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-l...@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. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Everything List" group. To post to this group, send email to everything-l...@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 -~----------~----~----~----~------~----~------~--~---
