Okay.
lmf:
There’s a related and even worse spanner in the works of elementary particle physics: particles (or fields, strings, or whatever) are supposed to be fully quantum-mechanical entities. But the people who work on them only ever construct classical, single-universe theories. Why? Because they think that the quantum part of the theory necessarily has to be trivial.
Deutsch is just dead wrong about this.
I don’t know exactly what a “fully quantum mechanical” entity is, but I don’t think that the particles in QFT count (nor do I think strings or branes count, but that’s a more complicated discussion).
It seems odd to me that you say you don’t understand something and then continue to write without asking questions. A longer explanation of what DD is talking about can be found here: