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Quantum Weirdness II:
Schroedinger's Cat
The Role of the Observer
Measurement operations on a quantum system always give
some definite answer, informing us that the system is in one
of the definite states detectable by the measuring device.
For example, an exposed grain of photographic film testifies
that the electron was there and nowhere else when it
deposited its energy in the detector. All subsequent
observations will be consistent with the first. So, for
example, if an electron is actually observed traversing slit
1, any future observations made on it will show it behaving
as a small pellet that has gone through that slit, not as
part of a superposition of slit-1 and slit-2 waves. The
measurement observation changes the state of the system,
replacing a superposition with a definite state.
What is it about the act of observation that causes such
a change? Quantum systems are microscopic, simple, and
have only small amounts of energy. Energy must be
transferred to the measuring device if the quantum is to be
detected at all, and that energy will be a significant
amount of the total available, so it is obvious that
measurement will be disruptive. But how is it that
superpositions never survive an act of measurement?
And what kind of interaction does it take to remove a
superposition and put a quantum system in a definite state?
Many attempts to probe these questions have centered around
Erwin Schroedinger's famous thought experiment involving a
cat in a box.
The Cat
From the amount of ink that has been spilled on this
subject, one might assume that Schroedinger had devoted a
whole book or at least a lengthy paper to the thought
experiment when he introduced it. In fact, his mention of it
is hardly more than an aside. In a general paper on quantum
mechanics, he discusses and rejects the interpretation that
a single quantum is somehow phyiscally "spread out" or
"blurred" among the different parts of a superposition (for
example, that the electron in the double slit experiment
somehow makes itself into a cloud and manages to go through
both slits). He then emphasizes this as follows:
One can even set up quite ridiculous cases. A
cat is penned up in a steel chamber, along with the
following diabolical device (which must be secured
against direct interference by the cat): in a Geiger
counter there is a tiny bit of radioactive substance,
so small that perhaps in the course of one
hour one of the atoms decays, but also, with equal
probability, perhaps none; if it happens, the counter
tube discharges and through a relay releases a hammer
which shatters a small flask of hydrochloric acid. If one
has left this entire system to itself for an hour, one
would say that the cat still lives if meanwhile no
atom has decayed. The first atomic decay would have
poisoned it. The [wave] function of the entire
system would express this by having in it the living and
dead cat (pardon the expression) mixed or smeared
out in equal parts.
It is typical of these cases that an indeterminacy
originally restricted to the atomic domain becomes
transformed into macroscopic indeterminacy, which can
then be resolved by direct observation. That
prevents us from so naively accepting as valid a "blurred
model" for representing reality. In itself it would not
embody anything unclear or contradictory. There is a
difference between a shaky or out-of-focus photograph and
a snapshot of clouds and fog banks.
Schroedinger's thought experiment has suffered much abuse
at the hands of poorly informed writers. In many treatments,
for example, it is taken for granted that a real cat placed
in a box like the one described would be in a quantum
superposition state until someone opens the box and looks at
it. This is not true, and it confuses the lesson to be
learned from the thought experiment. Unfortunately,
Schroedinger was writing in the earliest days of quantum
mechanics, before much attention had been given to the
distinction between superposition and mixture states. Let's
look first at what actually happens to Schroedinger's cat,
and then return to the questions raised by the thought
experiment.
Coherence
The distinguishing feature of a superposition state is
the possibility of interference between its two components.
If there were no inteference between the electrons passing
through the two slits in the double-slit experiment, they
would act just like classical particles and there would be
no quantum weirdness. Interference, though, depends on some
rather delicate conditions. The two waves must be in some
way "keeping time" with one another in order for their
crests and troughs to combine in the periodic, predictable
manner needed to produce an interference pattern. If one of
the waves is being constantly disrupted, so that there is no
reliable periodicity in its wave forms, we would observe
only a jumble of wave energy, not a geometrically regular
interference pattern. Imagine, for example, that we decided
to amplify one of the water waves from the double-slit
experiment to make it easier to detect. We might do this by
placing thousands of motorized paddles in the water around
slit 2, with their "on" switches attached to very sensitive
triggers so that any small disturbance in the water around
the paddle will start the motor running and launch strong
waves into the water in all directions. Now when the first
water wave traverses the slit and brushes the first
motorized paddle, it sets off a chain reaction. Before long,
all the thousands of motors are running, and thousands of
waves are being launched in all directions, none of them
synchronized with the others, and all starting in different
spots at different times. A huge amount of energy would be
carried to the detector by this cascade of secondary waves,
but there would no orderly pattern of wave crests at all,
and the interference pattern between the slit-1 and slit-2
waves would be completely lost.
