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I am debating with a person about questions of causes and effects in the quantum world, and some questions came into our minds, hence I would be very happy to know what is "your opinion" that is, what physics says about causes and effects in the quantum world.

In particular: take a process like spontaneous emission, or virtual particles fluctuations in the quantum vacuum. Could we claim the cause is undetermined or is there an explicit cause? Does Heisenberg rule the whole things, and would the same Heisenberg principle be a "sort of cause"?

Does spontaneity preclude its need for a direct cause (event A directly effecting event B) at all?

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closed as primarily opinion-based by Emilio Pisanty, David Z Feb 9 '18 at 2:58

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ @BySymmetry Uhm, I actually would prefer a "true physicist" answer, since it has to do with reality and the laws of the universe, rather than opinions by non scientists like philosophers xD $\endgroup$ – Les Adieux Feb 8 '18 at 13:53
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    $\begingroup$ @BySymmetry, it's not a general philosophical question he's asking, such as the meaning of life. He's asking a question about physics. $\endgroup$ – Steve Feb 8 '18 at 14:50
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    $\begingroup$ @VonNeumann I would urge you to think of this a bit differently -- physicists know physics, but the question you're asking isn't (purely) physics, but also about how we should interpret physical models and how we, humans, should think about and describe reality. "Non-scientists like philosophers" are going to have a lot of very useful input, possibly more than your average physicist -- we have an unfortunate tendency to not recognize the boundaries of our expertise. $\endgroup$ – zeldredge Feb 8 '18 at 14:51
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    $\begingroup$ Heisenberg's principle is not a statement about cause and effect: It is a statement about the precision with which we can measure the state of a system. $\endgroup$ – Solomon Slow Feb 8 '18 at 15:00
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    $\begingroup$ @Steve Physicists absolutely have plenty to contribute -- philosophers, after all, are frequently guilty of misrepresenting physical theory because they don't have the physicists' detailed understanding of its mechanics. I was pushing back on this idea that philosophers have nothing to contribute because they're "not scientists." For instance, the answer to this question hinges largely on how "causation" is defined, a question with a rich philosophical history from Aristotle to Hume to today. $\endgroup$ – zeldredge Feb 8 '18 at 18:05
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The premise of the question that "cause" or "effect" are well-defined physical notions at all is simply wrong, already classically:

Norton's dome is a perfectly fine classical Newtonian system, yet it has indeterminacy: It is consistent with the equations of motion that the ball balanced at the top of the dome can start rolling down at any point in time, without any external influence. This shows that classical Newtonian mechanics already does not admit straightforward notions of "cause" or "effect" in the sense that effects would necessarily need causes.

Therefore, physics says nothing about causes or effects, neither in the classical nor in the quantum world. Talking about causes or effects is an interpretation of the physical model, an ontology you impose on it, and not intrinsic to physics as such.

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  • $\begingroup$ Very interesting! This seems to be among the bigger misconceptions right? People commonly think of cause and effect as a very fundamental physical fact, where in fact as you put it, it is not at all the case. To better understand, would quantum fluctuations (which as far as we know are purely random and spontaneous, e.g. radioactive radiation) also be good examples that not all observed effects have a cause? But then again the time evolution in QM is unitary, I don't know how to think about this :( how do you usually see these things? $\endgroup$ – user929304 Mar 21 '18 at 15:12
  • $\begingroup$ @user929304 Drop by in chat sometime if you'd like to discuss this! (I'm usually around in the European evenings) $\endgroup$ – ACuriousMind Mar 21 '18 at 20:11
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The standpoint of standard Quantum Mechanics is very simple. Single processes, like the emission time of a photon from an excited atom, or the location of impact of a photon or electron on a screen after passing a double slit, are not causal, but random. There is no cause for a single particular outcome of the experiment. When considering the probabilities of such processes by averaging over many such single events, there is a causal law for the probabilities, which is determined by the quantum mechanical wave function obtained from the Schrödinger equation.

