Since then, scientists have applied the principle more broadly to suggest that humans have no special privileged view of the universe. We are just ordinary observers sitting on an ordinary planet in an ordinary part of an ordinary galaxy.

This form of thinking has had profound consequences. It led Copernicus to the idea that Earth orbits the sun and Einstein to his general theory of relativity. And it regularly guides the thinking of physicists, astronomers and cosmologists about the nature of the universe.

Now, Tom Westby and Christopher Conselice at the University of Nottingham in the U.K. have used the Copernican principle to come up with a new take on the existence of extraterrestrial civilizations. They point out that the principle implies there is nothing special about the conditions on Earth that allowed intelligent life to evolve. So, wherever these conditions exist, intelligent life is likely to evolve over about the same timescale as it evolved here.

This “astrobiological Copernican principle” has important implications for the way astronomers estimate the number of extraterrestrial civilizations that might be capable of communicating with us. Indeed, Westby and Conselice have crunched the numbers and say that, given the strongest limits they can place on the numbers, there are probably about 36 civilizations in the galaxy right now with this capability. But the numbers come with a significant caveat that also throws light on the Fermi Paradox, which famously suggests that if intelligent aliens exist, surely we ought to have seen them by now.