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In a recent question on superpositions of different quarks it was explained, that the superpositions of different electric charged particles cannot exist, in contrast to strangeness quantum number. It was reasoned that this is because of decoherence that comes from the QED vacuum - the photon interacts ("measures") with charged particles -> thus their charge is known -> destroys decoherence.

But of course, double slit experiments with single electrons have been performed already in the 70s.

My questions:

Why do virtual photons not cause decoherence of the spatial coherence of single electrons?

If they would cause decoherence, it would result in reduction of the contrast in single electron double-slit experiments.

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Mario Krenn
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  • Virtual photons are mathematical, not physical objects (and real photons aren't physical objects, either, they are properties). That electric charge is conserved is a symmetry property and does not depend on measurement. Why nature should care about some folks making tiny slits into metal foils is philosophically as questionable as it's physically irrelevant. – CuriousOne Jun 11 '16 at 20:05
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    @CuriousOne Your comment on virtual photons seem to ignore things like Casimir effect, Zitterbewegung etcetcetc. Your second sentence, thanks - that was indeed misleading, which I corrected now. Your 3rd sentense seem very dubious, you say that the electron-double-slit experiment is physically irrelevant (even though it has been elected as "the most beautiful experiment" in physics?). I guess/hope i misinterpret what you are saying, could you please clarify? And why did you downvote? – Mario Krenn Jun 11 '16 at 20:23
  • The Casimir effect is caused by the matter in the experiment, it doesn't happen without it. Saying that virtual photons are mathematical objects doesn't mean they don't matter, it just says that there is no relevant experimental ontology assigned to them. How would nature know that a homo sapiens is performing a double slit experiment? Nature is not a person and it doesn't keep a big book with our activities. In general there is a lot of unsupported philosophy here. – CuriousOne Jun 11 '16 at 20:31
  • As for your question... electron diffraction has been performed on structures as small as the internal structure of a proton with no noticeable deviation from the predictions of QFT, as far as I know, and the there is no competing model that would or could predict deviations, as far as I know, either. – CuriousOne Jun 11 '16 at 21:11
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    It may be worth noting that the usual electron one experiments with (i.e. the one that has a mass of $m_e \approx 511,\mathrm{keV/c^2} \approx 9.11 \times 10^{-31},\mathrm{kg}$ and a g-factor of $−2.00231930436182 \pm 0.00000000000052$) is 'dressed'. That is, it already has the effects of the QED vacuum folded in. No one has even gotten to play with the hypothetical, pristine, unicorn-attracting, 'bare' electron. – dmckee --- ex-moderator kitten Jun 11 '16 at 21:35
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    @NiceDean These two answers are both connected to your question. Also read the comments on the first one. – Oscar Cunningham Jun 12 '16 at 14:01
  • I don't understand your question. The answer you are talking about explicitly talks about an actual light wave passing by, not some "virtual photons in the QED vacuum" being responsible for the decay of the state. – ACuriousMind Jun 12 '16 at 17:35
  • @ACuriousMind The question is simple and written explicitly: Why do virtual photons not cause decoherence of the spatial coherence of electrons? (That they dont decohere the single electrons is visible in the high contrast of electron double-slit experiments). Let me write that more concretely. Thanks. – Mario Krenn Jun 12 '16 at 18:57
  • Virtual photons is a Coulomb field. If you charge the screen and other "environment", it will affect the interference picture. – Vladimir Kalitvianski Jun 12 '16 at 19:10
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    Who are the people who vote to close a question like this? There seems to be a cohort on this site, whose response to a question with slightly mistaken premises, is to just close the question! It's the only way I can understand some of the closures that occur. – Mitchell Porter Jun 12 '16 at 19:43
  • @VladimirKalitvianski Thanks for the answer. Unfortunatly I dont understand completly what you want to say. Is it that there are QED effects on the electron interference pattern? If yes, do you know of references to this? – Mario Krenn Jun 12 '16 at 21:06
  • @NiceDean : Yes, quasi-static (i.e., of relatively low frequency) electric field in the old television tubes was used to control the electron beam position. The same effect - deviation of the electron path may happen in the double slit experiment due to static fields (= virtual photons). It is not a decoherence. – Vladimir Kalitvianski Jun 13 '16 at 08:43

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