Accepting QM without collapse, I am not sure you can dump your memory
in the environment in any truly irreversible way.
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
> the result of a new measurement is not pre-determined in either case.
> ----- Original Message -----
> From: "Brent Meeker" <meeke...@dslextreme.com>
> To: <firstname.lastname@example.org>
> 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
>>> 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
>>> about the result will yield a version of me who is in a similar
>>> that earlier version of me who had yet to make the measurement. If
>>> 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
>>> the observer measured the z-component, the outcome would not be a
>>> determined, despite the observer having measured it earlier. So,
>>> branches are real. But then this is true in general, also if the
>>> 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
> 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,
> the spin
>> state of the particle without reversing what is known about it by
> rest of
>> the world".
>> If it was a known state (to someone) the particle can easily be put
> in that
>> state. But to do so for a general, unknown state, after a
>> require invoking time-reversal invariance of the state of whole
> (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
>>> branches is accepted, then each time you make a measurement and you
>>> know the outcome, the outcome is not fixed (proovided, of course,
>>> indeed more than one branch).
>>> ----- Original Message -----
>>> From: "Jack Mallah" <jackmal...@yahoo.com>
>>> To: <email@example.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:
>>>> 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
>>> 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
>>> could end up in a different branch due to memory erasure.
>>> You admit the 'effect' has no observable consequences. But it has
>>> unobservable meaning either.
>>> In fact, other than what I call 'causal differentiation', which
>>> 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
>>> 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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