Deep Time and Bug Eyed Monsters

It's a big universe out there, and among the billion-trillion or so stars in the observable part of it, it seems that lots have planets. So, as Fermi asked, where are the little green men or bug eyed monsters from outer space? Is it plausible that no technological civilizations that can cross interstellar space exist?

It's a hard question, since it depends on a lot of unknowns, expecially upon the three probabilities that a star with planets will have a habitable planet, on the probability that life will originate on a habitable planet, and on the probability that, once having originated, life will evolve a technological and space faring civilization. We don't know the answers to these questions or to some other relevant questions but at the moment, phase space for this reaction seems to be increasing.

A decade or two ago, we knew almost nothing about other planetary systems - today it is clear that they are common. We still don't know how life arose on this planet, but we know that it didn't take long. Earthly life appeats to have been around almost from the time that the planet was cool enough to support it.

It did take a long time for life to evolve technological civilization, apparently at least 3.5 billion years.

The observable universe appears to be about 13 billions years old, and the oldest stars nearly that old. Those oldest stars aren't much like ours though, and probably could have planets like ours. Sun like stars had to wait perhaps a few billion years, until enough supernovas had blown up to produce heavier elements. It likely took a few billion years for enough galactic and stellar evolution to occur for Sun like stars to become common. Still, it seems likely that the oldest population I (Sun like) stars are 3 billion or so years older than the Sun, which suggests that they have had plenty of time to evolve technological civilization.

So where are they?

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