Physics and Reality

Did quantum physics end determinism?

Classical determinism says that, given the full state of a system at one moment and the laws governing it, its future is fixed. The fact that we may be unable to calculate that future in practice does not change the claim. Complexity may hide the outcome from us, but it does not make the outcome open.

Quantum physics changes some aspects of this picture, but in an interesting way. What it changes is the notion of a state itself.

In classical physics, a state is a definite configuration. A particle has a position and a velocity. A field has a definite value at every point in space. The laws then tell us how that configuration evolves. In quantum physics, the state is not of that kind. Even when it is specified as fully as possible, it does not generally assign definite values to all the quantities that classical physics would want to specify at once.

A quantum state still evolves deterministically. But the state does not evolve into one future classical outcome. It evolves into a state that contains different possible outcomes, each with its own weight.

This is where probability enters differently from the classical case. In classical physics, probabilities usually reflect ignorance of the initial state. In quantum physics, probability is already built into the role played by the state itself. Even a fully specified quantum state does not determine the future in the same way that a fully specified classical state does.

This means that quantum physics did not simply replace determinism with randomness. The quantum state still evolves in a precise and highly constrained way. What disappears is the classical expectation that one complete present must contain one definite future in the same sense as before.

Whether one wants to say that determinism survives therefore depends on what one means by the word. If determinism means that the state evolves according to the laws of nature, then something like determinism remains. If it means that one actual present fixes one actual future outcome in the classical sense, then quantum physics breaks that picture.

Quantum physics therefore did not simply end determinism. It changed the meaning of determinism.

To summarize, a fully specified quantum state evolves into one unique quantum state at a later time. A generic quantum state, however, is a superposition of alternatives that are perceived as definite outcomes.

What this means for free will is a separate question.