I’ve wanted to talk about the apologetic worth of the Fermi Paradox and the Drake Equation for sometime. Now a recent paper on arXiv (https://arxiv.org/abs/1806.02404) gives me the excuse, and some fascinating numbers to motivate discussion. First a warning though, this paper is a preprint. While submitted to a reputable journal (Proceedings of the Royal Society of London A), it has not undergone peer review. As such one should take the results cautiously as the community is still coming to terms with this topic, as the paper shows.
So what is the Fermi Paradox? Originally posed by Enrico Fermi in 1950 it goes something like this. First assume that the 4.5 billion years (Gyr) it took our planet to evolve to its current stage of development is the average amount of time it takes for an intelligent species to arise. If this is the case then there should be plenty of opportunities for intelligent life to evolve elsewhere in the galaxy as the galaxy is approximately 9 Gyr old. After all there are about 250 billion stars in the galaxy and plenty of time to fit in the 4.5 Gyr needed to get to our current state of affairs. In addition to traverse all the stars in the galaxy would only take roughly 10-1000 Myr (million years) assuming sub-light travel speeds. As such there is plenty of time for an intelligent species to traverse the galaxy multiple times over. Given all this we should see signatures of alien intelligent life all over the place but we only see ourselves. So where are they?
From the opposition direction comes the Drake Equation. It tries to estimate how many intelligent space faring civilizations there should be in the galaxy. By combining estimates of how many earth-like planets there are, how many have given rise to abiogenesis, how many have had organisms evolve to be intelligent, and so on you can get a ball park figure as to how many civilizations you would expect to find.
Both of these are classic SETI (Search for ExtraTerrestrial Intelligence)/astrobiology thought experiments. I taught them both to undergrads in graduate school. They are great thought experiments because both are exceedingly sensitive to your biases. This is because we lack hard data to back up the numbers plugged into the Drake Equation, and the Fermi Paradox causes all sorts of ideas as to why we don’t see aliens.
The paper I alluded to above by Sandberg, Drexler, and Ord, called “Dissolving the Fermi Paradox” takes a survey of all the numbers used for estimates of the Drake Equation and instead of picking one, gives a range of probable answers and sees how likely it is statistically that we are in a universe with alien life. The name of the paper comes from the fact that the Fermi Paradox goes away if we find out its probably that there to be no life besides us in the universe. After looking at a variety of options they come to this conclusion:
We have seen that a Fermi paradox arises if we combine a high and extremely confident prior for the number of civilizations in our galaxy with the absence of evidence for their existence. The high confidence that causes this clash typically results from applying a Drake-like model using point estimates for the parameters. These estimates, however, make implicit knowledge claims about processes (especially those connected with the origin of life) which are untenable given the current state of scientific knowledge.When we take account of realistic uncertainty, replacing point estimates by probability distributions that reflect current scientific understanding, we find no reason to be highly confident that the galaxy (or observable universe) contains other civilizations, and thus no longer find our observations in conflict with our prior probabilities. We found qualitatively similar results through two different methods: using the authors’ assessments of current scientific knowledge bearingon key parameters, and using the divergent estimates of these parameters in the astrobiology literature as a proxy for current scientific uncertainty.When we update this prior in light of the Fermi observation, we find asubstantial probability that we are alone in our galaxy, and perhapseven in our observable universe (53%–99.6% and 39%–85% respectively). ’Where are they?’ — probably extremely far away, and quite possibly beyond the cosmological horizon and forever unreachable.