What would one use a theory of quantum gravity for

big-bangblack-holesgeneral-relativityquantum mechanicsquantum-gravity

I am generally wondering how useful new more ambitious theories would be considering that even with standard non-relativistic electrostatic QM one usually has to employ unsatisfyingly crude approximation, if one wants to calculate anything for a system with more then 2 particles, like ignoring quantum effects, ignoring pair interactions, assuming ground state or guessing some exchange correlation potentials.

More recently I was wondering though: Independently of the complexity of the calculation, for what kind of system would a description by a quantum gravity theory actually be needed?

If we want to describe quantum particles extremely weakly interacting via gravity, QM with static Newtonian potentials should yield perfectly fine results. If we want to describe stronger gravitational actions generated by allot of particles, we should be far away enough from them to ignore their quantum nature. If we want to describe a particle close to a black hole, doing QM in curved space time should be sufficient, so no quantization of gravity itself would be needed. Maybe QCD in a neutron star would be a use case? But shouldn't there also QM in curved spacetime be sufficient as the gravitational retroaction of individual QM-particles on the neutron star should be negligible? Maybe if one wants to describe singular quantum particles with macroscopic masses, perhaps some kind of high energy photons, but do such particles actually exist? Do they play a significant role in our universe?

I understand that one might hope to get some kind of insight over dark matter or dark energy problems. But shouldn't a theory of quantum gravity by construction reproduce general relativity at large distances and therefore yield the same dark matter and dark energy problems? In other terms to make general relativity work with QM one would need a modification of the theory at small distances, but to solve dark matter and dark energy problems one would need a modification at large distances?

So: For the description of what kind of systems would one actually need a theory of quantum gravity?

Best Answer

The two standard answers are:

  • Resolving the singularity at the Big Bang
  • Resolving the singularity in Black Holes

Both of these have large spatial curvature within very small regions.