Date: Wed, 5 Jun 1996 05:54:16 -0500
Subject: Schrodinger's Cat


Actually, Barkley, the reason I haven't replied is that it took me a hell
of a long time to find any account of this experiment. I finally tracked
down the location of an actual article about it (the May 24th issue of
Science, if anyone's interested), but I haven't been able to find that.
What I have read was two short, unclear accounts, I imagine much like those
in the NYT (all the news printed in a fit) or Washington Post except
undoubtedly less idiotic.

Here goes, Ralph. I'm not sure you'll find this very satisfactory, but
sometimes life just sucks. What has been done is that a beryllium ion was
trapped and supercooled (i.e., its velocity was reduced to near zero) by
some stationary electric fields and a bunch of laser beams. It was placed
in a superposition of two different electronic states (i.e., states of
vibration of the remaining electrons about the nucleus) by said beams . One
account said they were coherent states (a very special kind of state). Then
they "jiggled" the laser beams in the right way so as to "separate" the
states. What does this mean? Each of the states in question is centered
around some particular point in space. To put it very simply, since the two
states have different properties, they respond differently to the new set
of laser beams, i.e. as they evolve in time their centers may travel at
different speeds. By the principle of linearity, the result of the system
on the sum of the two states is the sum of the results on the two states
independently. Thus, the two states can get separated. Furthermore, the
separation was extended to 80 nm, which is far larger than the ion in
question, so basically the wave function of the ion had two fairly sharply
separated peaks. Then, according to the accounts I read, the ions were
brought back together by using a different configuration of lasers and "the
researchers detected interference patterns which provided evidence of the
original separation." Unfortunately, I don't know how they produced said
interference patterns.

First of all, what does the experiment mean? Well, the business about an
atom "being in two places at the same time" is kind of silly. Any
wavefunction that describes an atom, unless it's a very restricted kind
called a position eigenfunction, will give you nonzero probabilities of the
atom's being in various different places. All the statement means is that
the magnitude of the wavefunction had two noticeable peaks instead of 1, 3,
4, whatever. The wavefunction produced was the best quantum approximation
to such a classical picture (coherent states being considered to be
somewhat more like classical states than other quantum states for reasons
that I won't go into), but it's still not the same thing. The fact that an
interference pattern was produced when the states were brought back
together means that, since no measurement was performed, the atom never
knew which state it "was in." This is exactly what one would expect from
the fact that no measurement was made of the atom's position. The
experiment tells us absolutely nothing new. It's simply a confirmation of
standard quantum mechanics. It does seem to be very ingenious and a fine
new test of the fundamentals of QM, which have of course already been
tested six millions ways from Sunday. I would probably never have thought
of separating the two states like that, which may be why I'm not an
experimentalist.

So reality is as screwy as we knew all along. However, this most
emphatically does not mean that Schrodinger's cat is out of the bag. The
example of the cat was intended to be a reductio ad absurdum. After all, we
know that in the macroscopic realm the ordinary epistemology we grew up
with works just fine. This means that in the macroscopic limit, at the very
least, an object must be in a superposition of very nearby states, unlike
the ion in the experiment. Thus, if it were true that quantum mechanics
required the cat (or the whole black box for that matter) in the experiment
to be in a superposition of two macroscopically different states it would
mean that QM was just plain wrong. This point, unfortunately, is not very
widely appreciated. Note that I am not saying that QM can have no
macroscopic differences from classical physics, another common but
nonsensical view -- to take a basic example, classical physics predicts
that atoms are massively unstable to electromagnetic decay, whereas QM and
our experience tells us that the form of matter we know is fairly stable.
On the other hand, the reason QM seems so strange to us is that we know if
you throw a baseball at a wall that has another wall in front with two
holes cut in it and the baseball hits the back wall, then it has definitely
gone through one of the two holes, no matter what -- on the other hand,
this is not true of an electron unless you hit it with something to
determine which hole it went through. Not only does every physicist know
that the particular effects involved in the SC experiment wash out in
macroscopic objects, I believe there are arguments that have been given to
that effect. I've never gone through any, and I don't know if they're
rigorous or not.

One caution, Ralph. I know there's been a lot of mystical bullshit printed
about QM, some of it unfortunately by physicists, but it's still true that
quantum reality is very different from classical reality. We still don't
understand exactly what constitutes a measurement, i.e. what will collapse
the wavefunction, and it seems that any naive notion will be wrong. The
idea that some people got that there's some involvement of consciousness in
a measurement seems totally nonsensical to me, and in fact derived from a
silly essentialist interpretation of Heisenberg's original use of the term
"observer." Why a cat or a rock shouldn't be as good an observer as a human
being is beyond me. In any case, I think the question of the observer is
misleading and should be ignored. The real question is what constitutes an
observation, and I see no a priori reason that an observation implies an
observer of any kind. This question is unfortunately not addressed by,
e.g., the Copenhagen interpretation. We do know, however, that classical
determinism is just not good enough, and we are sure as we can be (but not
completely) that the statistical formulation is not an artifact of
ignorance, as Einstein thought. That view is simply not tenable any more.
As we go to deeper levels, our epistemology may have to be revised further,
but there's just no way it's going to nicely come back to what it was. This
is my opinion, but I couldn't really explain my reasons to you --
certainly, there are physicists who think differently. The idea that
reality must have the same kind of structure in the small as that we
observe around us, never particularly warranted, has been completely
exploded.

Rahul




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