A few years ago we talked about the chance that the first known extrasolar visitor — Oumuamua — might be a derelict solar sail. That notion has been picking up steam in the popular press lately, and it made us think again about the chances that the supposed rock was really a solar sail discarded or maybe even a probe flying with a solar sail. At the same time, Mars is as close as it ever gets so there is a gaggle of our probes searching the red planet, some of them looking for signs of past life.
All this makes us think: if we did find life or even artifacts of intelligent life, would we realize it? Sure, we can usually figure out what’s alive here on Earth. But to paraphrase Justice Potter Stewart, “We know it when we see it.” Defining life turns out to be surprisingly tricky, recognizing alien technology would be even harder.