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Shimony, Abner. “The Reality of the Quantum World."

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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