There’s no such thing as a “many worlds theorem.” Many worlds is an interpretation. There’s a genuine case for it but the case is philosophical, and remains argued about by people who understand everything there is to know about the subject. ...I challenged him on this, and he replied:
No, many-worlders and non-many-worlders make exactly the same predictions for what QCs will and won’t be able to do. That’s why many worlds is an “interpretation,” rather than a competing empirical theory!
I’m aware of all of this. The hardcore many-worlders think that non-many-worlders have a nonsense theory from which one shouldn’t be able to make predictions at all, and the hardcore Copenhagenists, QBists, etc. think exactly the same of many-worlders. Nevertheless, they do make the same predictions, regardless of whether they should! 🙂: The I no longer agree with calling many-worlds an "interpretation". It would be if they accepted the empirical recipe of QM, but they don't.
At least, they do to whatever extent they accept the empirical recipe of QM. People who deny the empirical recipe are (I’d say) neither many-worlders nor Copenhagenists nor QBists nor etc., but believers in a rival physical theory (whether or not they have clear ideas about what the rival theory is).
And as for the QC skeptics who accept QM, but believe some yet-to-be-discovered principle “censors” or “screens off” scalable QC? I’d hope that even they could still make the same conditional predictions: “yes, if it weren’t for our yet-to-be-discovered principle, then this is how a QC would operate, and this is the class of problems it could solve in polynomial time.”
The empirical recipe gives unique outcomes to experiments, but the many-worlders deny that. David Deutsch is a many-worlder, and he says a QC would prove the parallel universes.
I guess I am the skeptic who accepts QM, but believes some yet-to-be-discovered principle “censors” or “screens off” scalable QC. Scott has become a many-worlder, so he would probably also say I must believe in some yet-to-be-discovered principle that collapses the wave function.
This is not exactly my view. I am a positivist, and do not believe in yet-to-be-discovered principles. I accept what Scott calls "the empirical recipe of QM". How could I not? It works amazingly well.
I am skeptical about scalable QC.
I am also skeptical about intelligent life on other planets. Not because I believe in a yet-to-be-discovered principle. It just seems unlikely to me.
Scott's main point is to attack Kaku's new book on quantum supremacy, which I also attacked on May 6.