The universe is so big and full of stars that it seems obvious some must have evolved intelligent life. But it turns out we know so little we can’t know what’s obvious. Quite likely we are alone

Are we alone in the universe? Of all the billions of stars out there, is there none around which intelligent life has arisen, no other conscious beings who have looked at their sky and asked themselves whether there was anyone else out here? All we can know is that we don’t know of any others. But that has not stopped more or less well-informed speculation. The universe is so unthinkably enormous and old that it seems almost impossible that only one of the quintillion or so stars in the universe has actually developed intelligent life.

So where are they? So asked the physicist Enrico Fermi in 1950. If other intelligent species are out there, why haven’t we seen them yet? The mismatch between what we’d expect from the numbers, which is a universe full of spacefaring civilisations, and what we observe – nothing – is known as the Fermi paradox. Few of the explanations proposed for it are cheering. Perhaps all civilisations advanced enough to develop space travel are also technologically capable of annihilating themselves as well, and perhaps they all do. Perhaps the first culture to develop interstellar travel has already snuffed out all its rival species as they emerge, and is at this moment watching our first tentative explorations of the solar system as a cat might watch a fledgling on the ground. Or perhaps we have simply got the numbers wrong.

This last explanation comes from three Oxford philosophers, whose recently published paper examines the equations that make the Fermi paradox look real. These have to do partly with the number of stars with Earth-like planets in the universe, and, more crucially, with the probabilities of life evolving there, then becoming intelligent, and finally exploring space. None of these are known and we don’t yet know enough to make well-informed guesses. So the numbers that are plugged into all seven terms of the equations that estimate the likelihood of other life in the universe can vary by 12 or more orders of magnitude.

The Oxford paper shows that when you take these uncertainties into account and run hundreds of thousands of simulations exploring them, the probability that we are alone in our galaxy, and perhaps in the universe, rises to entirely reasonable levels. The Fermi paradox vanishes. There is quite probably no one out there to rescue or to care about us. What happens to our species is in our hands alone. We had better get on with it.