Randomness in Quantum Mechanics

Is the quantum world really random?How can one be sure that there are no variables that can actually predict the outcome like they do in Newtonian physics?

• In Newtonian physics, can you predict whether an unbiased coin toss will land heads or tails? You can predict average ~50% heads and ~50% tails. That is what also quantum mechanics does, it predicts averages and they are found experimentally true. Formula for averages are different though. – kpv Sep 9 '17 at 18:19
• This question has been asked a bazillion times and is famously addressed by the Bell theorem which has been experimentally verified. – DanielSank Sep 10 '17 at 0:08
• This is a wonderful question in principle, but please search for answers for at least a few seconds before posting. – DanielSank Sep 10 '17 at 0:09
• kvp you can actually predict outcomes of a coin toss.it is taken 50-50 just because there are to many factors affecting the outcome,so it seems that it is random. – spatialdelusion Sep 10 '17 at 3:27
• @daboss: Yes, and those factors are practically insurmountable and I do not think you can really reliably predict outcome of a coin toss. In QM, the factors can be thousand times more difficult, and due to the scales involved, can be really impossible factors, so individual outcomes can not be predicted. You have to settle with averages. – kpv Sep 10 '17 at 4:26

I think J.G.'s answer is good, but doesn't fully address the question since you have the 'quantum-interpretations' tag set (maybe this was done by Qmechanic?), but this is really an unresolved question and highly metaphysical.

In brief, some interpretations of quantum mechanics rely on true randomness (such as most of the collapse interpretations), e.g. Copenhagen and the GRW models.

Other interpretations, such as Bohmian mechanics, only have apparent randomness, which is rather similar to classical effects such as Brownian motion with incorporated non-locality, where the way it works out it is impossible to ever know with certainty the result of measurements, but this isn't due to randomness but rather the observer being a part of the system they are measuring (thus affecting results of measurements---which aren't random---in unpredictable ways).

You then have a loose third set of interpretations that either make no assertion either way, or it's unclear whether randomness exists or not within then.

As I said though, this is all metaphysical; most people can just pick the interpretation they like the most (or none) and forget about it. In this case, J.G.'s answer is all you really need. If you'd like elaboration on anything feel free to ask.

• It is not an "unsolved problem" any more than the magic of charged particles pushing on each other at a distance is an "unsolved problem". – DanielSank Sep 10 '17 at 0:44
• While it depends on how you define 'really random', the question asked ('Is the quantum world really random?') is unresolved in the sense that deterministic quantum interpretations exist that give identical experimental results to standard quantum mechanics. I'm not sure how you can argue with this, but I'd be happy to hear your response. – Toby Hawkins Sep 10 '17 at 1:21
• which interpretation of qm is most accepted? – spatialdelusion Sep 10 '17 at 3:28
• Copenhagen is the one taught in most textbooks and is (arguably) the oldest (really a broad class of interpretations), although many people take a purely statistical viewpoint and don't worry about the underlying process. Many-worlds has also started gaining some traction more recently. Among people who concern themselves with quantum foundations it's far from settled though. See bit.ly/1VLiONu – Toby Hawkins Sep 10 '17 at 3:41

A common misconception is that the difference between classical and quantum mechanics is that only the latter uses probability, and that we might therefore get away with "hidden variables" that explain the apparently stochastic behavior of particles. But actually the difference is that probability obeys different rules in the two theories (see here for a full explanation). In particular, Bell's inequalities apply to classical theories and are in general invalid in quantum mechanics. A simple explanation is given here.