# What experimental results lead us to conclude that the universe is nondeterministic?

I've recently been learning about Bell's Theorem and test experiments (e.g. as described here) and feel I understand why they imply that we cannot have local hidden variables.

However, I have heard some people say that these experiments also prove the universe is nondeterministic (that the state of the universe at time $$t+1$$ is not a function of its state at time $$t$$). I don't see why this is the case -- can't we have nonlocality and also determinism?

What, if any, experiments have we performed that would indicate the universe is nondeterministic?

• "I have heard some people say that these experiments also prove the universe is nondeterministic" Those would be people who take localism as a postulate. I've also met people who take realism as a postulate (and take experiments on Bell's Inequality as implying that the universe is necessarily non-local). – dmckee Jan 5 at 19:55
• Well, you could have neither, but I don't know anyone who likes that idea... – dmckee Jan 5 at 20:03
• @EliRose Quantum mechanics is a pretty reasonable nondeterministic but local theory that explains the Bell experiment. – DanielSank Jan 5 at 20:12
• Beware that "(non)local" is an overloaded word. Quantum field theory is "local" in the sense that it does not accommodate faster-than-light communication, but it is "non-local" in the weaker sense that it allows entanglement. String theory, on the other hand, is "non-local" in the stronger sense -- in fact a better word for string theory might be "alocal": spacetime isn't even well-defined in string theory, except as an approximation. So it's important to be clear about what we mean by "local" in discussions like this. – Chiral Anomaly Jan 5 at 20:37
• @EliRose In order to engender a constructive answer, could you perhaps precisely define what you mean by "is nondeterministic" ? That way we don't have answers using various definitions. There's a long philosophical tradition of people talking past each other on these points :) – N. Steinle Jan 8 at 22:18