When Wheeler first drew the “big U” when Wald was a student in the 1970s, Wald didn’t think much of it. “Wheeler’s idea struck me as not that solidly grounded,” he said.

And now? “A lot of the stuff he did was enthusiasm and some vague ideas which later turned out to be really on the mark,” Wald said, noting that Wheeler anticipated Hawking radiation long before the effect was calculated.

“He saw himself as holding out a lamp light to illuminate possible paths for other people to follow.”