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In everyday life, most of us assumes every event and object has a cause in some sense. I am wondering if the same is true for quantum physics.

Does the random nature of quantum phenomena mean they have no cause or does the theory say that causes of quantum randomness are unknown?

The word "random" is used here in its ontological sense.

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    $\begingroup$ It's important to realize that even though it's sometimes assumed, it's not true, even in everyday physics. Aristotle's insistence that "everything that is in motion must be moved by something" famously derailed the progress of mechanics in physics for 2000 years. $\endgroup$ – David H Sep 11 '13 at 9:16
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    $\begingroup$ @DanielPark would like to comment:" At the sub microscopic level time has no preferred direction and the arrow of time only starts to take on the forward direction when particles interact and "acknowledge" each other by doing things like sharing electrons and so forth. Their wave functions collapse into a coherent state where superposition is not only possible but the norm. Slowly as more particles take on their expected values gravity creeps into the picture allowing macroscopic beings like us to evolve and ask the question of causality." $\endgroup$ – anna v Sep 11 '13 at 16:49
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    $\begingroup$ philosophy.stackexchange.com/q/8119 $\endgroup$ – Kenshin Sep 12 '13 at 10:34
  • $\begingroup$ @IstvánZachar - I think your comment would make a perfect answer, or at least a better answer than the other two so far. $\endgroup$ – Observer Sep 13 '13 at 12:08
  • $\begingroup$ duplicate of physics.stackexchange.com/q/45597 , but i am not sure that that is most early of the duplicates $\endgroup$ – qdinar Jan 15 '17 at 8:58
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Per request, made to an answer from a comment:

It was John Stuart Bell in 1964who proved by simple arithmetics that there are no hidden local variables behind the statistical nature of quantum processes, and behind the spooky non-locality displayed by entangled particles. Consequently, the paradox presented in the 1935 Einstein-Podolsky-Rosen paper upon which they claimed that quantum physics cannot be complete ("since it relies on statistical laws, it cannot give the ultimate full description of nature") is inherently wrong.

We understand causality as a relation that links post-events (effect) to prior-events (cause) (note that this does not necessarily mean similar chronological sequence, see here). In this sense, observable phenomena are dependent on deeper, possibly hidden variables, that nevertheless can be usually uncovered, at least at the macroscopic level. However, as Bell has proven, there are no hidden variables responsible for lowest-level quantum processes e.g. the random decay of radioactive elements. Therefore I would say that there exist no lower-level, ultimate cause for these processes.

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  • $\begingroup$ nice answer. Could you please say what experiments exactly proved the Bell theorem ? $\endgroup$ – user46925 Jul 19 '16 at 19:13
  • $\begingroup$ @igael Since I'm not a physicist and I have no time to research this in detail, I only leave a pointer to the Bell test experiments on Wikipedia. $\endgroup$ – István Zachar Jul 20 '16 at 20:50
  • $\begingroup$ it wasn't an innocent question ... $\endgroup$ – user46925 Jul 20 '16 at 22:31
  • $\begingroup$ @igael Wasn't it? Well, I'm not very good in picking up subtle hints : ) $\endgroup$ – István Zachar Jul 21 '16 at 6:01
  • $\begingroup$ as i know, bell test experiments only prove that particles do not have hidden variables. while some properties randomly appear according to wave function. but are not they pseudo-random? bell experiments, as i understand, do not prove the full randomness. @Observer maybe you should unselect this as correct answer $\endgroup$ – qdinar Jan 15 '17 at 8:27
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Seems to me there is a misunderstanding here:

Does the random nature of quantum phenomena mean they have no cause or does the theory say that causes of quantum randomness are unknown?

Quantum phenomena are not random. They have been found to obey strict dynamical equations which are different than classical mechanics etc equations, but still the phenomena are constrained by the bounds given by the solutions to these equations.

We would not exist if it were not for the beautiful atomic energy levels. Nothing random about them.

Maybe you are confused by the probabilistic nature of quantum mechanics, probabilities do not mean randomness. There are dynamical systems behind the functions giving the probability distributions in space or energy etc. and a causal direction.

Edit after discussions in comments:

Randomness in individual measurements at levels commensurate to $\hbar$ are bounded by the Heisenberg Uncertainty principle so dynamical equations do not enter explicitly. Randomness in the energy levels for individual particles are bounded by the width of the energy level given by dynamical equations. In general though, the solutions of quantum dynamical equations are deterministic for the ensemble of measurements from many individual particles/setups. The distribution for the ensemble is predictable from the dynamics, and causal. For example: if one measures a lifetime from an ensemble plot of the order of $10^{-8}$ seconds one knows it cannot be an electromagnetic interaction, it is the weak interaction that is responsible for the decay.

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  • $\begingroup$ I'm not sure I agree with "probabilities do not mean randomness", would you not agree that, e.g., that the result of a coin toss is a random variable? I think the OP is asking a deeper question related to hidden variables etc. $\endgroup$ – innisfree Sep 11 '13 at 20:52
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    $\begingroup$ @innisfree Randomness means randomness. Equal probabilities for whatever one is trying to measure. Quantum mechanical probabilities are functions of the space, time, energy and momentum variables and give specific weights for specific measurements. This is confusing the word "random" with the word "probability". In addition there are causal paths in these probabilities. $\endgroup$ – anna v Sep 12 '13 at 2:39
  • $\begingroup$ I am not sure I get this - if quantum phenomena are not random then what is? Can you give us an example of a natural process or a natural phenomena that is random? $\endgroup$ – Observer Sep 12 '13 at 7:35
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    $\begingroup$ Randomness doesn't mean equal probabilities at all. e.g. the sum of two dice rolls is a random variable, the outcome is random, but the probabilities are not equal. Quantum mechanics does include randomness, the OP's question is fine. $\endgroup$ – innisfree Sep 12 '13 at 8:26
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    $\begingroup$ Thanks, but this is too philosophical for me, an experimental physicist. For me in physics the opposite of random is "dynamically predictable" . The class that can be predicted is not the class of individual measurements, but of statistical distributions of a large number of measurements. In contrast to classical mechanics/electrodynamics/ etc that predict dynamically individual measurements. So your "randomness" in quantum mechanics is on individual measurements, and my dynamically predicted is on ensembles of measurements, which last is also quantum mechanics. Not all of QM is random. $\endgroup$ – anna v Sep 13 '13 at 11:05
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Currently, we don't even know why the laws of quantum physics exist or what they are dependent upon, we only observe their properties, their results. The "random" decay of an atom is not without cause, it's machinations are simply unpredictable, too complex to predict accurately and therefore we call them "random" but then prove they are not random through statistical evidence of billions of such events.

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