Schrödinger’s Cat and the Meaning of Quantum Theory
One of the basic problems of quantum theory is the
relationship between measurement and reality. This is illustrated by the EPR
Paradox. A good way to picture the problem is via a famous thought-experiment
involving a hapless cat. The cat is in a box together with a canister of
poisonous gas connected to a radioactive device. If an atom in the device
decays, the canister is opened and the cat dies. Suppose that there is a 50-50
chance of this happening. Clearly when
we open the box we will observe a cat that is either alive or dead. But is the
cat alive or dead prior to the opening of the box?
Quantum orthodoxy (Copenhagen interpretation)
The dominant view in quantum mechanics is that quantum
probabilities become determinate on measurement - that the wave function (see
The Schrödinger Wave Equation) is collapsed by the intervention of classical
measuring apparatus. This means that the cat is neither alive nor dead until
the box is opened. The cat is in an indeterminate state. It merely has some
specifiable probability of being
alive, or, on the other hand, dead.
This interpretation is usually allied with a tendency to
extreme instrumentalism. On such a view the probabilities generated by the
Schrödinger Wave Equation do not correspond to any physical reality. There
simply is no reality to be described until an act of measurement collapses the
wave function. Quantum mechanics is merely a useful calculating device for
predicting the possible outcomes of such acts of measurement.
In spite of its dominance in the textbooks, this
interpretation is hardly satisfactory. To begin with, it may be regarded as
proposing a dualism in physical reality: two worlds - an indeterminate quantum
world and a determinate classical world. Then there is the problem of what
constitutes classical measuring apparatus. At what level does the wave function
actually collapse?
The act of measurement that collapses the wave function cannot
be limited to scientific instruments. After all, why should we assume that our scientific measurements are solely
responsible for collapsing the wave function? This would give rise to a most
peculiar world - one that was indeterminate until the evolution of hominids.
Other interpretations
Some physicists, e.g. Wigner and Wheeler, have identified the
classical measuring apparatus of the Copenhagen interpretation with consciousness. If so, they must be using
a much broader definition of consciousness than is usual. What level of
consciousness would be needed to make something determinate? Is the cat
sufficiently conscious to determine the outcome of the experiment? Would
earthworms do? What about viruses? The effect of pursuing this line of inquiry
is to move towards a form of panpsychism - the doctrine that every part of the
natural world no matter how humble is in some sense conscious!
An alternative might be to ask does God collapse the wave
function? (Click on this related topic to see the problems with this point of
view.)
Returning to the classical measuring apparatus, perhaps we should
put the emphasis on ‘classical’ rather than ‘measuring’ - stressing not so much
our intervention in the system as a transition from the world of the very
small, in which quantum principles operate, to the everyday world of classical
physics. This ‘neo-Copenhagen’ interpretation has the merit that it avoids the
absurdities of the consciousness-based approaches. However, we are still faced
with the difficulty of identifying an acceptable transition point. One
suggestion is that we choose the level at which physical phenomena become so
complex that they are irreversible.
Two other interpretations are the hidden-variable theory of
David Bohm and the many-worlds interpretation.
Email
link | Feedback | Contributed by: Dr.
Christopher Southgate
Source: God, Humanity and the
Cosmos (T&T Clark, 1999)
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