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" <marchal.domain.name.hidden>
To: <everything-list.domain.name.hidden>
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" <meekerdb.domain.name.hidden>
> > To: <everything-list.domain.name.hidden>
> > 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" <jackmallah.domain.name.hidden>
> >>> To: <everything-list.domain.name.hidden>
> >>> Sent: Thursday, March 12, 2009 03:47 AM
> >>> Subject: Re: Changing the past by forgetting
> >>>
> >>>
> >>>
> >>>
> >>> --- On Tue, 3/10/09, Saibal Mitra <smitra.domain.name.hidden> 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/
>
>
>
>
> >
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Received on Tue Apr 21 2009 - 19:37:45 PDT