Double slit with particles so heavy that they curve spacetime

[greypilgrim]

Hi.

I vaguely remember an article in a popular science magazine that tried to illustrate the problem of the reconcilability of QM and GR with the a double slit experiment with particles that are so heavy that they substantially curve spacetime, and since this is nonlinear, a quantum superposition description wouldn't work (I can't remember the exact conclusion though, and also can't find the article anymore).

Is that a valid point? What would string theory or loop quantum gravity predict?

 

[PeterDonis]

I vaguely remember an article in a popular science magazine

Which would not be a good source for actual science.

 

[PeterDonis]

Moderator's note: Thread moved to the beyond the standard models forum.

 

[PeterDonis]

What would string theory or loop quantum gravity predict?

I don't think either of these is developed to the point where they make any definite prediction for such a scenario.

 

[Demystifier]

Electrodynamics (interaction of EM field with charged fields) is also nonlinear, but quantum electrodynamics is linear and the superposition principle is valid. The point is that electrodynamics, gravity, etc. are non-linear in the sense that Hamiltonian is a non-quadratic function of fields, but the corresponding quantum theory is linear in the sense that the equations are linear in the quantum state. For example, the Wheeler-DeWitt equation of quantum gravity is something like
$$H[\pi,g]|\psi\rangle =0 $$
which is linear in ##|\psi\rangle##, but ##H[\pi,g]## is a complicated (non-quadratic) function of metric fields ##g##.

 

[greypilgrim]

Trying to find that article, I came across this lecture for the general audience:
https://scientifica.ch/event/vom-versuch-das-groesste-mit-dem-kleinsten-zu-vereinen/
It's in German, but the slides at 30:10 illustrate the point the professor is trying to make: If the double slit experiment is performed with structures that count as observers themselves, the "equations of quantum mechanics" predict that they will see interference, while an outside observer will not.

I'm not sure what "equations of quantum mechanics" he exactly means, but are those conclusions generally accepted? After all, the professor is co-author of the (in)famous Frauchiger-Renner paper which seems to be pretty controversial.

Also, are Wigner's friend experiments like this one really considered a major challenge in the reconcilability of QM and GR?