Hi!

I am going to explain this theory, but the question is are you going to understand it, when I tell you first that the first time we really thoroughly explain it to our own physics students is when they are in their third year of graduate physics, and you think the answer's gonna be No. And that's correct. You will not understand. But the students do not understand it either. And that's because the professor doesn't understand. And that's not a joke, but very interesting.

Richard Feynman, QED: Photons - Corpuscles of Light (Sir Douglas Robb Lectures, 1979)