The idea that intelligent life on Earth is a cosmic oddity strikes many as unwarranted terrestrial exceptionalism. There are some 300 billion stars in the Milky Way galaxy besides the sun and, by the latest estimates published earlier this month in Nature, each has, on average, at least one planet orbiting it.Even if only a tiny fraction could, in principle, sustain life, and only a tiny fraction of those actually do, that should still leave an awful lot of neighbours. Some of them would surely have called on man by now.
Why, then, haven’t they? The question, first posed explicitly in 1950 by Enrico Fermi, an Italian-American physicist, has elicited a plethora of responses. Perhaps civilisations just do not feel like chatting, or fear that humans could not handle it, or invariably destroy themselves before reaching the technological threshold at which interstellar communications become feasible? Alongside such inherently untestable proposals, however, are some more tractable ones. One is that although civilisations exist, they are few and slow to expand—and so have yet to reach Earth. Another is that galaxy is teeming with intelligent lifeforms, but they are unevenly distributed; Earth just happens to find itself in a bare patch.
Scientists calculated that any galactic empire would have spread outwards from its home planet at about 0.25% of the speed of light. The result is that after 50m years it would extend over 130,000 light years, with zealous colonisers moving in a relatively uniform cloud and more reticent ones protruding from a central blob. Since the Milky Way is estimated to be 100,000-120,000 light years across, outposts would be sprinkled throughout the galaxy, even if the home planet were, like Earth, located on the periphery.
And though 50m years may sound a lot, if intelligent life did evolve more than once, it could easily have done so billions of years before this happened on Earth. All this suggests that humans really do have the Milky Way to themselves. Either that or the neighbours are a particularly timid bunch.