I'm interested to understand the interpretation of gravity as a result of exchange of force particles (ie gravitons) vs General Relativity-based warping of space-time. Related to this is while a constraint to speed of cause and effect of c would apply to the graviton-based concept, why should this limit apply to space-time warping ? Re this latter point, I understand that while gravitational waves have been postulated there is yet no evidence of them (that in fact recent 2014 findings appear now to have been caused by interstellar dust). So I think my question can be distilled down to how can the theories of gravity as a particle-based force vs a space-time warp be reconciled, and what are the key implications of this ?
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3$\begingroup$ Quantum Gravity is an area of active research. $\endgroup$– MyridiumCommented Feb 18, 2015 at 0:56
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2 Answers
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There is no tension between the two viewpoints. In fact, the situation isn't much different from electromagnetism. The analogy is photon $\leftrightarrow$ graviton and electromagnetic field $\leftrightarrow$ metric tensor.
Both of these theories, EM or GR, are field theories, and exhibit the phenomena of radiation. Gravitational waves have not been detected yet but they surely exist :) In fact, they may be found very soon with Advanced LIGO coming online.
In a quantum theory, the excitations of the field are quantized, and this has been tested well in the EM theory but of course not at all in the GR theory. Now the full quantum theory of gravity is not known, and it has been the subject of intense research for many decades, but it surely possess something that "looks" like a graviton, the quanta associated with gravitational radiation.
answered Feb 18, 2015 at 1:07
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$\begingroup$ My question why the limit would not apply to the gravitational (as opposed to eg electromagnetic) field theory (as opposed to graviton particle force theory) is due to the former being a movement of space-time itself, which is not limited by c. (This concept as I understand has been explored/utilised in the Alcubierre metric). So if one views graviity as a bending/movement of space-time itself why is the speed of impact of gravity constrained by c ? $\endgroup$ Commented Feb 19, 2015 at 3:09
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$\begingroup$ Which limit do you speak of? Your question is confusing to me and I'm not sure what you're asking. I suggest reading a textbook on GR, for example Hartle's to bring some clarity to your thoughts on GR. Einstein's theory of general relativity clearly predicts that gravitational waves, or spacetime ripples, travel at the speed of light. $\endgroup$ Commented Feb 19, 2015 at 3:13
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$\begingroup$ I will do that thanks. My question was predicated on the notion that there is no "speed of light" limit on space-time expansion (eg the inflationary period) and that this limit only applied to movement within space-time, not of the space-time fabric itself. So therefore if gravity is bending the fabric, rather than moving through the fabric, why is there a speed of light limit. I will read up more on this, but was seeing if it could be explained in succinct terms. Thanks for the direction. $\endgroup$ Commented Feb 19, 2015 at 3:36
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There is huge tension between the viewpoints.
The current 'Firewall' debate - that QM predicts a huge amount of physics happening at the horizon where GR says that nothing at all should be happening is an example of the problems being faced in this area.
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1$\begingroup$ This is a very misleading statement. There is tension in the sense that we are very comfortable with QED, but are still searching for a fully consistent theory of quantum gravity that describes our universe. However, at the level of understanding radiation and effective field theory in the semiclassical limit, there is no tension. The firewall debate is simply a manifestation of the first fact that there are some aspects of quantum gravity we still don't understand. Another example is the beginning of time. But neither of these issues really play a role in the current discussion. $\endgroup$ Commented Feb 24, 2015 at 15:24