From: lydick@SOL1.GPS.CALTECH.EDU (Speaker-to-Minerals)
Newsgroups: alt.callahans
Subject: Re: Pair-allel Universes
Date: 17 Dec 1992 00:54:47 GMT
Organization: HST Wide Field/Planetary Camera
Distribution: world
Reply-To: lydick@SOL1.GPS.CALTECH.EDU
NNTP-Posting-Host: sol1.gps.caltech.edu

In article <1g7lpnINNe53@GRIZZLY.ZOO.CS.YALE.EDU>, lower-tony@cs.yale.edu (Tony Lower) writes:
>  "OK... just my standard dig on parallel universes as portrayed by
>SF authors... and it goes like this:
>
>    AN INFINITE SET DOES _NOT_ NECESSARILY CONTAIN EVERYTHING!
>
>  "It's much easier to see with numbers.  The even numbers (those
>divisible by 2) form an infinite set.  According to the way SF
>writers deal with universes, we can therefore prove that every
>conceivable number is a member of that set, thus showing that
>1,3,5, etc., are even.
>
>  "Therefore, it is a fallacy to say that in an infinite number
>of universes, ANYTHING must have a representation.

That's true, if you just arbitrarily choose an infinite set of universes. 
However, that's not what's going on with the many world interpretation of QM
and the science fiction stories based on it.  The equations of quantum
mechanics yield a result which, stated in English, goes more or less:
	Given a system in state S0 at time T0, there is a set of possible states
	S which the system might be in at time T1.

	Further, there's no way to predict with certainty which state in S the
	system will occupy at time T1.  All that can be predicted is the
	probabilities of the various states.
The equations themselves take us that far.  Then there's the question of what
to make of this.  At the macroscopic level, we don't see the universe as a
bunch of probability functions.  We never see Schrodinger's cat in a
superposition of states.  So how do we reconcile macroscopic observations and
the results of the equations of QM (which do an excellent job of predicting the
behavior of sufficiently simple systems, and which, combined with statistical
mechanics do a good job of predicting the behavior of more complex systems as
well)?

One way of reconciling things, called the Copenhagen Interpretation, is to say
that when the system is "observed," that causes the collapse of the wave
function, forcing the system into a unique state.  Of course, then there's the
question of whom (or what) constitutes a qualified observer.

Another interpretation is the "Many Worlds Interpretation."  This
interpretation says that the system really does occupy every *POSSIBLE* state,
but you get a new universe for each of the possible states.  That's right, this
interpretation of QM, and not the science fiction writers, is the source of the
item that Manx finds objectionable, at least in one sense.

On the other hand, remember that the Many Worlds interpretation does *NOT* say
that every conceivable state will occur, just that every POSSIBLE state will
occur.  For example, the  "...other universe which is exactly the same, except
that I'm left-handed," while perhaps conceivable, is not possible (by virtue of
being left-handed, you'd've done some things differently, and those
differences, however small, would leave traces).  If you didn't do some things
differently, then you wouldn't be left-handed.

>It is quite reasonable to assume that there are some constants: the laws of
>physics, perhaps.

Why?  According to some theories which are taken seriously by physicists, the
laws of physics weren't decided until well after the big bang (remember, for
cosmologists, this means you can hang a reasonably small negative exponent off
the 10 when you're talking about how many seconds after the big bang).
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