Caltech Question of the Month: What do the laws of physics, and the Heisenberg uncertainty principle in particular, say about whether free will exists?
Submitted by Robert R. Belliveau.
Answered by John Preskill, professor of theoretical physics, Caltech.
This is a deep question and there is no simple answer. I am not a philosopher; nor can I speak for all physicists. I can only state my personal views.
The question of free will implicitly relates to the issue of consciousness. Free will usually means the ability of conscious beings to influence their own future behavior. Its existence would seem to imply that different physical laws govern conscious systems and inanimate systems. I know of no persuasive evidence to support this viewpoint, and so I am inclined to reject it. It seems likely to me that it is possible in principle to predict the behavior of a person in the same sense that we can predict the behavior of an electron; it is just tremendously more difficult in practice.
That said, I feel that it would be too facile to completely dismiss the concept of free will. As the questioner rightly indicates, the deterministic worldview spawned by Newtonian physics has been overturned by quantum mechanics. Even in the case of a simple electron, I can have "complete" knowledge of the state of the electron, and yet I am still unable to predict with certainty where the electron will be found the next time I record its position. So it is with the universe. Even if I knew "everything" that could possibly be known about the universe a moment after the Big Bang, I could not predict everything about today's universe; the details hinge upon the random outcomes of countless tosses of the quantum dice. And so it is with a person.
But randomness is certainly not the same thing as free will. The illusion of free will (if it is an illusion) is sufficiently pervasive that I cherish my own ability to make decisions, while I certainly would not value my "ability" to make random choices! Free will is more than a limitation on predictability; it is the notion that "effects" can be "caused" by conscious beings.
Some scientists hope that a deeper grasp of the concept of free will might emerge from a more complete understanding of quantum reality. An eloquent appraisal of these issues can be found in the recent book The Fabric of Reality by David Deutsch. It's not an easy book, but then it's not an easy question!