to shield gravity away.
Gravity being "a wave of particles" does not imply that it can be shielded, and gravitational wave detectors are unlikely to tell us anything about that issue.
Even if it were possible to "shield gravity" (very unlikely), it is almost certainly impossible to do it with any realistic technology, because we already have a thorough understanding of gravity on the scales that our technology can reach in the forseeable future.
A little realism: LIGO and its kin may teach us something new about gravity near neutron stars and black holes, but the most likely outcome is that it will simply serve as a telescope to probe astrophysical phenomena not detectable in visible light. It is very farfetched to think that it will lead to antigravity or any Star Trek type applications.
I am surprised nobody mentioned Einstein@home - http://einstein.phys.uwm.edu/.
This experiment uses distributed computing to process their results,
and you can participate.
Gravitational waves will exist in any theory of gravitation that requires the effect of gravity to propagate at a finite speed. Newton's theory of gravity assumes that the effect propagates at an infinite speed, so this theory does not predict gravitational waves.
i on
It should be noted that the primary purpose of the detection of gravitational wave since at least the 1970's has been the both the detection and interpretation of the information contained in these waves. Depending on the frequency of the waves, they contain information about the beginning of the universe (long wave) or the collision of massive bodies (short wave). The advantage of a laser interferometer is that it is a wide band detector, while a Weber bar is a narrow band detector.
It was still being argued as late as the 1960's whether Einstein Theory gravatation waves could transmit energy:
http://en.wikipedia.org/wiki/Gravitational_radiat
What would happen if you shot photons at a semi-transparent mirror just one at a time. Can the exact number of photons that takes one route over the other be predicted?
It can be predicted only statistically, in the sense that you can't predict the "exact" number of heads that you will get if you flip a coin N times.
I assume that it should be 50/50 if the mirror is 50% transparent, but the likelihood of any given photon taking one route over the other should be random unless maybe if the mirror is polarized?
It's random, and is 50/50 for a 50% transparent mirror.
If it can not be predicted, is this the limitation of accuracy for the GEO 600?
Uncertainty in the number of photons arriving at the detector is a limitation of the experiment's accuracy: it's called "quantum shot noise". However, that uncertainty would be present even without a mirror to split them. Perhaps the mirror exacerbates the effect.
OK. I wasn't going to get involved in this thread, but I really have to jump on that one.
It's not just about confirming Einstein's theory of general relativity. Or, in fact any of the other relativistic gravitational theories - most (if not all - been a while since I checked on the basic theory and they might have come up with some new ones) of which require the existence of gravitational waves. It's not simply a case of checking that the theory is correct - there are indirect measurements which have already done this, it's about directly detecting something we're sure is there. Don't get me wrong; in part, you're correct - if the outcome is negative, then we can set an upper limit (i.e. the waves must be of lower magnitude than X at frequency Y). This in itself allows corroboration with cosmological models and provides a valuable experimental check against predictions of numerical relativity such as the strain effect on space due to the merger of black holes.
But when a positive detection is made it will provide confirmation/empirical data on the processes involved in such violent astronomical phenomena. What are the physical processes involved in the inspiral of a binary system? Do pulsars with asymmetrical mass distribution really lose energy as gravitational waves? We know about the cosmic microwave background, what about the gravitational wave stochastic background?
It's not just a case of "There's a peak on the trace! Well, that's our job done! Who's for tea and biscuits?" The potential gains in knowledge of astronomy, astrophysics and even particle physics are vast. Not to mention the gains in laser technologies, control systems, material science and computational analysis that such a project brings. Just by designing and building these instruments we push the boundaries of what's known. Of course there will still be tea and biscuits (well, maybe beer and doughnuts) but that's half the fun right there.
OK. Rant over. Everyone back on your heads.
If you can't think of something nice to say then don't say anything at all. No, REALLY.
Technically, you're correct. If the distortion of space-time 'compresses' one arm of the interferometer and 'extends' the other in a step (or constant offset), the amount of time taken for the light to traverse the arms and recombine at the beamsplitter wouldn't change and no phase difference would be detected between the returning beams.
However, gravitational waves are not stepwise events and have a frequency. This means that if the beams are split and traverse the arms the 'compression/extension' experienced by the beam on the way out will have changed by the time the beam returns such that the frame of reference is different for the outgoing and incoming beam in each arm. In essence, the instruments detect 'changes' in space-time but are unable to measure static states.
The schematic you mentioned from the article has additional mirrors [e + f]. 'f' is the signal recycling mirror which allows the instrument to by optically 'tuned' to the frequency of optimal detection (i.e. the frequency of your expected gravitational wave event). That's not to say that by choosing one frequency you're ignoring all the others - it just means the device is more sensitive at one chosen frequency.
If you can't think of something nice to say then don't say anything at all. No, REALLY.
If the stick is also locally contracting/expanding, then it may remain locally motionless with respect with the bead, and hence no friction, and therefore no heat and no energy.
The stick does not contract and expand, for the same reason that if you push/pull on its ends a little bit, it doesn't contract/expand. Or rather, it resists the compression/expansion, because the molecules try to be at a certain distance from each other: they have a fixed equilibrium distance that they try to maintain. (If you push/pull too hard, of course the rod will deform.) The beads, on the other hand, can slide "freely", or almost freely, if there's only a little static friction.
This is related to the reason why atoms don't expand when the universe expands.