Newsgroups: sci.physics
Subject: Re: What does quantum have to do with it?
From: mcirvin@husc8.harvard.edu (Matt McIrvin)
Date: 22 Jan 93 00:41:32 GMT

daryl@oracorp.com (Daryl McCullough) writes:

>Scott of course is giving the standard line, that science is about
>prediction, and any discussion that does not have empirical
>consequences is therefore "wild speculation". I would prefer to use
>the word "philosophy". I think that Scott has somewhere lost touch of
>the purpose of science: besides the modern purpose of allowing us to
>build bigger and better gadgets, there is the much more ancient
>purpose of trying to understand the universe we live in. Experiment
>should certainly play an important role in testing our understanding,
>but it isn't an end in itself. And there are other tests of our
>understanding: the coherence, consistency, and perhaps elegance of
>our theories.

Still, physicists will take a theory more seriously if one can 
cook up a potential way to test it.  This is even the case for
string theorists-- a subindustry in string theory seems to be the
quest for experimental tests using, say, 25th-century technology
instead of 159th-century technology.  The ultimate goal is to find
something truly feasible.

I am much more sympathetic to the search for good interpretations of
QM than are many physicists.  The problem is that the search is
sticky and time-consuming, and most physicists have enough interesting
things to work on as it is.  Research in the subject should go on,
but we shouldn't expect all physicists to be intensely interested.
QM as an uninterpreted recipe contains enough puzzles and surprises
to keep the vast majority of them working hard-- and one can obtain
a remarkably usable working understanding of how to use QM without
ever worrying about what the foundations are.

>That is one thing I find incoherent in quantum mechanics; it
>simultaneously requires that observers be given special status in the
>theory and denies that they have such a special status. Another source
>of incoherence is the nature of the wave function. Physicists often
>deny that the wave function is an objective physical quantity, and
>also will deny that anything *other* than the wave function is an
>objective physical quantity. They will say that nothing is real except
>observation, and then say that observation is simply a special case of
>a quantum mechanical interaction.

I suspect that you (or perhaps they) are treating a number of
different interpretations as the same thing.  Personally, I am all for
using more precise language when talking about these things, and
I've always found the smugness that exists in some textbook descriptions
of the subject a bit unnerving, as if the author were trying to paper
over basic ignorance in order to get on with the math.  But I doubt
we can really expect the matter to be settled definitively one way
or another.  There will mostly likely always be a few consistent
interpretations simultaneously compatible with the data, and different
ones may even be more useful for thinking about different problems.

It's, IMHO, likely that when more people try harder to state their
ideas precisely and the dust settles, we may end up with a situation
like the difference between the Heisenberg and the Schrodinger pictures.
The operators seem to have something to do with reality, but is it the
operators that are *really* evolving with time, or is it the state
vector?  Nobody cares, because the formulations are logically equivalent,
nothing you do is going to tell you which one is *really* moving,
and both (as well as hybrids) are useful in various situations.  So
are the probabilities real and the state vector just a calculational
device, or is the universe nothing but an evolving state vector?  Should
a measuring device be given special status, and if so, what's a
measuring device?  It may well turn out that the precisely stated
approaches will turn out to be logically equivalent, and the answer
will depend on what problem you're tackling.

>The bottom line for physicists is of course prediction, and in spite
>of the incoherent mess that quantum theory is, one can identify a
>solid enough core to make uncannily accurate predictions. That's
>enough for some people, but not for everyone.

The "incoherent mess" may lie more in the minds of the physicists
you're talking to than in the theory as a whole.  I agree that people
who deny the legitimacy of the study of interpretations, or who regard
it as a solved problem, are deluding themselves.
-- 
Matt McIrvin