How would you react to the discovery of extraterrestrial life?

A discovery [of extraterrestrial life] would be of tremendous scientific significance. What could be more fascinating than discovering life that had evolved entirely independently of life here on Earth? Many people would also find it heartening to learn that we are not entirely alone in this vast cold cosmos.

But I hope that our Mars probes will discover nothing. It would be good news if we find Mars to be completely sterile. Dead rocks and lifeless sands would lift my spirit.

Conversely, if we discovered traces of some simple extinct life form - some bacteria, some algae - it would be bad news. If we found fossils of something more advanced, perhaps something looking like the remnants of a trilobite or even the skeleton of a small mammal, it would be very bad news. The more complex the life we found, the more depressing the news of its existence would be. Scientifically interesting, certainly, but a bad omen for the future of the human race.

These are the words of Oxford philosopher Nick Bostrom, in his 2008 essay Where are they? Why I hope the search for extraterrestrial life finds nothing. His reasoning follows that of Robin Hanson's groundbreaking 1998 piece The Great Filter - are we almost past it?. In a blog post a few years ago, I offered a gentle introduction (in Swedish) to Hanson's Great Filter view of Fermi's Great Silence (i.e., of the fact that we seem not to have encountered, or even seen any signs of, extraterrestrial life) and how it leads to the conclusion that "dead rocks and lifeless sands" would be uplifting. Very briefly, the argument is as follows.

Let N denote the number of potentially life-supporting planets in the observable universe; N is a huge number, perhaps something like 10²⁰ or 10²². Let p denote the probability that a randomly chosen such planet goes on to develop life, and not only life but intelligent life and a technological civilization on the level of present-day humanity. Finally, let q denote the probability, conditional on having come that far, of going on to develop a supertechnological civilization visible to astronomers all over the observable universe. Then Npq is the expected number of such supertechnological civilizations arising, and Fermi's Great Silence strongly suggests that Npq is not very large, because if it were, the probability of at least one such supertechnological civilizations having come about would have been overwhelming, and we would have seen it. But if N is huge and Npq is not very large, then pq must be very small, so at least one of the probabilities p and q is very small. If we're hoping for humanity not to self-destruct or otherwise go extinct before we get the chance to conquer the universe, q had better not be very small. A discovery of extraterrestrial life would suggest that the emergence of life is not quite as unlikely, and p not quite as small, as we might otherwise have thought. But if p is not so small, then q must be very small, and we are pretty much doomed.

This argument makes good sense to me, but it does contain a good deal of handwaving, and it might be interesting to find out whether, e.g., a more rigorous statistical treatment of the same problem leads to the same conclusion. This is what my Chalmers colleague Vilhelm Verendel and I set out to do in our paper Fermi's paradox, extraterrestrial life and the future of humanity: a Bayesian analysis, which has been accepted for publication in the International Journal of Astrobiology. Our findings are a bit inconclusive, because it is by no means clear what is a sensible choice of prior distribution in our Bayesian analysis, and the end result does depend quite a bit on this choice. Quoting from the concluding section of our paper:

In summary, we still think that the intuition about the alarming effect of discovering extraterrestrial life expressed by Hanson (1998) and Bostrom (2008) has some appeal. In our Bayesian analysis, our first two priors (independent uniform, and independent log-uniform) support it. The third one (perfectly correlated log-uniform), however, contradicts it, and while we find the prior a bit too extreme to make a very good choice, this shows that some condition on the prior is needed to obtain qualitative conclusions about the effect on q of discovering extraterrestrial life.

A final word of caution: While a healthy dose of critical thinking regarding the choice of Bayesian prior is always to be recommended, the case for epistemic humility is especially strong in the study of the Fermi paradox and related "big questions". In more mainstream scientific studies, circumstances are often favorable, either through the existence of a solid body of independent evidence in support of the prior, or through the availability of sufficient amounts of data that one can reasonably hope that the effects of the prior are (mostly) washed out in the posterior. In the present setting we have neither, so all conclusions from the posterior should be viewed as highly tentative.

Read our full paper here!