# Can randomness exist?

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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– Qmechanic Apr 25 '12 at 15:09
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. – Raskolnikov Apr 25 '12 at 17:35
@Raskolnikov It's a logical deduction... – user8791 Apr 25 '12 at 17:53
What is a logical deduction? – Raskolnikov Apr 25 '12 at 21:19
[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? – Terry Bollinger Apr 26 '12 at 0:53

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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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? – Raskolnikov Apr 25 '12 at 17:27
Decoherence is part of quantum mechanics, that why I say the randomness is inherent. – John Rennie Apr 25 '12 at 17:39
But then classical mechanics is inherently random as well, because molecular chaos is a part of classical mechanics. – Raskolnikov Apr 25 '12 at 21:17
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. – John Rennie Apr 26 '12 at 5:42
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. – Raskolnikov Apr 26 '12 at 7:35

"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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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. – John Rennie Apr 25 '12 at 17:07
@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. – Martin Beckett Apr 25 '12 at 18:00
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. – John Rennie Apr 25 '12 at 18:47
+1 to all of the above--this exchange was incredibly insightful :-) – adamdport Nov 16 '12 at 14:16

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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## protected by Qmechanic♦Mar 25 '13 at 5:48

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