That's correct. It is not really irreversible. The point is that it doesn't matter as you end up in a state where the outcome of finding out what happened is not pre-determined.

Saibal ----- Original Message ----- From: "Bruno Marchal" <marc...@ulb.ac.be> To: <everything-list@googlegroups.com> Sent: Tuesday, April 21, 2009 07:27 PM Subject: Re: Changing the past by forgetting > > 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. To post to this group, send email to everything-list@googlegroups.com To unsubscribe from this group, send email to everything-list+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/everything-list?hl=en -~----------~----~----~----~------~----~------~--~---