every particle interaction creates n parallel universes which never physically interfere with each other”
Although a fairly standard way of explaining MWI, this is an example of conflating coherence and decoherence. To get branches that never interact with each other again, you need decoherence, but decoherence is a complex dynamical process..it takes some time...so it is not going to occur once per elementary interaction. It’s reasonable to suppose that elementary interactions produce coherent superpositions, on the other hand, but these are not mutually isolated “worlds”. And we have fairly strong evidence for them..
quantum computing relies on complex coherent superpositions....so any idea that all superpositions just automatically and instantly decohere must be rejected.
“David Deutsch, one of the founders of quantum computing in the 1980s, certainly thinks that it would. Though to be fair, Deutsch thinks the impact would “merely” be psychological – since for him, quantum mechanics has already proved the existence of parallel uni- verses! Deutsch is fond of asking questions like the following: if Shor’s algorithm succeeds in factoring a 3000-digit integer, then where was the number factored? Where did the computational resources needed to factor the number come from, if not from some sort of “multiverse” exponentially bigger than the universe we see? To my mind, Deutsch seems to be tacitly assuming here that factoring is not in BPP – but no matter; for purposes of argument, we can certainly grant him that assumption. It should surprise no one that Deutsch’s views about this are far from universally accepted. Many who agree about the possibility of building quantum computers, and the formalism needed to describe them, nevertheless disagree that the formalism is best interpreted in terms of “parallel universes.” To Deutsch, these people are simply intellectual wusses – like the churchmen who agreed that the Copernican system was practically useful, so long as one remembers that obviously the Earth doesn’t really go around the sun. So, how do the intellectual wusses respond to the charges? For one thing, they point out that viewing a quantum computer in terms of “parallel universes” raises serious difficulties of its own. In particular, there’s what those condemned to worry about such things call the “preferred basis problem.” The problem is basically this: how do we define a “split” between one parallel universe and another? There are infinitely many ways you could imagine slic- ing up a quantum state, and it’s not clear why one is better than another! One can push the argument further. The key thing that quantum computers rely on for speedups – indeed, the thing that makes quantum mechanics different from classical probability theory in the first place – is interference between positive and negative amplitudes. But to whatever extent different “branches” of the multiverse can usefully interfere for quantum computing, to that extent they don’t seem like separate branches at all! I mean, the whole point of inter- ference is to mix branches together so that they lose their individual identities. If they retain their identities, then for exactly that reason we don’t see interference. Of course, a many-worlder could respond that, in order to lose their separate identities by interfering with each other, the branches had to be there in the first place! And the argument could go on (indeed, has gone on) for quite a while. Rather than take sides in this fraught, fascinating, but perhaps ultimately meaningless debate...”..Scott Aaronson , QCSD, p148
Although a fairly standard way of explaining MWI, this is an example of conflating coherence and decoherence. To get branches that never interact with each other again, you need decoherence, but decoherence is a complex dynamical process..it takes some time...so it is not going to occur once per elementary interaction. It’s reasonable to suppose that elementary interactions produce coherent superpositions, on the other hand, but these are not mutually isolated “worlds”. And we have fairly strong evidence for them.. quantum computing relies on complex coherent superpositions....so any idea that all superpositions just automatically and instantly decohere must be rejected.