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I understand how the wave equation works, and therefore how light moves through the EMF; this question is not about that mechanism.

But when a charge moves in space, information about its location travels outward at the speed of light, altering the field strength of the EMF. What is the mechanism by which this information moves? Is it also the wave equation?

In my classical electromagnetism texts, Maxwell's equations simply state that the information about changes in charge density/location travels through the field at the speed of light, but do not explain the mechanism.

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    $\begingroup$ What would you consider an acceptable description of "the mechanism"? $\endgroup$ – Robin Ekman May 30 '14 at 0:40
  • $\begingroup$ consider a point p in the EMF local to charge q in an isolated system containing only that charge. That point has a field strength E given by Coulomb's law. Now the charge q moves. Coulomb's law shows that p now has a different field strength, say E'. How was the information about the change in the location of q communicated to p such that E changed to E'? It cant be a virtual photon; that notion is from QED; It cant be action-at-a-distance; that is what the EMF concept was invented to avoid. So how does the information propagate through the field? As an electromagnetic wave? If not, as what? $\endgroup$ – Yubadoobadoo May 30 '14 at 1:11
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    $\begingroup$ Oh, then I think you are looking for these: en.wikipedia.org/wiki/Li%C3%A9nard%E2%80%93Wiechert_potential $\endgroup$ – Robin Ekman May 30 '14 at 1:20
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The moving charge is exchanging photons with its surroundings. Photons are the force carrier for EM interactions. The "information" moves with the "speed of light" because it is carried by "light".

In classical electrodynamics the charges have fields radiating out like spokes. When accelerating the charge from a (relative) zero velocity to a positive velocity, the spokes then develop kink in them that propagates out at the speed of ... wait for it ... "light" ... as a consequence to Maxwell's Equations. Stationary charges have spokes that radiate out with a very uniform density, while moving charges with a constant velocity have spokes that are "dragged back" in proportion to the velocity. The spokes at at a sufficiently large distance are located where they were at an earlier time. The distant observer then actually "feels" the charge as though it were in a location from the past.

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  • $\begingroup$ I should have been more specific: I want to confine my question to classical electrodynamics. Your answer is from quantum electrodynamics, which posit "virtual photons" as the carriers of information about charge location/magnitude; there is a good deal of doubt philosophically speaking about whether these virtual photons are anything more than mathematical devices (most think they are not). But in any event how is this process explained in classical electromagnetism, within the confines of Maxwell's equations? $\endgroup$ – Yubadoobadoo May 30 '14 at 1:06

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