Re: MWI, Copenhage, Randomness

From: Bruno Marchal <>
Date: Fri, 6 Sep 2002 17:54:23 +0200

Jesse Mazer wrote

>Ok, I think I see where my mistake was. I was thinking that
>"decoherence" just referred to interactions between a system and the
>external environment, but what you seem to be saying is that it can
>also refer to an internal effect where interactions among the
>components of a system with many degrees of freedom cause
>interference terms to become negligible. If that's correct, then
>when Wigner decided that interference would cause the wavefunction
>of the cat or Wigner's friend to "collapse" even before the box or
>the room was opened, then he was probably referring to this sort of
>internal effect, so my argument about using quantum computers to
>simulate truly impenetrable boxes would not make a difference.

"decoherence" refers to anything interacting with what you are, as
observer, describing by a wave function, and which is not currently described
by your wave function. (-> need of a tensor product).
IMO, it has been discovered by Everett and it explained why we don't feel the
split or the differentiation. Decoherence is just entanglement with the
the environment, it is the contagion of the superposition state, the
linearity of the tensor product.

>But this makes me wonder about the thought-experiment by David
>Deutsch which Hal Finney brought up, in which interference shows
>that an isolated A.I. was splitting into multiple versions which
>experienced different outcomes. Presumably a simulation of an
>intelligence would have a lot of degrees of freedom too, so why
>wouldn't decoherence ruin things?

Ok, but then SWE is wrong at some point. Where do you think?

>Maybe since this is a computer simulation where we know the
>dynamical rules and initial state precisely, we would know just
>where to look for even the smallest interference effects, unlike in
>an ordinary macroscopic system where we don't have such detailed
>information. Also, we could run such a simulation over and over
>again from the same initial conditions, which would also help to
>detect small statistical deviations from classical predictions. I
>once read a comment by Deutch about decoherence where he said
>something like (paraphrasing) "saying the interference terms are
>'almost' zero is like saying someone is a little bit pregnant." His
>argument would probably be that although decoherence may explain why
>the world looks approximately classical in the many-worlds
>framework, it doesn't remove to postulate those other worlds in the
>first place.

I don't understand your last sentence.

Received on Fri Sep 06 2002 - 08:58:01 PDT

This archive was generated by hypermail 2.3.0 : Fri Feb 16 2018 - 13:20:07 PST