system
Given the following post, respond as if you're commenting on Reddit:
human
What are we missing before we finally have a quantum theory of gravity?

I know the motivations behind string theory and other models of quantum gravity (e.g. unification of fundamental forces) but what I don't understand is what is missing from the current proposed models. Do we have all the details worked out and we're just waiting for experimental verification? Are there still theoretical issues that these models cannot address? I remember reading part of *Quantum fields in curved spacetime* by Birrell & Davies and in the introduction they discussed issues with renormalization of infinities in the ground state energy of the quantum harmonic oscillator and how certain tricks to skirt that issue don't work with gravity. Is that still a problem?
gpt
It's not a problem anymore for string theory. String theory is a consistent UV completion of gravity. It doesn't have the incurable divergences of General Relativity, and is indeed healthy as a quantum theory. Moreover, it reduces to GR at low energies.

And it is extremely constrained, i.e. there's only one free parameter and a discrete small set of possible superstring theories. Even the spacetime dimension is constrained.

The downside is that string theory has a large number of vacua and virtually infinite ways of getting out the standard model, and no dynamical process to select one. It is therefore pretty difficult to guess all that happens inbetween and so to make any kind of low-energy prediction. One could say this is a problem of *any* quantum gravity proposal.

Other quantum gravity approaches such as LQG instead have yet to be proven to reduce to GR in the classical limit. There's considerable difficulty building a classical limit, and there is no reasonable argument for its existence. Maybe if there's someone working on the LQG side and correct this he can chime in.