From: jbaez@riesz.mit.edu (John C. Baez)
Newsgroups: sci.physics
Subject: Re:  Simple observation about Quantum Kitty Cats
Date: 28 Sep 92 21:44:20 GMT
Organization: MIT Department of Mathematics, Cambridge, MA

In article <1992Sep27.073401.3486@asl.dl.nec.com> terry@aslws01.asl.dl.nec.com (Terry Bollinger) writes:

>SOME SNIPES AT MWI
>
>I was amazed to see several apparently totally serious statements about
>MWI such as "many bizarre worlds are predicted, too!" (not a direct quote).

>Hmmm...  call me old fashioned, but I always thought "predicted" means
>"there is some way, even if it is very difficult, to verify its existence."
>
>If there is any mechanism whatsoever for getting a single bloomin' idea for
>a REAL prediction out of MWI, I'd love to hear about it.  (Is there?  Maybe
>I honestly *have* missed something.  Dr. Baez?)

>I sometimes wonder whether one factor in why Dr. Bell (a very great man in my
>opinion) did not receive a Nobel prize before his untimely death was simply
>because he refused to get caught up in abstractions that have no physical
>meaning.  How dull.  How bland.  How un-front-coverish.  How spectacularly
>successful at finding a genuine, bona-fide *testable* prediction of a truly
>strange implication of QM.  Ironic, is it not, that a fellow who thought
>very little of his own rather considerable musings in quantum cosmology was
>nonetheless able to out-predict the showier front-cover QM cosmologies that
>riveted the imagination of pop physics?

>Again my question to MWI advocates:  Does MWI say *anything* at all that has
>the courtesy of being testable in *any* conceivable fashion?  If not, how can
>you seriously claim the MWI to have any meaning?

The thing about "interpretations of QM" is that for the most part
they are not really physical theories.  QM is a physical theory and
makes testable predictions which seem to work well.  One can
think of this theory as consisting of two parts: the part that's pure
mathematics, and the part that tells us how to use this math to
calculate predictions from data we input.  One could call the latter
part the "interpretation" of QM, but usually interpretations of QM
do more.   "Interpretations" usually dress up this theory with a bunch
of language to give us a "picture" of the world QM is describing; this 
picture has the pragmatic effect of allowing us to match up the math of
QM with things we observe, but one can't typically experimentally test
all the elements of the picture.  Indeed, most of the decent
interpretations of QM are designed to be "operationally equivalent,"
that is, to be indistinguishable when it comes to concrete predictions.
That's why people will argue about interpretations of QM forever (or
until a still more unintuitive theory supplants it :-)), while
physicists who *use* QM seem to do fine for the most part without ever
worrying about such issues. 

Then why do I even bother writing about this topic?  Believe me, 
I usually manage to avoid it.  I have already mentioned my "shut up and
calculate" interpretation of QM.  This says that any attempt to make
QM seem obvious using English is like trying to write software
documentation in Aramaic; the language is simply not set up to handle
it easily.  From this point of view one should provide only enough
English to say how to use the math of QM to make predictions from
input data, but not try to round this off with a "picture" of what
the world is "really like" according to QM.  Many physicists find
this very upsetting since they think there is more to physics than
making predictions: to them, what is really fun about physics is the 
"picture".  This is fine with me as long as one doesn't expect all
elements of the picture to be operationally falsifiable.  

Unfortunately, *bad* interpretations of QM exist; these present pictures
that make it harder to do calculations correctly, by confusing the heck 
out of you.  It is likely that all existing interpretations of QM are
bad to some extent.  

Now, I like Everett's relative state formalism quite a bit.  This paper
of his, his PhD thesis in fact, was the basis of what people like to
call the "many worlds interpretation of QM".  In fact it appears now in
a book by that title.  But one will note that most of the more dramatic
aspects of the MWI do not appear in Everett's paper.  Namely, he
doesn't not talk about "many worlds" branching off like stalks of 
some great tree.  This exciting picture, which a lot of people have in
mind when they discuss the MWI, is very misleading in my opinion, just
as is the picture of the "collapse" of a wavefunction when a physical
process of a special type knows as "measurement" takes place.

So I would think that Everett would be shocked by the idea that
his relative state formalism should yield new testable predictions.
It was just devised to address the problem of how to reconcile
the math of QM to what we see without postulating "collapse".

I should add that, just as the "many worlds interpretation" is a
a silly spinoff of Everett's interpretation, the "collapse
interpretation" is not the same as the "Copenhagen interpretation"
worked out by Bohr and company.  These conflations only add to the
confusion in a subject which has plenty of intrinsic confusion.
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