I suspect that he was considered a crackpot at first, but now that we have 100s of millions of dollars being spent on these dead-ends of physics, he is revered as an insightful genius. He has been mentioned as a candidate for a Nobel Prize, if anyone ever finds any evidence for anything he says.
The MWI has two fatal flaws. First, there is no empirical evidence for it, and there can never be any such evidence. Second, it destroys the probabilistic predictions that are at the heart of quantum mechanics and every other science.
Deutsch has posted a new paper addressing these issues. There is no physics in the article; just philosophical hand-waving:
Claims that the standard methodology of scientific testing is inapplicable to Everettian quantum theory, and hence that the theory is untestable, are due to misconceptions about probability and about the logic of experimental testing. Refuting those claims by correcting those misconceptions leads to various simplifications, notably the elimination of everything probabilistic from fundamental physics (stochastic processes) and from the methodology of testing ('Bayesian' credences).By "Everettian", he means MWI, and he shortens it to just "quantum theory", as if that were the most sensible interpretation. Copenhagen and other textbook interpretation are called "collapse" variants. The collapse is the idea that you refuse to consider the alternative (unobservable) universes.
Deutsch flips the arguments with a philosophical sleight-of-hand. He credits Karl Popper's rejection of positivism that a good scientific explanation is much more important than a crucial experiment. He agrees with the philosophers who say that there is no such thing as the crucial experiment. MWI doesn't explain any experiments but it does give a good explanation, so he says that it is philosophically superior to collapses.
He goes further and denies that any probabilistic theories are truly testable, and only something like MWI, which says that anything can happen without any probability estimates, should be considered testable. He concludes:
By adopting Popper’s explanatory, conjectural conception of science, and his objective, problem-based methodology of scientific testing (instead of ones that are subjective, inductivist, positivist, ‘Bayesian’ etc.), and bearing in mind the decision-theoretic argument, we can eliminate the perceived problems about testing Everettian quantum theory and arrive at several simplifications of methodological issues in general.I cannot make any sense of this. All of science involves some sort of comparison of theory with experiment. The measurements never match up exactly, so we are always left with the problem of deciding whether the observations are within the margins of what the theory said was likely. There is no other way to do science, as far as I know.
In particular, I have shown that the claim that the standard methods of testing are invalid for Everettian quantum theory depends on adopting a positivist or instrumentalist view of what the theory is about. The claim evaporates, given that science is about explaining the physical world.
Even ‘everything-possible-happens’ theories can be testable. But Everettian quantum theory is more than an everything-possible-happens theory. Because of its explanatory structure (exploited by, for instance, the decision-theoretic argument) it is testable in all the standard ways. It is the predictions of its ‘collapse’ variants (and any theory predicting literally stochastic processes in nature) that are not genuinely testable: their ‘tests’ depend on scientists conforming to a rule of behaviour, and not solely on reality conforming to explanations.
If a theory gives probabilities and error estimates, as all good scientific theories do, then Deutsch says that it is not testable. If a theory says that everything possible happens, as MWI does, then Deutsch says that it is testable.
There is no merit to anything Deutsch says. Popper was wrong in his rejection of positivism. Duhem-Quine were wrong in their rejection of the crucial experiment. MWI is incoherent. Quantum computing is a pipe dream. You can reverse almost everything he says, and get closer to the truth.
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