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Physics, Astronomy & Cosmology / Re: What is the speed of gravity?
« on: 16/02/2016 10:52:21 »Quote from: chris
I have heard it stated many times that the velocity at which gravity propagates is the speed of light.This is the common view (as assumed in Einstein's General Relativity), but you can't say that it is proven.
Quantum theory and General Relativity currently have irreconcilable differences, but researchers are hoping that mediation may still prove successful.
If there is to be a reconciliation, it will probably be related to the hypothetical particle carrier of gravitation, the "Graviton".
If you look at the Wikipedia page on the graviton, you can see some of the open conflicts, eg:
- The information box states that the rest-mass of the graviton is zero. Since we know that gravitational waves can deliver momentum into a detector, this implies that the graviton must have momentum, and the only way a zero-mass particle can have momentum is if it travels at the speed of light (as per Einstein).
- From the recent detection of gravitational waves, researchers placed an upper bound on the rest mass of a graviton: < 10-22 eV/c2. This is miniscule, but it doesn't prove that it is exactly zero. If the mass is ever-so-slightly greater than zero, then the graviton cannot travel at c (but it could be extremely close to c).
- "gravitational waves must propagate slower than c in a region with non-zero mass density if they are to be detectable". I assume that this is analogous to the fact that light travels at speeds < c in a volume containing matter. So the speed of light (and possibly gravitation) is only c in a true cosmic void.
- String theory has been shown to be compatible with General Relativity, and to have a particle like a graviton. But it, too cannot be reconciled with the Standard Model of particle physics at this time.
- More speculatively, slight differences in the properties of gravitons (like traveling slightly slower than c, or leaking into other dimensions) might explain some of the characteristics of dark matter
An Upcoming Experiment?: As of this month, we have demonstrated an ability to detect gravitational waves. By 2020 we should have enough operational detectors worldwide to be able to locate a source like colliding neutron stars, and point our telescopes in that direction. In this case, we should be able to compare the time of arrival of the gravitational waves with the arrival of the optical/radio signal. This will give us a direct comparison of the speed of gravitational waves and light. It would be a bonus if that mass of decaying neutrons sprayed into space gave us a simultaneous pulse in the world's neutrino detectors!
A similar analysis with Supernova SN1987A allowed us to place fairly tight limits on the speed of neutrinos compared to the speed of light (it was very close to c; now we know from neutrino oscillations that it is ever-so-slightly slightly less than c).
Quote from: CPT ArkAngel
But curvature of spacetime, like a gravitational wave, in my opinion; should add proper mass to a box in a similar way as a photon.We know how to confine light in a reflective box. I don't know of any way to confine a gravitational wave inside a box.
Quote from: Space Flow
But Gravity does not propagate.
The fact that last September researchers detected a gravitational wave oscillations that seemed to come from merging black holes about a billion light-years away suggests that sudden changes in gravitational fields do propagate through space "to infinity", as predicted by Einstein.
Just like sudden changes in electric fields propagate through space "to infinity", as predicted by Maxwell.
I don't think it's any harder to believe in propagating gravitational fields any more than it is to believe in propagating radio waves.
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