I am a physics high-school student so my knowledge is not very deep on the subject. We started learnning about quantum mechanics and on some processes that my teacher described as random. I began to think on the concept of randomness and question it, thinking how can a process or an outcome be determined without any cause, how an outcome be determined at all in complete randomness?

I searched the internet and figured the scientific community does not agree with me. I'd really like to understand how can true randomness exist? Why the scientific community rejects the idea that ''random'' events may just have a cause we are not aware of? What am I missing?

Thanks for answers and sorry for bad English.

  • $\begingroup$ But isn't it different? Saying that we can't measure something precisely and saying the state is random? I'll tell you where exactly I began to think about it so you could maybe explain why I am wrong in those spesific things and then in the general sense. One time is when we learned about matter-decay, we were taught the process is random. Second time (I hope my description will be understandable with my vocabulary) is with the process that photon destabilize an electron and ''bounce'' it to other level, then the electron got back to the original level through sevral ways in a random manner. $\endgroup$ Commented May 28, 2015 at 15:51
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    $\begingroup$ "Random" means "practically unpredictable." An individual tritium atom will undergo beta decay if you wait long enough, but we have no theory that can predict when it will happen (i.e., it is "random" as far as we are concerned). That doesn't mean that there is no cause: It only means that if there is a cause, the cause is beyond our present knowledge. $\endgroup$ Commented May 28, 2015 at 16:15
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    $\begingroup$ @ james large : according to Bohr, there is no hidden story , nor hidden variable , for pure random. Not only unknown, it can't be known if any $\endgroup$
    – user46925
    Commented May 28, 2015 at 19:10
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    $\begingroup$ "True randomness" doesn't exist. A finite sequence of numbers does not have that property and an infinite sequence is not physically realizable. Like with every other philosophical/mathematical idea one has to check first if it satisfies trivial existence requirements before wasting any time on it in the context of physics. This one does not. Having said that, so far we have not seen any violations of the laws of quantum mechanics, but that has nothing to do with "true randomness". Quantum mechanics is about uncertainty, not randomness. $\endgroup$
    – CuriousOne
    Commented May 28, 2015 at 19:36

3 Answers 3


First of all, welcome to the site. What quantum mechanics tells us is not that processes are random; but that there is a "fundamental" (at least from the QM point of view) impossibility of having certain informations (for example the trajectory of a particle) with perfect precision. That means that the maximal informations we can have of the state of a system is of probabilistic nature, id est we are only able to say "the outcome $a$ of a measure about the quantity $A$ have a $x\%$ probability of being measured".

That does not mean, however, that the processes happen randomly, they are governed anyways by physical laws; nevertheless these same laws forbid a completely precise "classical" description of the system. So I may say that randomness, in this particular context, is not a good word, also because it may be misleading since the study of random processes is an important branch of mathematics and physics (but not strictly related to basic quantum mechanics).

Related to the OP comment above: those processes are not random from a QM point of view. Simply QM introduces, as I said, the concept of probability of a measure as opposed to exact knowledge. Take the specific example of the photon interacting with an electron. What we can say, very roughly speaking, is that there is only a certain probability that the photon would interact with the electron (and that probability depends on the state of the system photon+electron); and that this causes the electron to absorb the energy of the photon. After that, there is a certain probability that the electron would lose energy and emit a photon, perhaps of different energy. That does not mean that the events are random, they are dictated by the laws of quantum electrodynamics; it simply means that the maximal information that the QM description can give is this, i.e. which is the probability of a certain event.

It is a different type of prediction, that changes radically the point of view from the classical one: if in classical mechanics we would have predicted, given the initial conditions, exactly if the photon and the electron (seen as classical particles) would collide (as in a billiard); on the contrary in quantum mechanics we do not know a priori if the collision happens, we know only which is the probability of the collision. However there is nothing random about that probability, it is determined exactly by the quantum theory (as the trajectory was in classical mechanics).

  • $\begingroup$ How do you define randomness in the first place because wikipedia clearly says "During a measurement, on the other hand, the change of the initial wavefunction into another, later wavefunction is not deterministic, it is unpredictable (i.e., random)." Of course there might be non-local hidden-variables that determine the events but that is not the issue here. $\endgroup$
    – Gonenc
    Commented May 28, 2015 at 16:52
  • $\begingroup$ @gonenc There is a very long dispute about the quantum measurement process; there are mathematical models of it that have nothing unpredictable (at least in principle). Nevertheless, also from a naïve point of view, when you do a measurement you know a priori the probability of the possible outcomes, and given the effective outcome, it is postulated by QM that the original state is projected on the eigensubspace corresponding to the obtained measurement. This seems a quite predictable description, within the indeterminacy proper of quantum measurements (that is again predictable). $\endgroup$
    – yuggib
    Commented May 28, 2015 at 17:09
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    $\begingroup$ This bit "those processes are not random from a QM point of view. Simply QM introduces, as I said, the concept of probability of a measure as opposed to exact knowledge." is interpretive and not backed up by clear experimental data. The data tell us that it is either actually random or practically random because we can't know the hidden variables, but it doesn't distinguish the two cases. You can have either a local truly random theory or a non-local hidden-variables theory. You just can't have a local hidden-variable theory. $\endgroup$ Commented May 28, 2015 at 17:15
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    $\begingroup$ Hmmm ... on further reading I see that you are trying to draw a strong denotative difference in the works "random" and "probabilistic". I see where you're coming from on that, but I don't believe that this distinction is universally recognized. $\endgroup$ Commented May 28, 2015 at 17:18
  • $\begingroup$ @dmckee exactly, I think that there is a strong semantic difference between the two terms, however as you may see also in chat now there is not much consensus on that...;-) $\endgroup$
    – yuggib
    Commented May 28, 2015 at 17:27

Bell proved that, if there exists such an unknown cause, then it surely must violate special relativity (Information must travel faster than the Speed of Light).

Taken from Wikipedia

"Realist interpretations of quantum mechanics are possible, although, such interpretations must reject either locality or counter-factual definiteness."




Quantum mechanics is a theory where randonmness is inherently present. According to it, it is possible to desing experiments where we can only predict the probabilities of the different outcomes.

It has been argued, in the beggining of the theory, that this must happen becasue QM is an incomplete theory of nature. So, there must be hidden variables than the theory ignores, and that it is in this ignorance that the randonmness originates. These critics where done in the 30's.

Years later, a physicist called Bell, discovered that any theory that would include these hidden variables will inevitably be incompatible with QM. Since then, every experiment done has been in agreement with QM, so today it is quite accepted that QM implies the presence of pure randomness.

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    $\begingroup$ Random means that happens unpredictably; while the probability of an outcome in quantum mechanics is perfectly predictable. Therefore I think the word "random" to describe a quantum process is wildly inaccurate. $\endgroup$
    – yuggib
    Commented May 28, 2015 at 16:22
  • $\begingroup$ So flipping a coin is not a random process? $\endgroup$ Commented May 28, 2015 at 16:42
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    $\begingroup$ Flipping a coin can be approximated by a mathematical random process, however it is not the case for a quantum wavefunction... $\endgroup$
    – yuggib
    Commented May 28, 2015 at 17:03

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