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Considering every cause has an action, how can anything be random? For something to happen, it must have a cause and through that definition it can't be random.

Considering this why are many quantum mechanical phenomena attributed to "randomness"?

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    $\begingroup$ Related: physics.stackexchange.com/q/317/2451 $\endgroup$
    – Qmechanic
    Apr 25, 2012 at 15:09
  • $\begingroup$ I'd like to point out that there is no set principle in physics that stipulates that "every cause should have an action" to begin with. So I also vote to close the question as not appropriate for this site. May be more suitable as a philosophical question. $\endgroup$ Apr 25, 2012 at 17:35
  • $\begingroup$ @Raskolnikov It's a logical deduction... $\endgroup$
    – user8791
    Apr 25, 2012 at 17:53
  • $\begingroup$ What is a logical deduction? $\endgroup$ Apr 25, 2012 at 21:19
  • $\begingroup$ [Humor alert] This strikes me as a rather random question... :) ... and quickly, the serious part: It really is. All of us are! How do we manage to have the diversity that makes this discussion possible, when the most obvious universe, if any at all, would just be smooth and simple? $\endgroup$ Apr 26, 2012 at 0:53

3 Answers 3

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You should look at the link that Qmechanic gives, as it is closely related to your question.

The "randomness" in quantum mechanics is widely misunderstood. There is nothing random in the wavefunction (or quantum field theory description) and as long as all interacting systems stay entangled the behaviour is completely predictable. We only see randomness when the system decoheres, which typically happens when when we make an observation.

The randomness when making an observation is normally considered built into quantum mechanics i.e. there is no explanation for it. It's just the way quantum mechanics works. It may seem a bit unsatifactory to just have to accept the randomness, but remember that quantum mechanics is only a mathematical model - one that so far successfully describes the universe we see around us. All mathematical models are based on some assumptions, and the randomness in making measurements is one of these assumptions. It's possible that some deeper model will be developed one day, and this model will explain why the randomness occurs. However there is no such model at the moment, nor even a hint of one.

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    $\begingroup$ On the one hand, you say that QM is deterministic and randomness is only an apparant effect due to decoherence. In the next paragraph you say QM is inherently random. What have I misunderstood? $\endgroup$ Apr 25, 2012 at 17:27
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    $\begingroup$ Decoherence is part of quantum mechanics, that why I say the randomness is inherent. $\endgroup$ Apr 25, 2012 at 17:39
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    $\begingroup$ But then classical mechanics is inherently random as well, because molecular chaos is a part of classical mechanics. $\endgroup$ Apr 25, 2012 at 21:17
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    $\begingroup$ Chaos is not randomness, it is insufficient knowledge about the initial conditions. Admittedly, you'd need perfect knowledge of initial conditions, but leaving aside QM this is possible in principle if not in practice. It's only when you include QM that you find perfect knowledge of the initial conditions is impossible in principle. $\endgroup$ Apr 26, 2012 at 5:42
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    $\begingroup$ Molecular chaos $\neq$ dynamical chaos. But maybe I should have used another example. Decoherence is usually achieved because of compling with some heath bath. But in classical mechanics, we also have coupling with heath baths which lead to statistical descriptions. Therefore, by your reasoning, classical mechanics is inherently random. My point still stands. $\endgroup$ Apr 26, 2012 at 7:35
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"Considering every cause has an action, how can anything be random?"

That's not really true. What happens is lots and lots of things happen at random and 'on average' you get the same predictable result.

Even classical areas like thermodynamics and fluid dynamics are really just the statistical
behaviour of lots of random individual effects.

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    $\begingroup$ That's not really true. Leaving aside quantum mechanics, fluid dynamics and thermodynamics are in principle deterministic i.e if we knew exactly where in the phase space the system was we could calculate it's future trajectory. $\endgroup$ Apr 25, 2012 at 17:07
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    $\begingroup$ @JohnRennie - only statistically. You can't prove that all the air molecules won't decide to fill one corner of the room - you can just calculate that it's a very very low probability distribution. It's not the same randomness as quantum decoherence - but in general case->effect is just averaging on a macroscopic scale. $\endgroup$ Apr 25, 2012 at 18:00
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    $\begingroup$ You mistake my meaning. If you know q and qdot for all the molecules you can calculate them at any future or past time because the system obeys the Euler-Lagrange equations. In practice this is impossible but in principle it could be done. $\endgroup$ Apr 25, 2012 at 18:47
  • $\begingroup$ +1 to all of the above--this exchange was incredibly insightful :-) $\endgroup$
    – adamdport
    Nov 16, 2012 at 14:16
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Randomness is just a word to compensate our lack of proper understanding. It's logical that actions cause reaction. Everywhere and always.

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