Purported UFO, Passoria, N.J., 1952. Enthusiasts point out that with 250 billion stars in the Milky Way and 70 sextillion in the visible universe, it’s overwhelmingly likely that there are other intelligent, communicating beings out there.

But over a lunch discussion in 1950, physicist Enrico Fermi asked a telling question: “Where are they?” The universe is 13 billion years old, and it’s been estimated that an advanced civilization could colonize our whole galaxy in 5 million years. That’s a flash, as cosmologists reckon time — even if the aliens themselves couldn’t survive an interstellar journey, surely they could design a self-replicating spacecraft that could.

So how come we haven’t met our galactic neighbors? Opinions vary. Maybe we’re in a zoo. Maybe they’re so alien that even detecting them is impossible. Or maybe they don’t believe in us.