Trying to test Many-Worlds

David Deutsch is a big proponent of the Many Worlds Interpretation of quantum mechanics, and writes The Logic of Experimental Tests, Particularly of Everettian Quantum Theory

By adopting a conception – based on Popper’s – of scientific theories as conjectural
and explanatory and rooted in problems (rather than being positivistic, instrumentalist and rooted in evidence), and a scientific methodology not involving induction, confirmation, probability or degrees of credence, and bearing in mind the decision-theoretic argument for betting-type decisions, we can eliminate the perceived problems about testing Everettian quantum theory and arrive at several simplifications of methodological issues in general.

This is what you get when you reject positivism. You can decide to believe in parallel universes with no evidence, because only the positivists insist on being rooted in evidence. Deutsch even claims that the parallel universe theory is testable, because of some philosophical misdirection.

n explanation is bad (or worse than a rival or variant explanation) to the extent that…

(i) it seems not to account for its explicanda; or
(ii) it seems to conflict with explanations that are otherwise good; or
(iii) it could easily be adapted to account for anything (so it explains nothing).

It follows that sometimes a good explanation may be less true than a bad one (i.e. its true assertions about reality may be subset of the latter’s, and its false ones a superset).

This sort of reasoning allows him to accept explanations that are not really true.

Scientific methodology, in turn, does not (nor could it validly) provide criteria for accepting a theory. Conjecture, and the correction of apparent errors and deficiencies, are the only processes at work. And just as the objective of science isn’t to find evidence that justifies theories as true or probable, so the objective of the methodology of science isn’t to find rules which, if followed, are guaranteed, or likely, to identify true theories as true.

This is anti-positivism.

I take the positivist view that science has established the truth of theories like Newtonian gravity for celestial mechanics, within a suitable domain of applicability. The theory lets you can make observations, fit a model, and make predictions of orbits with error estimates.

Yes, general relativity makes more precise predictions in some extreme cases, and gives a more satisfactory causal explanation, but the original Newtonian theory is still valid within its limits.

But one thing that cannot be modelled by random numbers is ignorance itself (as in the ‘ignorance interpretation of probability’).

Deutsch and other many-worlds advocates are very unhappy with probability, because they have to idea how to say that some of the parallel worlds are more likely than others, and hence cannot make sense out of probabilistic experiments.

So Deutsch says that the answer is to reject positivism and be very broad-minded about what is accepted as a explanation.