I Is QM Deterministic in MWI and Time Reversible?

[A. Neumaier]

to find a consistent description of all phenomena (including a quantum description of gravity)

How could this ever be done without treating the universe as a quantum system?

 

[vanhees71]

We have a classical model of the universe without having ever observed it as a whole. There's also some philosophical debate, whether cosmology is really science since it assumes the Copernican principle without being ever able to test this hypothesis by observation or at least only to a certain extent.

That's why it is so complicated to get a clue about the quantum theory of gravitation: To get an idea, how to describe it, I think we'd need some phenomena related to it, and such phenomena are available to us only in a quite small piece of the universe, i.e., sufficiently close to us. Maybe we need strong gravitational fields to see deviations from classical behavior like better and better observations of black holes in our and other galaxies. Who knows?

 

[A. Neumaier]

We have a classical model of the universe without having ever observed it as a whole.

Therefore we can also expect to work successfully with a quantum model of the universe without having ever observed it as a whole. The thermal interpretation gives it a workable interpretation without weird, counterintuitive features.

 

[vanhees71]

You are always referring to the "universe as a whole" not I ;-)).

 

[A. Neumaier]

You are always referring to the "universe as a whole" not I ;-)).

Sure, we can model the universe as a whole without having observed the whole universe.

We also model a univariate function as a whole already when we observed it only at a fairly small number of points. Otherwise we couldn't use the concept of a function in physics. Modeling an infinitely extended universe is not really different in principle from modeling an infinitely extended function.