Graduate Gravity in the Thermal Interpretation

[PeterDonis]

TL;DR

Does gravity need to be quantized in the thermal interpretation of QM?

This question is mainly for @A. Neumaier, but I post it in public in case others are interested.

The usual reason given for needing to quantize gravity is, heuristically, that, in the presence of quantized stress-energy where there can be a superposition of different stress-energy tensors, there must also be a superposition of different spacetime geometries.

One proposal for avoiding having to do this, at least as an approximation, is the "semiclassical" Einstein Field Equation, where the spacetime geometry is still treated classically, and the source on the RHS of the Einstein Field Equation is the expectation value of the stress-energy tensor. This is normally considered an approximation because in QM expectation values are normally not considered as fundamental.

In the thermal interpretation of QM, however, q-expectations are considered fundamental "beables". That raises an obvious question: would the "semiclassical" EFE even need to be treated as an approximation in the thermal interpretation? Or could it be considered as the fundamental equation of gravity, since the q-expectation of the stress-energy tensor is considered a fundamental "beable" in this interpretation? In short, would this remove the need to quantize gravity in the thermal interpretation?

[1] https://www.physicsforums.com/threads/the-thermal-interpretation-of-quantum-physics.967116/

 

[A. Neumaier]

In the thermal interpretation of QM, however, q-expectations are considered fundamental "beables". That raises an obvious question: would the "semiclassical" EFE even need to be treated as an approximation in the thermal interpretation? Or could it be considered as the fundamental equation of gravity, since the q-expectation of the stress-energy tensor is considered a fundamental "beable" in this interpretation? In short, would this remove the need to quantize gravity in the thermal interpretation?

Semiclassical gravity is consistent in the thermal interpretation, as is any mixed quantum-classical dynamics following standard patterns. (See Section 7.8 of my book.)

Thus there is no absolute need for quantizing gravity. However, since all other quantum-classical dynamical systems studied are approximations of a more fundamental purely quantum dynamical system, it appears very likely (to me, on the grounds of beauty and uniformity) that the same holds for quantum gravity.