Understanding General Relativity & Quantum Gravity

[Timothy S]

Hello,

I have a conceptual problem. How can both General Relativity and a theory of Quantum Gravity simultaneously exist? GR describes gravity as the curvature of spacetime, while QG is most likely a gauge theory. Furthermore, if gravity is indeed describedby particle interactions-what does that say about other aspects of GR such as time dilation?

 

[PeterDonis]

How can both General Relativity and a theory of Quantum Gravity simultaneously exist?

The current belief among most physicists is that GR is the classical limit of whatever quantum theory describes gravity. So GR is not itself a "final theory"; it's only an approximation. The underlying quantum theory could indeed be something quite different; as long as its classical limit is GR, that's all that's necessary.

GR describes gravity as the curvature of spacetime, while QG is most likely a gauge theory. Furthermore, if gravity is indeed describedby particle interactions-what does that say about other aspects of GR such as time dilation?

If the underlying quantum theory of gravity describes it as "particle interactions", then that is a description at that level. But it certainly doesn't preclude a classical description at a higher level (the level at which GR is a good approximation). Quantum electrodynamics does not preclude a classical description of electromagnetism as classical electric and magnetic fields as an approximation, with no trace of the underlying quantum "particle interactions" that QED describes. The connection between quantum gravity and classical gravity would work the same way.

 

[Timothy S]

Thank you for the explanation. It seems to make sense, however, it is apparent that the gauge theory for gravity when it is discovered will be quite odd.

 

[PeterDonis]

it is apparent that the gauge theory for gravity when it is discovered will be quite odd.

Not necessarily. It ls already well established that the "obvious" way to construct a quantum field theory of a massless, spin-2 field has GR as its classical limit. The problem is that this "obvious" theory is not renormalizable. But as a gauge theory, it simply says that the gauge freedom corresponds to choosing coordinates, i.e., that it's just diffeomorphism invariance.

 

[Evan Fox]

String theory attempts to unify them into one "theory of everything"