VN AfterTaste

This anecdote from “Surely you’re Joking Mr. Feynman” an autobiography by physicist Richard Feynman:

“I don’t know what’s the matter with people: they don’t learn by understanding; they learn by some other way– by rote or something. Their knowledge is so fragile!

I did the same kind of trick four years later at Princeton when I was talking to an experienced character, an assistant of Einstein, who was surely working with gravity all the time. I gave him a problem: You blast off in a rocket which has a clock on board, and there’s a clock on the ground. The idea is that you have to be back when the clock on the ground says one hour has passed. Now you want it so that when you come back, your clock is as far ahead as possible. According to Einstein, if you go very high, your clock will go faster, because the higher something is in the gravitational field, the faster its clock goes. But if you try to go too high, since you’ve only got an hour, you have to go so fast to get there the speed slows your clock down. So you can’t go too high. The question is, exactly what program of speed and height should you make so that you get the maximum time on your clock?

The assistant of Einstein worked on it for quite a bit before he realized that the answer is the real motion of matter. If you shoot something up in a normal way, so that the time it takes the shell to go up and come down is an hour, that’s the correct motion. It’s the fundamental principle of Einstein’s gravity– that is, what’s called the “proper time” is at a maximum for the actual curve. But when I put it to him, about a rocket with a clock, he didn’t recognize it. It was just like the guys in mechanical drawing class, but this time it wasn’t dumb freshmen. So this kind of fragility is, in fact, fairly common, even with more learned people.”