Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site sdcsvax.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!vax135!cornell!uw-beaver!tektronix!hplabs!sdcrdcf!sdcsvax!davidson
From: davidson@sdcsvax.UUCP (Greg Davidson)
Newsgroups: net.physics
Subject: Re: QM and Multiple Worlds
Message-ID: <1049@sdcsvax.UUCP>
Date: Mon, 19-Aug-85 17:52:20 EDT
Article-I.D.: sdcsvax.1049
Posted: Mon Aug 19 17:52:20 1985
Date-Received: Sun, 25-Aug-85 04:42:54 EDT
References: <486@talcott.UUCP>
Organization: EECS Dept. U.C. San Diego
Lines: 112

The replies to both of my previous articles have missed the point,
which may be more subtle than I thought.  Let me try again with more
care and detail, because I think that the point is an important one.  I
invite others to help me do so.  I apologize for the length of this
article.  I will first present the point in detail, then answer the
criticisms made by those who replied to my earlier articles.  You may
wish to skip the latter, but don't if you're going to post a followup
to this one.  Finally, I'll give some references.

First suppose that every elementary event occurs in all possible ways by
forking off a whole universe for each possible outcome of the event.  For
``observers'' along any world-line, it is irrelevant (because unobservable)
whether some outcomes are more probable, or whether all are equally likely.
It is also irrelevant whether outcomes are constrained by some ``laws of
physics'' or whether all conceivable outcomes generate universes.  Note
that the latter is a simpler theory, hence preferable by Occam's razor.

Now in such a system, the notion of observer is rather bizarre (hence
my earlier quotes).  Particle configurations identical to what we think
of as observers pop into and out of existence in all possible ways at
all possible points, complete with false memories (brain states not
corresponding to any past events along their world-line) false
sensations (sensory neuron states not corresponding to any external
events, if any, etc.  Even when observers are part of what we think of
as a normal universe, with true memories and true sensations, this is
entirely an accidental occurrance.  Only some successor universes will
preserve this kind of special relationship.  There is no reason to
attribute any significance to those world-lines which do preserve what
we think of as normality.

So to summarize, MWTs do violence to the notion of observers, and thus
to the practice of physics.  To belive in a MWT is to believe ourselves
to be illusions, and is therefore absurd.  (Please note that I'm
talking here about our physical continuity being illusory, not about
our psychological selves being so, the latter being quite another
issue.)  A MWT unconstrained by any laws of physics is preferable,
because simpler, to one incorporating laws of physics, hence MWTs
directly do violence to the notion of physics.  Now let me answer some
of the replies I've received.

Doug Gwyn wrote:
> [1]  Not all the alternate worlds are equiprobable!  [2]  There is no
> observable difference between the alternate-worlds QM and the
> Copenhagen QM.
[1] Maybe, but irrelevant to my point.  Also, Occam's razor would suggest
abandoning any theory of structure in the generation of successor worlds.
[2] Agreed.  Yet note that in an unconstrained MWT, there will be observers
who witness events consistent with any physical theory you want to posit.

Thomas M. Breuel wrote:
> [1] The 'multiple worlds interpretation' of QM is not a physical theory:
> you cannot design an experiment to disprove it, since it postulates that
> there is not interaction between its different worlds.
[1] Many physical theories give the same results.  Ptolemaic epicycles
give the same results as Keplerian ellipses in explaining observations
of the planets (and were embarassingly superior to Copernican circles).
Does that mean that Ptolomy's model of the universe as consisting of
crystalline spheres is as good a model of the Solar System as the
heliocentric model?  (Disregarding later information from telescopes,
spacecraft, etc.).  Choosing among equivalent models is part of physics.
As I understand it, MWT was offered because of philosophical problems with
CI, not because CI fails to explain events.  Unfortunately, both MWT and CI
have terrible philosophical problems.  Does anyone know of a third model
without such problems?
> [2a] It makes no sense to speak of 'observing differences' among worlds,
> or [2b] to generalise notions of normality to a fictitious ensemble of
> such separate worlds. [3a] WE are the dwelles of THIS world, and [3b] by the
> very defition of the word 'world' (in this context), [3c] the best thing
> that natural science can do is to describe statistically the physical
> laws that govern this world.
[2] I do not speak of physically observing such differences.  The examining
of any MWT is the act of mentally considering such differences.  Sorry to
have led you astray here.
[3a] The implication of an unconstrained MWT is that ``WE'', along with
``THIS world'' are illusions (except for unobservable instantaneous time
slices.
[3b] The definition of the word `world' merely reflects conventional,
pre-QM ideas.  The compound world-line is more modern, but is a bit
peculiar, since the word `world' is generalized to meaning a particle or
system of particles considered collectively.  I don't see that either
implies [3c].
[3c] Even this is not possible.  The best thing that science can do is
to construct theories, check them against observations and less ponderable
criteria (such as elegance), and investigate their consequences.

For some very interesting reading on these issues, I recommend some
fictional stories designed to present relevant thought experiments.
The earliest I know of are Jorge Luis Borges' stories ``The Garden
of Forking Paths'' which explores the nature of MWTs, and ``The Library
of Babel'', which explains how all possible writings (read `universes')
can be generated by a finite library of books (read `world states').
Both of these can be found in his collection ``Labyrinths'', New
Directions Pub. Corp., 1964.

A more recent, and more devastating exploration of MWTs is Larry
Niven's story ``All the Myriad Ways'', in his collection by the same
name [Ballantine Books, 1971], and probably in other collections as
well.

A scholarly and systematic examination of the notion of an observer,
with a good coverage of the above mentioned ideas of false memories and
false sensations is covered in ``The Story of a Brain'' in the
collection ``The Mind's Eye'' edited by the philosopher Daniel Dennett
and the physicist and computer scientist Douglas Hofstadter.  I cannot
recommend this latter collection too strongly (but can't give a full
citing - my three copies are all lent out - but its still in print).

As usual, I welcome comments, but if you don't see my point or follow
my logic, you may wish to reply to me directly.  I'll post any direct
replies which I find interesting.

_Greg Davidson			Virtual Infinity Systems, San Diego