Comments by "Lepi Doptera" (@lepidoptera9337) on "The 2nd Quantum Revolution -- Nobel Prize in Physics 2022" video.
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@JohnSmith-ut5th Don't get me wrong. Feynman's book is great. He gives a great insight into the phenomenon of how path integrals work and how they create semi-classical effects (by destructive interference of rapidly oscillating terms that are far away from the classical trajectory that minimizes the classical action).
One has to understand that he comes from high energy physics background, though. In that particular case, high energy quanta do behave, in a sense, like classical particles when they are being subjected to weak measurements in detectors. A weak measurements is one that does not remove the entire energy of the original quantum but instead only makes many small momentum changes while the energy is being slowly absorbed by a detector medium in many steps. How this happens form the standpoint of a pure wave theory has first been pointed out by Mott in 1929. It's basically an application of the equivalent of conditional probabilities in quantum mechanics: if we localize a quantum weakly at some coordinate x, then we incur a small momentum uncertainty, which for a next weak measurement will give us the same coordinate plus the classical estimate for the location change due to the initial momentum plus a stochastic term that came from the momentum uncertainty of the localization.
In essence, experimental high energy physicist can treat the quanta they are detecting in their experiments like classical particles that make a little random dance around the classical trajectory. In the detector the energy of an e.g. 1Gev "particle" changes in steps of hundreds of eV to MeV per interaction and hundreds and thousands of such interactions occur before all the energy is used up. That's what creates those nice particle tracks that show up in all the high energy experiments.
However, in the interaction point of the beams in an accelerator experiment, that is the point where the actually interesting physics happens during collisions, this weak measurement approximation does not apply. There quanta are exchanging all of their energy/momenta at once and no classical paths exist . That's where we have to go through the entire infinite sum of Feynman graphs to elaborate all physically allowed interaction processes. That is what the path integrals and their perturbation series (Feynman diagrams) were really invented (or shall we say discovered) for. At that point we are back to the same principles that apply in low energy quantum interactions like the photoelectric effect: quanta only exist as physically useful entities during the interaction of quantum fields, when the entire energy of a state is being exchanged. When these fields are not interacting, however, then they behave like wave phenomena.
End rant. :-)
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