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Theoretical physicist, Richard Feynman is often attributed with the quote

Anyone who claims to understand quantum theory is either lying or crazy.

Example of the claim.

Is this apocryphal, or did Feynman (or another notable physicist) actually say that?

Note, in November 1964 Feynman similarly stated

I think I can safely say that nobody understands quantum mechanics.

(This is sometimes quoted to lend authority to mysticism or anti-intellectualism, but in context it seems to be voicing his lack of an intuitive classical-mechanics based explanation for why nature obeys quantum laws. This was also the same month as Bell's theorem was published, so contemporary physicists still anticipated uncovering a classical explanation of quantum theory.)

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    Remember: Do not use the comments to post pseudo-answers.
    – Oddthinking ♦
    Jan 16 at 21:44
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    I'd like to note that you refer to or cite quantum physics by that term as well as quantum theory, quantum mechanics and quantum laws. Now these may all be synonymous (I don't know), but if they are not it may be better to update the question accordingly.
    – oriberu
    Jan 17 at 10:48
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    Note: even if people did not understand it in the 1960s, much time has passed. So this quote does not support adequately the statement that quantum physics is not understood now. Things that no one understood in the 1960s could well be standard material for undergrads these days.
    – Federico Poloni
    Jan 18 at 13:04
  • @FedericoPoloni as the 2019 opinion piece I linked to in my answer shows, Feynman's quote and the sentiment behind it are still very much relevant today. Although technically you are correct, as a general rule if something was not understood in the 1960s that does not prove it is not understood today.
    – Dan Romik
    Jan 19 at 21:23

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I can’t find hard evidence for the “lying or crazy” quote, but as OP already mentioned, Feynman said something very similar in content, if perhaps less colorfully phrased:

I think I can safely say that nobody understands quantum mechanics.

Here he is on YouTube saying it.

Here is a discussion of what he meant.

Here is a 2019 New York Times opinion piece with a modern take on the issue by Sean Carroll, a well known physicist.

Because of the similarity between the two quotes, I don’t think it’s of material significance whether he actually made the more colorful statement. It expresses an identical sentiment to what he is shown saying in the video.

As the discussions I linked to above illustrate, the common view among physicists and mathematicians is that while quantum mechanics works amazingly well as a way of predicting the results of experiments, and is a phenomenally successful model of how the physical universe behaves, there is still something fundamentally mysterious and unintuitive about what its predictions really “mean”. That’s likely the sentiment that Feynman was trying to capture with his quote. The point is that he meant something pretty specific.

The reason that this quote has such potential to be used out of context and in misleading ways is that there isn’t an objectively correct definition of what it means to “understand” something. At a practical level, if we can use quantum mechanics to build amazing technological inventions such as lasers, MRI machines, and much more, then we can claim to understand it pretty well. That doesn’t mean we understand everything that we would like to understand, or that it’s some kind of heresy or admission of failure to express frustration about the aspects of the theory we don’t understand, including through pithy, colorful statements.

The issue about the slippery and subjective nature of “understanding” in science is illustrated quite well by another quote from a famous mathematician and physicist, John von Neumann, who once said to a colleague: “Young man, in mathematics you don't understand things. You just get used to them.” See the discussion here.

Sorry for editorializing in the above couple of paragraphs, I realize some people might object that this is off-topic, but I thought it was important to discuss not just the literal question of whether Feynman said something, but also the implied question of whether what he said actually means what some of the people citing his quote seem to think it means.

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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Oddthinking ♦
    Jan 25 at 1:31
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There are multiple interpretations of quantum mechanics that seemingly contradict one another. (They don't.) The end results of a calculation, what the mathematics says regarding an experiment using various particular interpretations, must in the end agree with the results of the experiment. Those interpretations that markedly disagree with experimental results have been tossed. The interpretations that remain all agree with one another (and with experimental results) regarding the outcome of all experiments conducted to date.

Feynman apparently was of the Copenhagen interpretation camp, or perhaps even the "shut up and calculate" camp, which is closely aligned with the Copenhagen interpretation. This latter interpretation of quantum mechanics says that one should not pretend to understand quantum mechanics. Just shut up and calculate.

That said, a lot of quotes that have been attributed to Feynman, including "shut up and calculate", might or might not have been said by him. The same applies to Einstein, who supposedly made a lot of statements that he provably did not make. One statement that Einstein definitely did make was that he thought parts of quantum mechanics were downright spooky. It turns out that Einstein's derogatory claims that quantum mechanics involves a spooky action at a distance has been experimentally verified.

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    @doneal24 The two interpretations are fully reconcilable. No experiment has been performed that disproves one of those interpretations but not the other.
    – David Hammen
    Jan 16 at 19:48
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    While I agree that no experiment has ever disproven either interpretation, the two interpretations have different views on the measurement of a property. Copenhagen has the measurement irreversibly collapsing the wave function while many worlds does not collapse the wave function. This seems to be a fundamental difference even if we cannot prove or disprove either interpretation. Just because we don't know which is correct doesn't mean they're in agreement with each other.
    – doneal24
    Jan 16 at 19:58
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    @doneal24, "Just because we don't know which is correct doesn't mean they're in agreement with each other." — nor does it mean they're in disagreement. In grade 5 I learned to calculate square roots by hand using a method similar to long division. In university I learned to calculate roots using Newton's method. These are two fundamentally different ways of solving the same problem, yet they both produce identical answers. Is it meaningful to say that one is correct and the other isn't, or that they themselves (as opposed to the answers they produce) are in disagreement with each other?
    – Ray Butterworth
    Jan 16 at 23:50
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    @doneal24 the reason we talk about different “interpretations” of QM rather than talking about different theories altogether is that no one can think of an experiment that will distinguish one interpretation from the others. They are describing the same theory and all predict the exact same result for any experiment we can come up with but are merely coming up with different philosophical explanations of what the prediction “means”. So, the question about which one is “correct” does not appear to be a meaningfully answerable scientific question, at this point in time at least. AFAIK
    – Dan Romik
    Jan 17 at 0:37
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    @doneal24 Many-worlds has a concept of irreversible quantum decoherence that essentially replaces the Copenhagen interpretation concept of irreversible wave function collapse. The two interpretations lead to the same predictions regarding experiments, at least so far. If in the future an experiment can be devised that does distinguish between the two interpretations (or between those two interpretations and other interpretations), the loser(s) will be tossed on the huge heap of discarded scientific notions. But as of now, no such experiment exists.
    – David Hammen
    Jan 17 at 6:38

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