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But why does that unifcation

*have to*mean quantization of gravity?

Of course, there is the Einstein field equation. If the stress-energy tensor is the expectation value of some quantum state, but spacetime and the metric are classical entities, then this will lead to difficulties when we measure the quantum states of matter (see Wald's GR book p.382).

But on the other hand, since probing distances shorter than the Planck length is already forbidden by classical GR (creations of black holes in ultra-high energy scattering, all singularities are hidden via the censorship theorem for black holes and inflation for the big bang), might there not some conspiracy by Nature that sweeps these inconsistencies under the rug?

In Zee new GR book (p.766) I even read there is a "classificalization of gravity" program, which claims "..it maybe that when quantum gravity enters into Planck distance scales, quantum physics will inturn be replaced by classical physics somehow."

So my question again: why do we need to quantize gravity?

thank you!