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Electric field lines from a moving charged particle

  1. Sep 12, 2015 #1


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    The electric field emanating from charged particle travels at the speed of light; fine. To pose an unrealistic thought experiment (a more realistic, if more complicated, thought experiment could replace this one, but the unrealistic one gives the idea), if a proton were to suddenly pop into existence at a certain time T in a vacuum, and a stationary electron were situated 300,000 km away (and nothing else in the vicinity), then after one second, at time T+1 (ignoring the difficulty in synchronizing watches ), the electron would start to move towards the proton (and vice-versa, of course) in a straight line between the electron and proton. So far, so classic. However, if that proton is moving, would the electron at the moment T+1
    (a) start to move toward the position the proton was at time T, or
    (b) with a sort of quantum-eraser retroactive effect move toward the position the proton would be at time T+1?
    (ignoring the effect of any magnetic field created by the moving proton)
    Thanks in advance.
  2. jcsd
  3. Sep 12, 2015 #2


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    The appearing proton violates the laws of electrodynamics (charge conservation). It does not make sense to ask what physical laws predict in a situation where the laws are violated.

    "Where the proton was one second ago" is a better description, but it is not always exact.
  4. Sep 12, 2015 #3


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    Thanks for the reply, mfb.
    This is why I inserted the caveat that one could certainly make up a Gedankenexperiment which did not violate the laws of physics but which got the idea across. My experiment was in the spirit of 3:40-5:20 of Greene's . I could have perhaps better simply asked the question without thought experiment in this way: "assuming it makes sense to talk about the velocity and straight-line path of the carriers of the electric field, then it makes sense to ask about that path as a straight line between two space-time points. Are these points separated in time as would be expected in a classical theory, or not?"
    I presume that is the answer to my question, despite its impossibility. I also presume that the lack of exactitude you refer to is due to relativistic considerations, not to mention the magnetic field. Any further corrections will be appreciated.
    Thanks again.
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