# 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" <jackmal...@yahoo.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.

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