- 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.
- 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.