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A couple of years ago the eminent theoretical physicist Stephen Hawking warned that a human encounter with intelligent extraterrestrial beings could be dangerous for us, sort of similar to the way Europeans were dangerous to ancient Native Americans, “which didn’t turn out very well for the Native Americans.” In hindsight, it would not have been a good idea for the Native Americans to go searching for Europeans.

Still, there is a profound natural fascination for the idea that we are not alone. In the last 60 years or so, our collective thinking about the possibility of alien life has evolved from a state of dismissive skepticism to a sense of probability. Hawking himself has indicated the possibility of extraterrestrial life is perfectly plausible. There are so many stars in the observable universe — 100 billion or more galaxies containing an average of, let’s say, 100 billion stars each — that it seems less likely we’re unique, and more likely there are planets like ours with life forms. Astrobiologists nowadays construct detailed theories of how, where and when life — not necessarily intelligent, but any life — might arise in places other than Earth. The SETI Institute (“search for extraterrestrial intelligence”), which was founded in 1984 and is run by serious scientists, actively searches for intelligent extraterrestrial life by sending out and listening for coherent signals. It’s one of many such projects in the past 50 years, some associated with Harvard and NASA.

If intelligent life is common, the physicist Enrico Fermi asked in about 1950, then why haven’t we seen them yet? This was a hard question to answer, and became known in SETI studies as the Fermi Paradox, or in some lexicons the Great Silence. There are five basic replies to the question, and maybe a sixth:

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