[fspeech]

QM is essentially deterministic. Neither CM or QM allows for true randomness. Maybe at this stage it is only a philosophical issue. In the end Statistical Mechanics and Thermodynamics are what truly rule the world. There is no hard and fast rule on when QM should kick in. We adjust the rule based on the need, which is why we largely don't need to deal with foundational issues while interpreting the world in QM framework.

[AnotherGoodName]

>QM is essentially deterministic

No it isn't. Even with perfect information and unlimited compute resources you cannot determine the outcome of a simple Bell Test https://en.wikipedia.org/wiki/Bell_test_experiments

The same is not true for CM. QM absolutely allows for true randomness. This has been proven time and time again. I encourage you and the original author to read the above. Look at the Feynman lectures. There are no hidden local variables. This is true randomness.

[wnoise]

The only point at which randomness enters is with "measurement" -- interaction with a classical system. But, if you believe in quantum mechanics, there is actually no such thing as a classical system, only large quantum systems.

[btilly]

And with that observation, you're into the Everett interpretation. Also called Many Worlds.

Namely that when an observer observes a quantum mechanical system, you get a quantum mechanical system that can be described as a superposition of parallel observers who each think that they saw something different. Those parallel observers cannot meaningfully interact thanks to thermodynamic considerations.

Of course accepting this description involves believing in quantum mechanics a little more than most feel comfortable believing in it...

[wnoise]

Yes. It's been my defaultish interpretation since I've learned quantum mechanics.

There is a wonderful lecture by Harvard Professor Sidney Coleman, called "Quantum Mechanics in Your Face"[1]. In it, he essentially leads into precisely this. It's what happens when you take quantum mechanics seriously.

My issue is that I can't take quantum mechanics seriously, or expect that it's interpretational issues can be sorted out within itself. The problem is that quantum mechanics is "merely" an extremely excellent approximation to quantum field theory. It can be thought of as an "effective theory" in a very similar way to QFTs as low-energy versions of other QFTs. Which means naturally that we should expect the framework of QFT to answer the interpretational issues, especially to provide guidance as to how the changes due to the approximations change the interpretation.

This is all a fine idea, but quantum field theories have even worse interpretational problems.

[1] https://www.youtube.com/watch?v=EtyNMlXN-sw

[lisper]

> There are no hidden local variables

But there could be non-local ones. The Bohm theory, which is perfectly consistent with standard QM, is fully deterministic.

[lambdatronics]

The wavefunction is a nonlocal variable!

[wavegeek]

> The wavefunction is a nonlocal variable!

Important point. It is quite unfair to say "nya nya local hidden variable theories cannot replicate QM", when QM is itself radically non-local.

[lisper]

Yes, but it's not a hidden variable (at least not in a prepared system).

[kanzenryu2]

Unless locality is not true, which we don't know for an absolute fact.

[fspeech]

There is no "experiment" in a purely quantum world.

[wavegeek]

> QM is essentially deterministic.

No. The standard formulation of QM posits a measurement using a pseudo-classical device which generates a probabilistic outcome. This is at the very core. Source: any text on QM.

The wave function evolution is deterministic but that is only a part of the theory.