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Can the electron field be excited while the photon field is not? I'm guessing the answer is no, because electrons are supposed to interact with their own electric field. I don't know about fluctuations from vacuum, though.

But how does that work anyway? Does it all come from the coupling of the two fields? How?

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    $\begingroup$ I think it is somewhat analogous to the question if electric and magnetic fields can exist independently. Yes, but not really if you think about all observers rather than just the observer in one particular rest system. It's probably better to think of all these fields as components of one larger, single entity ("the physical vacuum"). Divide and conquer takes us only so far and in this case it might oversimply things a little too much. $\endgroup$ Jan 23 at 11:06
  • $\begingroup$ Are there even photons in the physical vacuum? $\endgroup$
    – K. Pull
    Jan 23 at 11:44
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    $\begingroup$ Quanta are the results of actual measurements, so, no, there are no individual photons/electrons etc. in the physical vacuum. There is only energy, momentum, angular momentum and charge that is distributed according to the dynamics of quantum fields. The question here is whether we can separate these fields in general. The answer is negative. We can separate them in many special cases, like along the lines of electromagnetic/weak/strong interaction or along energy/momentum (e.g. below/above pair production threshold). In general the vacuum behaves like a very complex single entity. $\endgroup$ Jan 23 at 22:31

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In addition to electromagnetic ("photon") field, electrons also participate in weak interactions with other particles. So, in principle it is possible to excite electron without photons. The simplest example is (electronic) beta-decay of e.g. neutron. Here among other particles, an electron is produced, i.e. the electron field has acquired an additional quantum of excitation with photons ${not}$ being involved.

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    $\begingroup$ Muon decay to an electron, a neutrino and an antineutrino is another common example. $\endgroup$ Jan 23 at 10:44
  • $\begingroup$ How are photons not involved when the renormalized mass of the electron contains contributions from the photon field? $\endgroup$
    – K. Pull
    Jan 23 at 11:43

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