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" <[email protected]> To: <[email protected]> Sent: Thursday, March 12, 2009 03:47 AM Subject: Re: Changing the past by forgetting --- On Tue, 3/10/09, Saibal Mitra <[email protected]> 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. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Everything List" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/everything-list?hl=en -~----------~----~----~----~------~----~------~--~---
