Richard P. Feynman, 1985
Penguin Books p 55-56, Note 3
This is an example of the "uncertainty principle": there is a kind of "complementarity" between knowledge of where the light goes between the blocks and where it goes afterwards—precise knowledge of both is impossible. I would like to put the uncertainty principle in its historical place: When the revolutionary ideas of quantum physics were first coming out, people still tried to understand them in terms of old-fashioned ideas (such as light goes in straight lines). But at a certain point the old-fashioned ideas would begin to fail, so a warning was developed that said, in effect, "Your old-fashioned ideas are no damn good when..." If you get rid of the old-fashioned ideas and instead use the ideas that I'm explaining in these lectures—adding arrows for all the ways an event can happen—there is no need for an uncertainty principle.