The history of human technological civilization is measured in centuries—and it may be only one or two more centuries before humans are overtaken or transcended by inorganic intelligence, which will then persist, continuing to evolve, for billions of years. This suggests that if we were to detect ET, it would be far more likely to be inorganic: We would be most unlikely to “catch” alien intelligence in the brief sliver of time when it was still in organic form.

What does this mean for how we search for ET? SETI searches are surely worthwhile, despite the heavy odds against success, because the stakes are so high. They typically seek some electromagnetic transmission that is manifestly artificial. But even if the search succeeded (and few of us would bet more than 1 percent on this), it would still in my view be unlikely that the “signal” would be a decodable message. It would more likely represent a byproduct (or even a malfunction) of some super-complex machine far beyond our comprehension that could trace its lineage back to alien organic beings (which might still exist on their home planet, or might long ago have died out). The only type of intelligence whose messages we could decode would be the (perhaps small) subset that used a technology attuned to our own parochial concepts.

Even if intelligence were widespread in the cosmos, we may only ever recognize a small and atypical fraction of it. Some “brains” may package reality in a fashion that we can’t conceive. Others could be living contemplative lives, perhaps deep under some planetary ocean, doing nothing to reveal their presence. It makes sense to focus searches first on Earth-like planets orbiting long-lived stars.