What do you think about question X in quantum gravity?

I don’t write a lot about quantum gravity, mainly because my interest in the subject is somewhat limited. The main reason for this is that the problem of just finding a quantum theory of gravity comes with the danger that even if you find one, you will have no way of knowing whether it has anything to do with reality. Quantum gravitational effects are unobservably small, so experiment likely will be of no help. Often the hope is that a unique and convincing way to quantize gravity will be found, but all evidence seems to be that popular ways of consistently quantizing gravity will lead to an arbitrarily large number of possibilities, with no way to tell between them. The string theory landscape is one example of this.

How to quantize gravitational degrees of freedom remains a deep and attractive problem, but my suspicion is that a successful solution to this problem will require understanding the relationship between gravitation and standard model degrees of freedom, unification if you will. Only with such unification is there likely to be enough connection between the rest of physics and a specific quantum gravity theory to get indirect evidence for it. String theory became very popular exactly because it promised to provide this sort of link, a promise which unfortunately did not turn out.

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