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In principle a gravitational wave will travel slower than $c$ when passing through matter, but in practice the reduction in speed is absurdly small. Consider first a light wave passing through a dielectric. You can explain what happens using either classical or quantum approaches, but we'll use a classical description since that's all we have available for ...


1

Your reasoning is based upon gravity being different to the other forces. However all the forces can be formulated in a geometric way. See for example the question Can all fundamental forces be fictitious forces? and the links within it. So there is no reason to suppose gravity is fundamentally different to the other forces, and no reason to suppose it is ...


2

I found the answer myself: requiring that the gravitational potential $GM^2/r$ is equal to the rest mass $Mc^2$, one obtains for r equal to a natural unit of length $Mc^2=1.22\times10^{19}$ GeV that is the Planck scale.


0

Well, an heuristic solution would be that it is actually impossible for anybody to cross the event horizon. As you noticed, Schwarzschild black hole is dual (at least locally, that is, if you don't make full turn) to Rindler's observer (the uniformly accelerated one). The stationnary observer in the Schwarzschild case is the uniformly accelerated one in ...


4

The question whether primordial B-mode detection says something about quantum gravity or not may be subjective, but one can give an unambiguous and objective answer why people were even bringing up quantum gravity in connection with B-modes (and BICEP2). Here, I will try to answer that question. My answer is mostly based on an excellent (guest) blog post by ...


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This is rather subjective. But I'll volunteer my answer: Would the presence of B-modes in the CMB provide evidence for quantum gravity? No. And recent argument is that they don't even provide evidence for gravitational waves. I don't know if you saw the recent Horizon program, but note that what didn't come out is that the gravitational waves date from ...


1

There seems to be some confusion. In the referenced paper there is no temporal direction, and even though there is curvature there, it has nothing to do with gravity (there is no time). Where as when you compare with $2D$ gauge theory, one of those $2$ dimensions is temporal in deed. So if you wanted to carry the analogy you should consider two dimensions, ...


1

Let us suppose the holographic principle is indeed correct and that there is a (3+1)-dimensional quantum gravity theory that explains our universe, which has an equivalent description as a (2+1)-dimensional system. This simply means that the two descriptions cannot be distiguished so there is no physical experiment that could determine if we "actually" live ...



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