Such loss of coherence through amplification is an
inescapable part of our knowledge of quantum phenomena.
Individual quanta are too small to enter our awareness
without what Bohr called "an irreversible act of
amplification" taking place at some point or other...in a
measurement apparatus or in our own sense organs. The
decoherence that invariably accompanies such amplification
destroys any original superposition state and leaves a
mixture in its place.
 In
the cat scenario, the amplification takes place in the
Geiger tube used to detect the radioactive decay. The
radioactive atom may indeed be in a superposition of
"decayed" and "undecayed" states, but when the
emitted alpha particle causes the Geiger tube to discharge,
the state of the system becomes a mixture. The state in
which the tube discharged and the state in which it did not
discharge cannot interfere with each other. From this point
on, quantum weirdness is out of the picture: the relay,
hammer, flask of acid, and cat all behave precisely as they
do in classical physics. Although it is technically true
that, until we open the box, we must describe the cat as a
mixture of live and dead states, this means nothing more
than simply saying that the cat may be dead or alive, and we
do not yet know which.
Schroedinger, although he does not speak of
superpositions and mixtures, clearly has this in mind when
he says that any uncertainty in the state of a macroscopic
system can be resolved by direct observation. He thinks it
is wrong to imagine the cat as being something like a
double-exposed photograph, dead and alive superimposed. The
cat, as common sense tells us, is one way or the other; only
our knowledge is incomplete.
So What's the Problem?
It might seem as if the process of decoherence can dispel
these questions of the role of the observer in quantum
mechanics. Decoherence through amplification is, after all,
a purely physical process. No consciousness is required to
make it happen. Our choice to look in the box has no effect
on the subsequent behavior of the system; it only affects
our state of knowledge about it.
In the Copenhagen Interpretation, all unobserved states
(superpositions and mixtures both) are viewed very much
alike from a philosophical standpoint. Because no
observation has yet been made on them, they are not
considered objectively real (since objectivity implies that
different persons have a shared experience of the
phenomenon, and before observation there is no experience to
share!). Some of these unobserved states (the coherent ones)
have interference terms in their descriptions, others do
not. Either way, our description incorporates the
incompleteness of our knowledge--knowledge that can only
become complete through actual observation. With this
philosophical orientation, one would indeed say that the cat
is neither alive nor dead as a matter of objective reality
until an observation is made.
From a philosophical orientation that is more
metaphysical and less epistemological, there is no great
difference between a pre-observation mixture and a
post-observation single state. From this perspective, the
objective reality of the cat's condition does not change on
opening the box. If the cat is objectively dead, it became
so the instant the flask broke, not at some later time when
the box was opened. These two philosophical viewpoints are
equally consistent with the facts; there is no experiment
that can distinguish between them. It is more a matter of
preference than of science.
The transition from superposition to mixture, and hence
the transition for quantum weirdness to classical
normalness, can indeed happen long before any human observes
the system. In fact, it usually does. But this fact alone is
not enough to completely separate the process of conscious
observation from the behavior of quantum systems. Although
we can have mixtures without observation, we still cannot
have superpositions with observation. We can only
directly observe states that are no longer in superposition.
The coherence can be lost long before the observation, or it
can be lost at the moment of observation, but it cannot be
lost after the observation. Whenever we look we see
something--something definite, the system in a
well-defined state. In classical physics, we are free to
assume that the system was in that state all along, before
we bothered to look. In quantum mechanics, this assumption
is sometimes possible (if we are observing a mixture), but
soemtimes not (if we are observing a state that was in
superposition).
Observation on a mixture state has no physical
consequences, and so we are free to endulge either an
epistemological or a metaphysical view of reality when it
comes to mixtures; it makes no difference. But if you follow
the metaphysical perspective, you enter a nightmare world
when you try to extend your picture of reality to encompass
the superposition states too. A dead cat and a live cat do
not interfere with each other, but an electron going through
slit 1 and an electron going through slit 2 do. Accounting
for the interference and maintaing a picture of a
single, localized, objectively real electron traversing the
experimental apparatus is impossible. It is this
impossibility that is demonstrated and put into
experimentally testable terms by Bell's inequality.
In no case, however, should one think of consciousness as
physically producing a change in the state of a
system, either forcing a superposition into a mixture or
forcing a mixture into a single state. Observing the cat
does not kill the cat. It is very unfortunate that many
popular writers on quantum mechanics have given this
impression of the role of the observer. It is inconsistent
with both the epistemological philosophy of the Copenhagen
Interpretation and the metaphysical philosophy of
hidden-variable approaches such as Bohm's.
Go to Quantum Weirdness III: Bell's
Inequality
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