In the spring of 2016 I was at a reception with Freeman Dyson, the brilliant physicist and mathematician, then 92 and emeritus at the Institute for Advanced Study in Princeton, N.J. He never says what you expect him to, so I asked him, “What’s new?” He smiled his ambiguous smile and answered, “Apparently we’re going to Alpha Centauri.” This star is one of our sun’s nearest neighbors, and a Silicon Valley billionaire had recently announced that he was funding a project called Breakthrough Starshot to send some kind of spaceship there. “Is that a good idea?” I asked. Dyson’s smile got wider:

“No, it’s silly.” Then he added, “But the spacecraft is interesting.”

The spacecraft is indeed interesting. Instead of the usual rocket, powered by chemical reactions and big enough to carry humans or heavy instruments, Starshot is a cloud of tiny, multifunction chips called StarChips, each attached to a so-called light sail. The sail would be so insubstantial that when hit by a laser beam, called a light beamer, it would a