Monday, 27 May 2019

Does the frac43 problem of classical electromagnetism remain in quantum mechanics?


In Volume II Chapter 28 of the Feymann Lectures on Physics, Feynman discusses the infamous 43 problem of classical electromagnetism. Suppose you have a charged particle of radius a and charge q (uniformly distributed on the surface). If you integrate the energy density of the electromagnetic field over all space outside the particle, you'll get the total electromagnetic energy, which is an expression proportional to c2. The energy divided by c2 is what we usually call the mass, so if we calculate the "electromagnetic mass" in this manner we'll get m=1214πϵ0q2ac2. If, on the other hand, you took the momentum density of the electromagnetic field and integrated it over all space outside the particle, you'd get the total electromagnetic momentum, which turns out (for $v<) to be proportional to the velocity of the particle. The constant of proportionality of momentum and velocity is what we call mass, so if we calculated the electromagnetic mass in this way we would get m=2314πϵ0q2ac2, which is 43 times the value we got before! That is the 43 problem.


Feynman claims that this fundamental issue remains when we move to quantum electrodynamics. Was he right, and if so has the situation changed since the 1960s when he was writing? I've seen claims on the Internet (I don't have the links) that the 43 problem is still there in QED, but instead of 43 the coefficient is something closer to 1. Is that true, and if so what's the coefficient? All of this is of course related to issues of self-energy and renormalization.


Any help would be greatly appreciated.




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