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    $\begingroup$ No known cause. It doesn't behove scientists to allege that things "have no cause", since the work of many generations of scientists has been precisely to investigate and find the natural causes of things. $\endgroup$ – Steve Feb 8 '18 at 15:29
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    $\begingroup$ @Steve - I am talking about the standpoint of the standard interpretation of quantum mechanics. You are free to elaborate in a separate answer on hidden parameter interpretations of quantum mechanics. The Bell experiments have experimentally excluded a very large class of hidden parameter theories. The interpretations of quantum mechanics are usually considered philosophical problems. $\endgroup$ – freecharly Feb 8 '18 at 16:26
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    $\begingroup$ It should be noted that this answer is extremely misleading. Quantum mechanical superpositions are fundamentally, inherently different from classical random processes. A photon in spontaneous emission isn't emitted at a random time: it is emitted at a coherent superposition of all relevant times, and it is only when it is projectively measured that one of those outcomes gets selected. However, there's always multiple incompatible possible projective measurements to be made (in the case of the photon, a high-resolution frequency measurement) and ... $\endgroup$ – Emilio Pisanty Feb 8 '18 at 19:03
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – ACuriousMind Feb 9 '18 at 17:38
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I think this question will be closed because it comes down to philosophy.

But FWIW, my take:

Causes and effects in the quantum world

is a very open topic, hence the many different interpretations of what the quantum word actually is.

since it has to do with reality and the laws of the universe.

I don't know what reality is either, except that the quantum world is more real as an accurate picture of physical processes, than the classical world, but how far does that get you? Again choose your favourite quantum world interpretation.

take a process like spontaneous emission, or virtual particles fluctuations in the quantum vacuum. Could we claim the cause is undetermined or is there an explicit cause?

Physics has a law that says these processes are random, say the emission of radioactivity. If we had a better understanding of this process, then hurrah!!, we can dump the random bit and replace it with a better explanation. As of now, random is all we've got, so that's real physics, in that it has a law "explaining" it, but obviously it's not a very deep explaination.

Does spontaneity preclude its need for a direct cause (event A directly effecting event B) at all?

We don't know if any process is truly random, so currently this seems to depend on what you consider/define spontaneinty as. "Without apparent cause" would seem to sum it up, until we know more, if ever.

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We do know that "spontaneous emission" and other fundamental things are not completely physically independent. For example, we know that they can change their rate when mass is moving.

But there is no agreed-on conception in physics of what is happening at the quantum level, although there are a variety of competing heuristics, and each time one side thinks it has staked the other through the heart and nailed the coffin lid down, the lid flings off and the creature lurches back out grinning - and thus it has been since the first half of the 20th century.

Not a great deal of progress has been made since, mainly because many physicists have tried to stop talking about it, or have even contended that they don't need to talk about it, and a generation of physicists have spent careers in an environment that attempts to suppress such discussion (or, when it involves the most prominent physicists, to disown it as obiter dicta and personal musings on "philosophy", rather than serious work in the field of theoretical physics, whether it be correct or not).

EDIT: to anyone who disagrees with any aspect of this post, it would be helpful (for all our sakes) if you were to review the further discussion that was had on it, and to leave such further remarks as you may feel justify its contradiction.

Right of Reply: @EmilioPisanty makes some forceful points about the clarity of my assertions.

He makes the point that work in "quantum foundations" is ongoing in the field even if a settled resolution of the multiple QM interpretations has not been achieved. He differentiates between "progress" defined in terms of furtherance toward a resolution of a question, with "progress" defined in terms of the achievement of a resolution and a moving-on from the question. I should be clear that I use the word in my answer here in the latter sense. He argues that, otherwise, the reader may well think no useful work is being done on the question at all!

He also argues that work was never totally halted in the field - a point on which I agree with him - whereas my original words are capable of implying that it stopped completely. On the contrary, it is only my position that work on "quantum foundations" was widely devalued in physics, and that therefore fewer physicists turned their time and attention to it than otherwise would have if it had instead been lauded as an important outstanding question in physics.

Emilio acknowledges that this "attempt to suppress" was made, but denies that it was successful, and denies that the attempt had any appreciable effects on the progress in this area of physics. Therefore, if the long-standing controversy over QM interpretations remains outstanding in physics, he argues is wrong for me to attribute this to "suppression". This remains a matter of difference between us upon which the reader would have to adjudicate.

EDIT: I'd also like to offer the following, from authors on either side of the issue, as particularly enjoyable reads:

Berthold-Georg Englert, On Quantum Theory:

https://arxiv.org/abs/1308.5290

Pablo Echenique-Robba, Shut up and let me think. Or why you should work on the foundations of quantum mechanics as much as you please:

https://arxiv.org/abs/1308.5619

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – ACuriousMind Feb 9 '18 at 23:19

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