> On 28 Jan 2021, at 06:58, Bruce Kellett <[email protected]> wrote: > > On Thu, Jan 28, 2021 at 3:42 PM Pierz Newton-John <[email protected] > <mailto:[email protected]>> wrote: > > On 28 Jan 2021, at 2:49 pm, 'Brent Meeker' via Everything List > > <[email protected] > > <mailto:[email protected]>> wrote: > > On 1/27/2021 5:11 PM, Pierz Newton-John wrote: > >> I’m not saying decoherence is reversible. I’ve corrected myself (or > >> accepted your correction) on that point. But my understanding of proposals > >> for disconfirming MWI involve extending quantum coherence to larger and > >> larger scales. Deutsch has argued that if we can get enough qubits into a > >> quantum computation, we’ve effectively “proved” MWI since “where did all > >> that information come from?". > > > > And Scott Aaronson has pointed out it all had to be in this world in order > > for interference to produce an answer. > > > That argument rests on the definition of world as a decohered branch, and > Deutsch would not accept that definition. > > > I agree. Deutsch has his own idiosyncratic definition that does not accord > with common usage. The trouble is that the word 'world' has an ordinary, > everyday use, and if you make a technical definition of something that is > quite different from the everyday meaning, but use the same word, you open > yourself to equivocation and invalid arguments. > > Deutsch is using a technical definition of 'world' which corresponds to the > notion of a distinct basis for a particular Hilbert space. So, in his terms, > there are separate 'worlds' for every possible basis. In common usage, the > word 'world' is reserved for bases that are singled out by decoherence as the > preferred, stable bases. If you use the word 'world' for any and every one > of the infinite number of possible bases for any given Hilbert space, then > you have deprived the word of any sensible referent or meaning. The word > 'branch' is likewise unavailable to Deutsch, because it has a similar > everyday meaning to 'world ' in this context. He could use the term 'Hilbert > space basis', but that robs his catch phrase of its impact : "Quantum > computers work because every possible calculation is performed in some basis > of the Hilbert space." Bases in Hilbert space do not evidently have the same > computational resources as other worlds. So Deutsch's argument rests on an > equivocation between the meanings of 'world'. His argument is patently > invalid. > > > I’m not sure if I agree with his argument, but I’m also not necessarily > convinced by that definition of “world”. I mean, it’s perfectly good as far > as it goes, but I’m not sure I’m happy with it being marshalled as an > argument in this way. If there is a world W which contains an electron in an > up/down superposition, then in the Deutsch picture, and I would say the > Everett picture in general, that means some observer in W is unaware of which > world he/she is in: the one where the electron is up or the one where it is > down. Or rather (and this is Deutsch not Everett), the stack of worlds where > it is up or the stack where it is down.The measurement leaks that information > via decoherence, and the worlds diverge irretrievably at that point. > > > This is certainly a problem for Deutsch's interpretation of 'world'. Because > there are an infinite number of equivalent sets of basis vectors available > for every Hilbert space, it makes little sense to claim that an observer is > uncertain as to which basis he is in. He could choose any basis whatsoever. > But if he wants his choice to make sense in his lived life, he would be wise > to choose the basis that is singled out by decoherence as stable against > environmental degradation. In other words, he has to rely on decoherence to > solve the basis problem. Deutsch has no way of resolving the preferred basis > problem in his approach since, to him, all bases correspond to equivalent > 'worlds’.
That is why it is preferable to abandon the idea of “world” (an idea which BTW belongs more to metaphysics than physics) and use the “relative state”, or the “history” notions instead. Decoherence is irreversible from inside the multiverse for the same reason that statistical physics is reversible, in Everett. The whole “universe” remains “in principle” reversible, bit not from inside, unless amnesia and ultra-sophisiticated technology (which doubtfully could ever exist). Obviously, something like “the whole universe” is not yet definable, as this would need a quantum theory of gravity. Bruno > > Bruce > > -- > You received this message because you are subscribed to the Google Groups > "Everything List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] > <mailto:[email protected]>. > To view this discussion on the web visit > https://groups.google.com/d/msgid/everything-list/CAFxXSLSRP6%3DOZrEeULtgp7DEZRKAeUWJ8r%3DZDaLaESQMm-64sw%40mail.gmail.com > > <https://groups.google.com/d/msgid/everything-list/CAFxXSLSRP6%3DOZrEeULtgp7DEZRKAeUWJ8r%3DZDaLaESQMm-64sw%40mail.gmail.com?utm_medium=email&utm_source=footer>. -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/FD5ED114-809D-4452-ABED-DCA863C947BF%40ulb.ac.be.
