Experimental Status
There are cases where semiclassical gravity breaks down. For instance, if M is a huge mass, then the superposition
where A and B are widely separated, then the expectation value of the stress-energy tensor is M/2 at A and M/2 at B, but we would never observe the metric sourced by such a distribution. Instead, we decohere into a state with the metric sourced at A and another sourced at B with a 50% chance each.
Read more about this topic: Semiclassical Gravity
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