> On 4 May 2018, at 15:56, Bruce Kellett <bhkell...@optusnet.com.au> wrote: > > >> From: Bruno Marchal <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>> >>> On 4 May 2018, at 01:03, Bruce Kellett < >>> <mailto:bhkell...@optusnet.com.au>bhkell...@optusnet.com.au >>> <mailto:bhkell...@optusnet.com.au>> wrote: >>> >>> >>> Actually, this is the basis of MWI -- everything in physics is based on >>> unitary transformations. The Schrödinger equation can be derived by >>> assuming time evolution is unitary. So, in the wider context, everything, >>> even decoherence into the wider universe, is reversible, in the sense that >>> there is a unitary transformation that, when applied to any final state, >>> restores the initial state -- just take the unitary operator that describes >>> the time evolution, say U, and then take its inverse, U^{-1}. >>> >>> The problem, of course, is that this unitary operator is formed in the >>> multiverse, so to form its inverse we have to have access to the other >>> worlds of the multiverse. And this is impossible because of the linearity >>> of the SE. So although the mathematics of unitary transformations is >>> perfectly reversible, measurements are not reversible in principle in the >>> one world we find ourselves to inhabit. >>> >>> So even Deutsch's quantum brain is likely to run into difficulties, since >>> it has to communicate with the real world. >> >> OK. But that is the same with any quantum computer. Are you saying that >> quantum computing is not possible in practice? > > No, quantum computing should be possible with sufficient protection against > decoherence. > >> There are quantum algorithm capable of “fighting by quantum error >> procedures” the effect of decoherence. Imo, the Deutsch experiment is as >> much possible as a working quantum computer. I am pretty sure this is >> technologically possible, although plausibly not even in a near future, but >> soon after :) > > The problem with Deutsch's thought experiment is that everything takes place > within the quantum computer, so no real measurement has ever been made. > Measurement involves decoherence and the effectively permanent splitting of > branches. No quantum computer can work in such circumstances. Calling the > unmeasured elements of a superposition "worlds" as Deutsch does, equivocates > on the orthogonality inherent in an operational concept of a "world". If > elements of a superposition can interfere, they are not separate worlds. > Deutsch's idea is sunk by the "preferred basis" problem. It is only > decoherence into the external world that can fix the basis (by einselection).
I agree with all what you say. Everett convinced me of the higher plausibility of MW only when I realise that the relative results of local measurements do not depend on the choice of the basis. Deutsch notion of world is too much naive (and does not make already sense only with computationalism). 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 everything-list+unsubscr...@googlegroups.com > <mailto:everything-list+unsubscr...@googlegroups.com>. > To post to this group, send email to everything-list@googlegroups.com > <mailto:everything-list@googlegroups.com>. > Visit this group at https://groups.google.com/group/everything-list > <https://groups.google.com/group/everything-list>. > For more options, visit https://groups.google.com/d/optout > <https://groups.google.com/d/optout>. -- 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 everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
