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I apologize if this question has been asked before, but I was unable to find it if it has.

I am watching this 1964 Character of Physical Law lecture (link is to minute 41), and in this portion Feynman discusses Heisenberg's uncertainty principle. As he describes it, it sounds like it is a limitation on measurement, not on the nature of reality (he talks about not being able to measure without "disturbing" the electrons too much). However, as discussed in other questions on this site, the uncertainty principle is much more fundamental than a limitation on measurement.

So, my question is: Am I misunderstanding Feynman, or has the understanding of Heisenberg's uncertainty principle changed since 1964? If I am misunderstanding, I'd appreciate a correction.

I'm aware that Feynman goes on to say that hidden variable theories don't work, but it seems that there could be a single trajectory, unpredictable at the onset, that we could still measure.

andars
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1 Answers1

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Some context is necessary here. Feynman was lecturing to undergraduate students in an introductory course of physics. Most of these students would have had no prior exposure to quantum mechanics (or any physics for that matter). As a teaching strategy, it is often useful to talk about experiments and other physical situations that are easy to imagine. With this in mind, it is easier for students to understand that in order to measure the position of an electron, you have to hit it with a photon. But, that transfers momentum to the electron, disturbing its prior motion.

Furthermore, the question of whether the Uncertainty Principle was fundamental or instrumental was still unsettled at the time of the Feynman lectures. In fact, the question had been unanswered for so long that most physicists considered it nonsense (in technical jargon, a "philosophical question"). It wasn't until 1964 (after the lectures) that John Stewart Bell published his theorem that allowed the question to be answered experimentally. That is, he found a way to measure whether a particle actually has a definite position (or any other property subject to the Uncertainty Principle) prior to measurement. The answer, confirmed in many experiments up to the present, is no.

Mark H
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