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 - 19:27:28 PDT