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After reading most of the electromagnetism chapters of Feynman's lectures on physics, I would like to understand in more detail, at least an idea, of what causes the electromagnetic fields. Not sure where to go.

For instance, an electric field is caused by a charged particle. The presence of another particle would mean a force would act upon it. But is there a better explanation for this force to occur? By wikipedia browsing, photons are "the force carrier for the electromagnetic force". What does this mean exactly?

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The force acts between bodies, the electric field determines the force into the equations of motion of bodies. The body interaction is generally retarded so the field depends on time in a retarded way. The total electric field of a given charge is a sum of a "near field" that depends on time, but decays with distance as $1/R^2$ (they say it is "attached" to the charge), and a propagating field decaying as $1/R$. The latter corresponds to real photons (waves propagating with light velocity) whereas both are involved into the charge interaction. Depending on the value of $R$, one or another term may dominate.

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Nice, thanks. And which field of Physics introduces these separate fields? I read that almost everything is explained by quantum mechanics+relativity, but if a middle theory introduces this, the better. (such as electromagnetism or mechanics, which are often "good enough") –  user7384 Jan 24 '12 at 16:45
    
You may find this discussion useful. Also the short chatty discussion on John Baez website –  twistor59 Jan 24 '12 at 17:17
    
Such a filed decomposition can be found in any textbook on classical electrodynamics that describes the radiation. In particular, Feynman lectures contain it too, see Chapter 21 in Vol. 2 and references in it. student.fizika.org/~jsisko/Knjige/Opca%20Fizika/… –  Vladimir Kalitvianski Jan 24 '12 at 17:18
    
All great answers, thanks :). ps: can't vote yet –  user7384 Jan 24 '12 at 17:43
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