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In this paper, Abner Shimony
describes two essential concepts in quantum mechanics. The first is the quantum
state or wavefunction, which specifies all the quantities of a physical system
“to the extent that it is possible to do so.” This caveat is crucial since,
according to the Heisenberg uncertainty principle, not all such quantities have
simultaneously definite values. The wavefunction does, however, give the
probability of each possible outcome of every experiment that can be performed
on the system. The second is the superposition principle, according to which
new quantum states can be formed by superposing any two allowable states of the
system. From these two basic ideas Shimony delineates three crucial features
which distinguish quantum physics and our ordinary experience of the world:
objective indefiniteness, objective chance, and objective probability. Thus
quantum quantities before measurement are objectively indefinite, their
definite but unpredictable value after measurement implies objective chance,
and the probability of finding that value after measurement is objective.
Shimony then describes in detail a fourth counterintuitive property:
non-locality. Here two particles which once formed a single system and have
been widely separated show an uncanny correlation between their properties,
challenging the relativistic concept of locality (i.e., that effects cannot
propagate faster than light).
How are we to handle these
remarkable features? One way would be to reject the premise that the
wavefunction gives a complete specification of the quantum state. Instead,
there might be as-yet unknown, or “hidden,” variables at work that explain
these strange features. In 1964, however, John S. Bell proved that the
predictions of local hidden-variables models are incompatible with the
predictions of quantum mechanics. Crucial experiments, such as those by Clauser
and later by Aspect (and proposed in part by Shimony), vindicated quantum
mechanics at the expense of local hidden-variables theories. Does this mean
that quantum mechanics involves unacceptable kinds of nonlocal
action-at-a-distance? Not according to Shimony, who points out that quantum
correlations between separated parts of a system do not allow one to send
information faster than light. Thus Shimony suggests that we think in terms of
“passion-at-a- distance” instead of instantaneous action-at-a-distance.
Shimony then describes other
experiments which reveal further elements of quantum strangeness.
Delayed-choice experiments underscore the difficulties in interpreting how and
when quantum properties become definite in the experimental context.
Schrödinger-cat type experiments raise the possibility of quantum
indefiniteness in the macroscopic world. Here something like an “irreversible
act of amplification” is involved, but for Shimony, we may need to discover new
physical principles if a full account is to be achieved. Finally neutron
interferometry and the Aharonov-Bohm effect underscore additional highly
nonclassical features of the quantum world. In sum, these highly nonclassical
features of quantum systems raise profound philosophical issues for our
understanding of the physical world.
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