This reminds me of something I read about the time invariance in QED. I forgets the details, but I think it was Feynman who said that when you learn QM, you lea

This reminds me of something I read about the time invariance in QED. I forgets the details, but I think it was Feynman who said that when you learn QM, you learn about these equations which are time invariant, they produce symeytical equations where the left side is a particle is travelling backwards in time, and the right side is travelling forwards in time. And your professor waves his hands and says we'll deal with that later. Then eventually at the end of your studies you just wipe the left side off the board and ignore it.