I am not a physicist but I've started studying the subject and noticed that terms like "random", "randomness", "randomly" are widely used when talking about nature. For example, random movement of atoms, random fluctuations, random path of an electron etc.

My question does not concern any of these examples themselves.

What I would like to know is whether physicists use the term "randomness":

A) Literally

That is, whether physicists believe that true randomness actually exists in nature: Do physicists believe that (in some cases) something that is in a certain state X, is so not due to some cause (or a collection of causes) that preceded it and caused it to be in that particular state X rather than in some other possible state Y. In other words do physicists believe that causality is not a rule.


B) Figuratively

Do physicists use the term "randomness" as simply a shorter way of saying:

"we know that there must be a trillion things that caused this state X rather than this other possible state Y, but because the number of things that constitute that cause is so great, we can never know it or grasp it in its whole as humans, so in those cases we'll call it 'random' for lack of a better word, but we don't actually mean 'random' in the literal sense of the word".


EDIT: I want to know how the term is used by physicists, so that I may know how to proceed with the study of the subject. I'm not interested in opening a discussion about whether nature actually is (or not) non-deterministic sometimes.

  • $\begingroup$ So you are asking whether or not quantum mechanics or even nature is deterministic? Or what are you asking? $\endgroup$ – AlmostClueless May 16 at 12:33
  • $\begingroup$ The question as expressed inside the actual post is simple and obvious but the title was indeed somewhat misleading. I have updated the title. However, I will rephrase and simplify even more here: I'm not asking whether nature is sometimes non-deterministic. I'm not interested in opening such a discussion because I know the answer regardless of any possible discussion. I'm asking whether physicists believe that nature is sometimes non-deterministic. $\endgroup$ – Sprout Coder May 16 at 12:42
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    $\begingroup$ Whether or not an individual physicist uses "random" in the literal or the figurative meaning is something we cannot tell you in generality - it depends on the context. If you ask about a specific instance of usage of 'random' we might be able to tell you which it is, but as it stands the only physics question related to your question is whether or not the universe is deterministic, which has been discussed on our site before (see the duplicate). $\endgroup$ – ACuriousMind May 16 at 12:51
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    $\begingroup$ The question as expressed in the post is not simple and obvious. A) equates 'true randomness' to acausality, B) equates 'non-true randomness' to non-predictability. You therefore imply that there is more to causality that empirical predicatibility, but this is in fact a specific philosophical stance about the relation between physical models and nature. Randomness is the attribute of a model - I have no idea what "true randomness" may be. $\endgroup$ – Stéphane Rollandin May 16 at 12:54
  • $\begingroup$ @ACuriousMind I think this is not a dupe of the linked question, although closely related. In particular, the question asks about the different meanings of the world randomness in physics rather than asking whether quantum mechanics is truly random or not (which is what the linked question is asking). I also thought it was a dupe of the same question for a while but came to this conclusion, I might be wrong, of course. $\endgroup$ – Dvij D.C. May 16 at 13:02

Literal Randomness

In quantum mechanics, outcomes of an experiment are fundamentally probabilistic, truly. I'd doubt that anyone would actually use the word "random" for this fact because it seems to suggest that we have no idea what would be the outcome. Whereas in quantum mechanics, we know the exact probabilities of the different possible outcomes of a measurement. So, a better word is "probabilistic", not random. But if one speaks of randomness in quantum mechanics, they mean to address this truly probabilistic nature of quantum mechanics.

Notice that causality is not violated because of this feature of quantum mechanics. We know exactly what causes a state to collapse, namely, its measurement. If we don't perform a measurement, the state will deterministically evolve according to Schrödinger's equation. Moreover, a concept that becomes intrinsically linked to causality in special relativity is locality which says that physical causes propagate through space at speeds no higher than the speed of light. And locality is also not violated in quantum mechanics when you include relativity. Relativistic quantum mechanical theories, known as quantum field theories, prohibit any and all faster-than-light communications in a beautiful way. There are some strong signs that locality is violated in a subtle way in theories of quantum gravity but that's beyond my paygrade at the moment.

Figurative Randomness

Apart from quantum mechanics, physicists talk of randomness in statistical mechanics or in the behavior of chaotic systems. In both these cases, the randomness is supposed to be arising out of our ignorance of some piece of knowledge about the system and is not intrinsic to the nature of the system.

However, in quantum statistical mechanics, it is often impossible to distinguish between statistical probabilities and quantum probabilities in an invariant way. However, we can always be certain as to whether the system contains both or only one.

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