From: daryl@oracorp.com (Daryl McCullough)
Subject: Re: What does quantum have to do with it?
Organization: ORA Corporation
Date: Mon, 25 Jan 1993 13:46:27 GMT

mcirvin@husc8.harvard.edu (Matt McIrvin) writes:

>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.

Oh, I agree. I'm not demanding that any physicists should abandon
their interests in string theory or loop quantization of gravity or
whatever in order to tidy up the foundations of quantum theory.

>>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.

The problem is that no self-consistent interpretation of QM is
adequate for all the purposes of physics. It's all very well to say
that QM says nothing about reality, but only predicts the results of
experiments, but then how can someone take seriously a quantum theory
of black holes, or neutron stars, or the Big Bang? Is talk about a
quantum theory of black holes really short-hand for "If in a lab, we
prepare a black hole with such and such a mass, and later perform such
and such a measurement...?

Perhaps what I should say is not that any particular interpretation is
incoherent, but that the de facto interpretation that physicists use
is incoherent. (Of course, the situation is similar in some other
fields, such as mathematics, where practitioners tend to believe
incompatible fragments of platonism, formalism, and contructivism.)

>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.

I agree.

>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.

If the different interpretations were like the Heisenberg and
Schrodinger pictures, I would be satisfied. Obviously there is a sense
in which the different mathematical formalisms for quantum mechanics
are simply different ways of looking at the same thing. In particular,
what can be done in one formalism can be done (perhaps more awkwardly)
in another. However, the contradictions in the interpretations of
quantum theory I think are more severe:

I don't think it is consistent to give a special status to measuring
devices (or observers). I don't think it is consistent to give special
status to macroscopic quantities. Either quantum mechanics is wrong,
or the properties of macroscopic objects and measuring devices should
follow from the properties of electrons and protons, etc. However, the
notion of measurement seems to be necessary for formulating the
probabilistic interpretation of quantum mechanics. So, QM seems
simultaneously to require and deny the specialness of measurement. I
don't think it is simply a contradiction between different (and
logically equivalent) formalisms; I think that QM doesn't have a
completely consistent formalism.

Maybe the situation is analogous to using coordinates to describe
locations in space. The coordinate description gives a special status
to this one location, the origin. However, the laws of physics don't
really treat the origin any different from any other point. Maybe
measurement is in the same position; Quantum mechanics doesn't really
treat measurement devices any different, but nevertheless it is
necessary to decide what are the measurement devices.

>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.

I agree that the incoherence is in minds, rather than the theory.  It
could be very well be the case that there is some way of looking at
things so that it all becomes perfectly clear and consistent. I don't
see any evidence that the way has been found yet, though.

Daryl McCullough
ORA Corp.
Ithaca, NY
Newsgroups: sci.physics,sci.misc,sci.space