Re: Changing the past by forgetting

From: Saibal Mitra <>
Date: Sun, 15 Mar 2009 17:42:50 +0100

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>]

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" <>
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 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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Received on Sun Mar 15 2009 - 12:43:08 PDT

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