Harold: It seems to me that there has been an enormous amount of resources spent on the various quantum gravity programs. Is there an actual proof that gravity has to be quantized at all?

Steph: Well, that depends on what you think would constitute a proof. Does classical mechanics have to be quantized? Apparently. Do we have a proof to that effect? No. We have a theory, it makes predictions and seems to agree with nature so we accept it. By the nature of the whole enterprise though, if we encounter a prediction that is wrong then we have to give up the theory. So, does gravity need to be quantized? I don't know. What I do know is that it is a fundamental interaction. The other three fundamental interactions all seem to have consistent quantum descriptions and, personally, I find that three out of four having quantum descriptions and only one being completely classical is unappealing. But maybe this is just the way nature is.

Harold: Where does the need to quantize gravity come from except for a belief in unification which may or may not be satisfied in reality?

Steph: I would say that it comes from a belief that at sufficiently small scales all the interactions exhibit quantum behaviour.