Indeterminism

In quantum mechanics there is deterministic evolution only of the wave function describing a situation: The present state of the wave function determines its future state uniquely and completely. However, the wave function is not directly observable. It determines the probability that measurements will have particular outcomes. This probabilistic aspect is not a consequence of an incomplete description, loss of information, or imperfect observing equipment. It is intrinsic to the nature of quantum reality. It is a manifestation of the limits of classical concepts, such as position, momentum, and energy that are used to describe nature.

The standard interpretation of quantum mechanics includes indeterminism in principle. Perfect knowledge of the present state of the world cannot be obtained even with perfect measuring instruments. In Isaac Newton's (1642–1727) picture of the world (which does not contain the quantum aspects of reality but which can be obtained from the quantum theory as a limiting case when the sizes of objects are much larger than their quantum wavelengths) there appears to be determinism in principle but not in practice. Newton's laws allow the complete prediction of the future from the present state of the world if it is known with perfect accuracy, as envisaged by astronomer and mathematician Pierre Laplace (1749–1827). However, it is impossible for the present state of the world to be determined with perfect accuracy and scientists know that many configurations of matter have the property that any small uncertainty in their initial state is amplified exponentially rapidly with the passage of time. Thus there is indeterminacy in practice. This feature of the Newtonian world is called chaos. There have been many attempts to arrive at a full understanding of the quantum version of this type of chaotic unpredictability, but a complete understanding is still to be arrived at.

See also Chaos, Quantum; Physics, Quantum

Bibliography

barrow, john d. the universe that discovered itself. oxford: oxford university press, 2000.

earman, john. primer on determinism. dordrecht, netherlands: kluwer, 1978.

john d. barrow