From Feynman's book QED The Strange Theory of Light and Matter. Because the theory of quantum mechanics could explain all of chemistry and the various properties of substances, it was a tremendous success. But still there was the problem of the interaction of light and matter. That is, Maxwell's theory of electricity and magnetism had to be changed to be in accord with the new principles of quantum mechanics that had been developed. So a new theory, the quantum theory of the interaction of light and matter, which is called by the horrible name of "quantum electrodynamics," was finally developed by a number of physicists in 1929. But the theory was troubled. If you calculated something roughly, it would give a reasonable answer. But if you tried to compute it more accurately, you would find that the the correction you thought was going to be small (the next term in a series, for example) was in fact very large--in fact it was infinity! So it turned out that you couldn...