Monday, September 12, 2016

Where are all the extraterrestrials?

Thousands of alien species could have set out for the galactic frontiers because our home galaxy has had at least ten billion years to create civilizations from habitable worlds. We're the new kids on the block.

In astronomy the simplest arguments are often the most profound. In the 1950s and 1970s, respectively, physicists Enrico Fermi and Michael Hart reasoned that any sufficiently motivated extraterrestrials have had more than enough time to explore all the worlds of the Milky Way. The fact that we see no evidence of alien civilizations forms the basis of the Fermi-Hart paradox.

The argument is straightforward. Our galaxy has had at least ten billion years to form habitable worlds around many of its now hundreds of billions of stars. If the tiniest fraction of those worlds supported intelligent life with the will to explore, thousands of alien species could have set out for the galactic frontiers. Even using "primitive" spaceflight technology of the sort we can develop now or within a generation or two, it should take only a hundred million years to cover all the worthwhile destinations.

That's a long time, but it's far less than ten billion years, or even the 4.6 billion that our planet has existed. In theory, Earth has been around long enough for us to be visited dozens of times. As Enrico Fermi famously asked, "Where is everybody?" Are we alone in the universe? Is intelligent life exceedingly rare?

Are we in some kind of cosmic zoo? Are we stuck in a great simulation? Is civilization doomed? There are lots of ideas out there, but without testable predictions, most aren't science. Still, they are fun to think about.

From National Geographic's "Are We Alone?" special magazine:

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