🚀 go-pugleaf

orourke@sophia.smith.edu (Joseph O'Rourke) writes:

>In article <1992Dec11.011556.17556@galois.mit.edu> jbaez@riesz.mit.edu (John C. Baez) writes:
>>Nada.  (Nothing.  Nichts.)  Everett's relative state formalism doesn't
>>involve any notion about "worlds splitting" - and thus doesn't require
>>any answer to your question.  

>	My understanding of Everett's work is shallow, and I certainly
>accept your answer, and am grateful for the reply.  But let me quote
>Penrose ("The Emporer's New Mind", p.295):

>	"The consciousness of each observer is regarded as 'splitting'...
>	Indeed, not just the observer, but the entire universe that he
>	inhabits splits in two (or more) at each 'measurement' that he
>	makes of the world."

>So you can see where non-physicists like me might pick up the notion
>of measurements causing world splits.

You can use that language to describe vaguely what is going on, but
it's misleading.  What happens is that the wave function of the
world ends up in a superposition of different states that evolve in
a more or less classical manner, with very, very little interference
between the two.  The "worlds" are just these different nearly-
classical regions of the wave function.  It's a kind of "world-
splitting," but it's important to realize that:

(1) The "other worlds" aren't anywhere "else" in space, or in
    another spacetime manifold, or extra dimensions, or any such
    thing; the world is just superposed.  It's misleading to refer
    to states as multiple universes because it implies that something
    is selecting out measurement processes as special and using them
    to spawn new universes like budding yeast.  Though such budding
    occurs in certain speculative cosmological theories, it has 
    nothing to do with the process going on here.

(2) No extra machinery is required to throw off these "other worlds";
    the superposition arises naturally from the same kind of time
    development we normally see in quantum mechanics, applied to
    everything consistently.

(3) The only way the existence of the other nearly-classical parts
    of the universal wave function could be detected would be via
    experiments that, roughly speaking, get exponentially difficult
    with time after the various states get macroscopically different, 
    with an extremely short characteristic time scale.
    Under such conditions one can, with an *extremely* small loss
    of accuracy, take one of the nearly-classical states as the
    state of the world, and not bother about the others.  If you
    like, you can call the others "unreal," and unless you do certain
    implausibly sensitive experiments it doesn't change the physics.
    This is roughly what corresponds to "collapse" in the Everett
    interpretation, but it's not an extra postulate, just a
    consequence of ordinary dynamics.
-- 
Matt McIrvin