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Mudassir Ali

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This isn’t the crazy question you might think it is.

However, there are consistency problems if one assumes gravity is purely classical and everything else is quantum mechanical. These problems are already present in the famous Schrodinger’s cat thought experiment, which tries to couple radioactive decay — a purely probabilistic event — to whether a classical cat is alive or dead. You end up with the absurd consequence that a cat can be in a superposition of being dead or alive and, so, there must be a quantum theory of cats. Similarly, there must be a quantum theory of gravity.

There is another problem though that indicates gravity must be quantum mechanical. We now know that quantum field theory near a classical blackhole predicts Hawking radiation. We know, from general conservation laws, that the emission of that Hawking radiation should cause black holes to evaporate as it loses energy (this is a good thing, otherwise even the tiniest black hole would grow larger). The problem is that there is no mechanism for this to happen in classical gravity. We are led to another inconsistency that we can only resolve by quantizing gravity.

EDIT: At the risk of weakening my answer, these consistency problems actually strongly suggest but do not prove outright that gravity should be quantized. This highlights, once again, that this is a good question. Apparently (see comments) there may be loopholes that would still allow a non-quantum gravity coupled to quantum everything else. Nevertheless, gravity being quantum mechanical seems like a good bet to me.