Is the universe deterministic? This is something I have spent many hours thinking about and debating with other physicists. Sean Carroll, in a response to a post by Massimo Pigliucci, provides a good introduction to some of the different view points taken in this debate. Much of it hinges on which interpretation of quantum mechanics you choose to hang your hat.

Sean makes several interesting observations including this one:

My personal suspicion is that the ultimate laws of physics will embody something like the many-worlds philosophy: the underlying laws are perfectly deterministic, but what happens along any specific history is irreducibly probabilistic. (In a better understanding of quantum gravity, our notion of “time” might be altered, and therefore our notion of “determinism” might be affected; but I suspect that there will still be some underlying equations that are rigidly obeyed.) But that’s just a suspicion, not anything worth taking to the bank.

Emphasis added by me. This is an excellent point that I had never considered before. There is a chance that our view of time and determinism our incomplete within the current quantum framework. This would be surprising (at least to me), but I have not spent much time think about issues in quantum gravity.

Sean also makes a good point about arguments concerning free will. Often determinism and free will are conflated with one another:

It matters, of course, how one defines “free will.” The usual strategy in these discussions is to pick your own definition, and then argue on that basis, no matter what definition is being used by the person you’re arguing with. It’s not a strategy that advances human knowledge, but it makes for an endless string of debates.

A better question is, if we choose to think of human beings as collections of atoms and particles evolving according to the laws of physics, is such a description accurate and complete? Or is there something about human consciousness — some strong sense of “free will” — that allows us to deviate from the predictions that such a purely mechanistic model would make?

If that’s your definition of free will, then it doesn’t matter whether the laws of physics are deterministic or not — all that matters is that there are laws. If the atoms and particles that make up human beings obey those laws, there is no free will in this strong sense; if there is such a notion of free will, the laws are violated. […] Quantum mechanics doesn’t say “we don’t know what’s going to happen, but maybe our ineffable spirit energies are secretly making the choices”; it says “the probability of an outcome is the modulus squared of the quantum amplitude,” full stop. Just because there are probabilities doesn’t mean there is room for free will in that sense.

Interesting and thought provoking. I am eager to see how this discussion unfolds.