"Surely You're Joking, Mr. Feynman!" - unam.

pronouncing the words, knowing what not to include in a paper or a seminar, and
detecting a weak technique in an experiment. But I love physics, and I love to go back to
it.
Monster Minds
While I was still a graduate student at Princeton, I worked as a research assistant
under John Wheeler. He gave me a problem to work on, and it got hard, and I wasn't
getting anywhere. So I went back to an idea that I had had earlier, at MIT. The idea was
that electrons don't act on themselves, they only act on other electrons.
There was this problem: When you shake an electron, it radiates energy, and so
there's a loss. That means there must be a force on it. And there must be a different force
when it's charged than when it's not charged. (If the force were exactly the same when it
was charged and not charged, in one case it would lose energy, and in the other it
wouldn't. You can't have two different answers to the same problem.)
The standard theory was that it was the electron acting on itself that made that
force (called the force of radiation reaction), and I had only electrons acting on other
electrons. So I was in some difficulty, I realized, by that time. (When I was at MIT, I got
the idea without noticing the problem, but by the time I got to Princeton, I knew that
problem.)
What I thought was: I'll shake this electron. It will make some nearby electron
shake, and the effect back from the nearby electron would be the origin of the force of
radiation reaction. So I did some calculations and took them to Wheeler.
Wheeler, right away, said, "Well, that isn't right because it varies inversely as the
square of the distance of the other electrons, whereas it should not depend on any of these
variables at all. It'll also depend inversely upon the mass of the other electron; it'll be
proportional to the charge on the other electron."
What bothered me was, I thought he must have done the calculation. I only
realized later that a man like Wheeler could immediately see all that stuff when you give
him the problem. I had to calculate, but he could see.
Then he said, "And it'll be delayed ­­ the wave returns late ­­ so all you've
described is reflected light."
"Oh! Of course," I said.
"But wait," he said. "Let's suppose it returns by advanced waves ­­ reactions
backward in time ­­ so it comes back at the right time. We saw the effect varied inversely
as the square of the distance, but suppose there are a lot of electrons, all over space: the
number is proportional to the square of the distance. So maybe we can make it all
compensate."
We found out we could do that. It came out very nicely, and fit very well. It was a
classical theory that could be right, even though it differed from Maxwell's standard, or
Lorentz's standard theory. It didn't have any trouble with the infinity of self­action, and it
was ingenious. It had actions and delays, forwards and backwards in time ­­ we called it
"half­advanced and half­retarded potentials."
Wheeler and I thought the next problem was to turn to the quantum theory of
electrodynamics, which had difficulties (I thought) with the self­action of the electron.