Accepting QM without collapse, I am not sure you can dump your memory in the environment in any truly irreversible way.
Bruno On 21 Apr 2009, at 15:22, Saibal Mitra wrote: > > Yes, I agree, and that's then why we cannot do this in practice. The > verification of the MWI would have to wait untilk we have artificially > intelligent observers implemented by quantum computers. > > However, ass uming that the MWI is indeed correct, it doesn't matter > if you > undo the measurement. If you just dump your memory in the nvironment > in an > irreversible way, you end up in a superposition like: > > |you>[ |universe_1| + |universe_2> ] > > As far as |you> are concerned, it doesn't matter if |universe_1> and > |universe_2> differ by one electron state or the state of 10^23 > particles: > the result of a new measurement is not pre-determined in either case. > > > ----- Original Message ----- > From: "Brent Meeker" <meeke...@dslextreme.com> > To: <everything-list@googlegroups.com> > Sent: Sunday, March 15, 2009 08:06 PM > Subject: Re: Changing the past by forgetting > > >> >> Saibal Mitra wrote: >>> If we consider measuring the spin of a particle, you could also >>> say that > the >>> two possible outcomes just exist and thatthere are two possible >>> future >>> versions of me. There is no meaningful way to associate myself with > either >>> of the two outcomes. >>> >>> But then, precisely this implies that after a measurement and >>> forgetting >>> about the result will yield a version of me who is in a similar >>> position > as >>> that earlier version of me who had yet to make the measurement. If >>> one > could >>> perform measurements in a reversible way, this would be possible to >>> experimentally confirm, as David Deutsch pointed out. You can >>> start with > a >>> spin polarized in the x direction. Then you measure the z-component. > There >>> then exists a unitary transformation which leads to the observer > forgetting >>> about the outcome of the measurement and to the spin to be >>> restored in > the >>> original state. The observer does remember having measured the > z-component >>> of the spin. >>> >>> Then, measuring the x-component again will yield "spin-up" with 100% >>> probability, confirming that both branches in which the observer > measured >>> spin up and spin down have coherently recombined. This then proves >>> that > had >>> the observer measured the z-component, the outcome would not be a >>> priori >>> determined, despite the observer having measured it earlier. So, >>> both >>> branches are real. But then this is true in general, also if the >>> quantum >>> state is of the form: >>> >>> |You>[|spin up>|rest of the world knows the spin is up> + |spin > down>|rest >>> of the world knows spin is down>] >> >> You're contemplating reversing three different things: >> >> 1) Your knowledge, by forgetting a measurement result. Something >> that's > easy to do. >> >> 2) The spin state of a particle. >> >> 3) The state of what the rest of the world knows. >> >> Because of the entanglement, I don't think you can, in general, >> reverse > the spin >> state of the particle without reversing what is known about it by >> "the > rest of >> the world". >> If it was a known state (to someone) the particle can easily be put >> back > in that >> state. But to do so for a general, unknown state, after a >> measurement > would >> require invoking time-reversal invariance of the state of whole >> universe > (or at >> least all of it entangled with the particle spin via the measuring > apparatus). >> >> Brent Meeker >> >>> >>> although you cannot directly verify it here. But that means that you > cannot >>> rule out an alternative theory in which only one of the branches >>> is real >>> when performing a measurement in this case. But if the reality of >>> both >>> branches is accepted, then each time you make a measurement and you > don't >>> know the outcome, the outcome is not fixed (proovided, of course, >>> there > is >>> indeed more than one branch). >>> >>> >>> ----- Original Message ----- >>> From: "Jack Mallah" <jackmal...@yahoo.com> >>> To: <everything-list@googlegroups.com> >>> Sent: Thursday, March 12, 2009 03:47 AM >>> Subject: Re: Changing the past by forgetting >>> >>> >>> >>> >>> --- On Tue, 3/10/09, Saibal Mitra <smi...@zeelandnet.nl> wrote: >>>> http://arxiv.org/abs/0902.3825 >>>> >>>> I've written up a small article about the idea that you could end >>>> up in > a >>> different sector of the multiverse by selective memory erasure. I >>> had >>> written about that possibility a long time ago on this list, but now > I've >>> made the argument more rigorous. >>> >>> Saibal, I have to say that I disagree. As you acknowledge, erasing > memory >>> doesn't recohere the branches. There is no meaningful sense in >>> which > you >>> could end up in a different branch due to memory erasure. >>> >>> You admit the 'effect' has no observable consequences. But it has >>> no >>> unobservable meaning either. >>> >>> In fact, other than what I call 'causal differentiation', which >>> clearly > will >>> track the already-decohered branches (so you don't get to >>> reshuffle the >>> deck), there is no meaningful sense in which "you" will end up in >>> one >>> particular future branch at all. Other than causal differentiation >>> tracking, either 'you' are all of your future branches, or 'you' are > just >>> here for the moment and are none of them. >> >>> > > > > http://iridia.ulb.ac.be/~marchal/ --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Everything List" group. 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