Re: Changing the past by forgetting

From: Saibal Mitra <>
Date: Tue, 21 Apr 2009 15:22:46 +0200

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" <>
To: <>
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
> > two possible outcomes just exist and thatthere are two possible future
> > versions of me. There is no meaningful way to associate myself with
> > 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
> > that earlier version of me who had yet to make the measurement. If one
> > perform measurements in a reversible way, this would be possible to
> > experimentally confirm, as David Deutsch pointed out. You can start with
> > spin polarized in the x direction. Then you measure the z-component.
> > then exists a unitary transformation which leads to the observer
> > about the outcome of the measurement and to the spin to be restored in
> > original state. The observer does remember having measured the
> > of the spin.
> >
> > Then, measuring the x-component again will yield "spin-up" with 100%
> > probability, confirming that both branches in which the observer
> > spin up and spin down have coherently recombined. This then proves that
> > 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
> > 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
> 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
> Brent Meeker
> >
> > although you cannot directly verify it here. But that means that you
> > 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
> > know the outcome, the outcome is not fixed (proovided, of course, there
> > indeed more than one branch).
> >
> >
> > ----- Original Message -----
> > From: "Jack Mallah" <>
> > To: <>
> > Sent: Thursday, March 12, 2009 03:47 AM
> > Subject: Re: Changing the past by forgetting
> >
> >
> >
> >
> > --- On Tue, 3/10/09, Saibal Mitra <> wrote:
> >>
> >>
> >> I've written up a small article about the idea that you could end up in
> > different sector of the multiverse by selective memory erasure. I had
> > written about that possibility a long time ago on this list, but now
> > made the argument more rigorous.
> >
> > Saibal, I have to say that I disagree. As you acknowledge, erasing
> > doesn't recohere the branches. There is no meaningful sense in which
> > 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
> > 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
> > here for the moment and are none of them.
> >

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Received on Tue Apr 21 2009 - 15:22:46 PDT

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