In the double-slit-experiment, one particle at a time (for example, the double-slit-experiment performed by Dr. Tonomura showing the build-up of an interference pattern of single electrons) can be observed in a detector or photographic plate by attenuating the source. At such a low rate of detection, I read that background radiation becomes a significant proportion of the particles detected. What I don’t read, is the effect virtual particles have on this experiment. Should they be factored into the experiment? If so, or if not, why?
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$\begingroup$ Background radiation and virtual particles are rather different things - you may want to clarify your point about these (my guess is that virtual particles are not relevant here). $\endgroup$– Roger V.Jul 25, 2020 at 9:52
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$\begingroup$ @Vadim Thank you for pointing this out. When I read about the experimental setup and interpretation of results, background 'noise', or more precisely, background radiation, is discussed and accounted for as it is present in the detectors / photographic plates in significant quantities in this kind of experiment. Virtual particles are different phenomenon. However, they are present in this kind of experiment (all experiments, really!). Yet they are not discussed in the papers. Like you, I assume they are not relevant but I wanted to ask and check my assumption. $\endgroup$– QubitJul 27, 2020 at 1:39
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$\begingroup$ The double slit is not even a quantum experiment. Planck's constant doesn't show up it in anywhere and we aren't looking at n-quantum correlations, either. For light the double slit "with photons" is simply a low intensity measurement on a system that obeys Maxwell's equations, which are classical. $\endgroup$– FlatterMannMay 10 at 0:52
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2 Answers
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The question implies that you don't know what virtual particles are. The name 'virtual particle' is arguably a bit of poor terminology. It refers to a disturbance in the field (for electrons the Dirac field, for photons the electromagnetic field) that is part of the complete calculation for how a particle moves from one place to another or interacts with another particle. The 'virtual particles' are a convenient way to break up a calculation into a sum of lots of pieces. Asking whether they influence things is sort of muddled because they are the influence (or part of it) when fields interact.
answered May 9 at 20:47
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Two-slit experiment is a thought experiment to demonstrate the wave nature of electrons in quantum mechanics. There exist many implementations of this experiment, which necessarily differ from the simple analysis given in the textbooks and compounded by many external influences, than need to be factored in.
E.g., observing electron wave interference in nanoscale Aharonov-Bohm rings is complicated by phase rigidity due to multiple scattering events. This necessitates opening the interferometer in order to allow scattered particles to escape, which necessarily lowers the visibility of the interference picture. In a way, this is just the opposite of the problem that you have in mind.
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$\begingroup$ Thank you for your answer. The experiment you reference is an interesting read and as far as I can tell, another ingenious way to add weight to Bohr's Complementarity. The discussion includes 'quantum shot noise' but does not make reference to virtual particles (VPs). You chose a good example. Had you chosen a double-slit exp. with photons and should VPs also be limited to photons, then we can be sure of no interaction as the Pauli exclusion principle does not apply. But seeing as fermions exhibit interference in the D-slit and VPs include fermions, then my question remains valid? $\endgroup$– QubitJul 27, 2020 at 6:41
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$\begingroup$ @user1575415 I am not sure what you mean by virtual particles - for me those are the intermediate states in the perturbative expansion, i.e., they exist in the calculations, but not in real life. In the Heiblum's experiment one could have virtual electrons. $\endgroup$– Roger V.Jul 27, 2020 at 7:23
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$\begingroup$ Yes, I should have been more specific: I did have in mind the perturbation of the quantum vacuum. But you made me think. Usually I would defer to a Feynman diagram for two real particles. However, perturbation should also apply to a single particle in the experiment we discussed, for example, the effective charge of the electron is different from the naked charge because of VPs. Are VPs real and not just a fudge factor for the calculations? Maybe vacuum birefringence could be observed? Otherwise, yes, they are virtual in QFT it would seem. $\endgroup$– QubitJul 28, 2020 at 4:07