"Where are they?" famously asked Italian physicist Enrico Fermi in the 1950s. By "they," he meant space aliens. Fermi figured that if the galaxy contained space-faring civilizations it would only take them a few tens of millions of years to populate it. So why hasn't there been a saucer-landing-take-me-to-your leader moment already? This is the Fermi Paradox.
In 1961, American astronomer Frank Drake devised an equation in which he tried to estimate the number of technological civilizations that might exist in our galaxy. Depending on the values plugged into it, the galaxy could be brimming with extra-terrestrials or we might be its only technologically advanced denizens.
A new paper in arXiv seeks to "dissolve the Fermi Paradox" by specifying various values for the parameters in the Drake Equation. The paper is by three researchers from the Future of Humanity Institute at Oxford University, research fellow Anders Sandberg, nanotechnologist Eric Drexler, and philosopher Tod Ord. As the researchers note:
We examine these parameters, incorporating models of chemical and genetic transitions on paths to the origin of life, and show that extant scientific knowledge corresponds to uncertainties that span multiple orders of magnitude. This makes a stark difference. When the model is recast to represent realistic distributions of uncertainty, we find a substantial ex ante probability of there being no other intelligent life in our observable universe, and thus that there should be little surprise when we fail to detect any signs of it. This result dissolves the Fermi paradox, and in doing so removes any need to invoke speculative mechanisms by which civilizations would inevitably fail to have observable effects upon the universe.
By "fail to have observable effects upon the universe," they are trying to address, among other issues, the prospect of a Great Filter that causes advanced civilizations to destroy themselves before they can colonize other stars or that aliens find watching television at home more edifying that traveling among the stars.
So what do the three conclude? From the article:
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 bearing on 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 a substantial probability that we are alone in our galaxy, and perhaps even 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.