[maskedSlacker]

It's been a long time since my QM course, but, assuming by unitary you mean deterministic, then the answer is that time evolution of a particle's state is NOT deterministic. It is probabilistic. For large collections of particles, the probabilities come to overwhelmingly favor a single average state, but the state of any single particle remains non-deterministic.

This is in contrast to, for example, classical statistical mechanics where individual particles are considered to behave deterministically, but we simply lack the ability to measure their behavior. In (the Copenhagen interpretation of) QM there is no underlying determinism at all.

You might not like this idea (it's certainly non-intuitive), but decades of experiments have shown that the alternatives are all, most likely, wrong.

[danbruc]

You are not too far off, as a first approximation you can say unitary means deterministic. But you are wrong with the time evolution not being deterministic, time evolution in quantum mechanics is deterministic. What seems not to be deterministic in the general case are measurement outcomes, that is where we take the Born rule, square the probability amplitudes, and get the probabilities for the different measurement outcomes.

But here lies the incompatibility I am asking about, time evolution is unitary and is in a sense incompatible with the Born rule stating that different measurement outcomes are possible. You can not have unitary time evolution and different possible measurement outcomes starting from the same initial state, at least not in a really obvious way.

[throwaway37585]

> decades of experiments have shown that the alternatives are all, most likely, wrong

How?

[maskedSlacker]

The Bell Theorem shows that any local hidden variable theory wouldn't match the experimental data we already have.

There's other options for getting around the weirdness of the Copenhagen interpretation that aren't ruled out by the data we have, but most of them are even weirder.

[throwaway37585]

What makes you think the Copenhagen interpretation is the least weird choice, given all its difficulties?

[maskedSlacker]

It's the simplest interpretation of the data we have.

[danbruc]

The many-worlds interpretation is arguably simpler than the Copenhagen interpretation because it gets rid of the collapse postulate. And depending on how literally you take the Copenhagen interpretation, it may not even be self-consistent. At best it provides no explanation how the apparent collapse of the wave function happens, at worst it suggests that the world evolves according to two fundamentally incompatible laws, unitary time evolution and probabilistic projection onto eigenstates during measurement.

I don't want to argue for the many-worlds interpretation, I don't really like - and I am aware that this is a somewhat stupid thing to say about a physical theory. But I also have to admit that it makes sense from a point of simplicity and consistency and compares favorable to other interpretations.

[throwaway37585]

How is an interpretation which violates unitary evolution and doesn’t even explain measurement “the simplest”?