Contrary to what you may occasionally hear, "we can do quantum field theory on the curved spacetime background of general relativity.
It is also possible to introduce quantum matter as a source of gravitation, but only in a rather inelegant way. Quantum matter is represented mathematically using quantities that do not behave as numbers.
What we would like to have is more than an ad hoc semiclassical equation, but a proper quantum field theory of gravitation or equivalent. The problem with gravitation starts with its coupling constant, Newton’s constant of gravitation. This is a dimensioned constant, that is, it has units attached. In units preferred by particle physicists, the gravitational constant has units of length squared or units of inverse mass squared. It is known that a theory with such a coupling constant is not renormalizable: that is, the usual technique of removing the infinities that arise in a quantum field theory and produce consistently finite results do not work for gravitation.
This is a problem that so far found no satisfactory resolution. Semiclassical gravity works but it is inelegant. For a while, there was hope that in gravitation the unwanted infinities cancel out each other anyway but that has not been the case. Many different approaches have since been tried, ranging from novel approaches to quantizing gravity to not quantizing gravity at all. Ultimately, I think the real problem is that beyond semiclassical gravity, Nature offered no hints so far. Much as we’d like to think that we are smart enough to figure out things on our own, that has never been the case: Physics is dead without data.
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