Fermi Paradox
Overview
The Fermi paradox is the tension between the apparent likelihood of extraterrestrial civilisations, given the number of stars and planets and the age of the galaxy, and the complete absence of evidence for any. It is named for a lunchtime remark attributed to the physicist Enrico Fermi at Los Alamos in 1950, "Where is everybody?", and was set out formally by Michael Hart in 1975: a civilisation with modest interstellar ambitions could cross the galaxy in a few million years, which is nothing against its age, so if such civilisations were common the evidence should be everywhere. The Drake equation (1961) is the usual way of laying out the unknowns: the rate of star formation, the fraction with planets, the fraction of those that produce life, intelligence, technology, and the length of time a technological civilisation survives.
Notes
- It is only a paradox if you assume the terms in the Drake equation are large. Most of them are unknown to within many orders of magnitude; the fraction of habitable planets on which life begins could be close to one or close to zero, and we have a sample of one. "Great Filter" arguments ask which step is the hard one and whether it lies behind us or ahead.
- The proposed resolutions are many: life is rare, intelligence is rare, civilisations are short-lived, interstellar travel is harder than it looks, they are here and hiding, they are not interested, or we have not looked for long enough or in the right way. Several of these are consistent with the evidence, which is to say that the evidence does not currently discriminate between them.
- My position: the honest answer is that we do not know, and the paradox is a good question rather than a crisis. I lean towards the view that the steps from chemistry to a radio telescope are each less probable than optimism assumes, so that life may be common and company rare. The one resolution I reject is the UFO one, "they are here, but only ever show up as a blurry dot", because it solves a puzzle about missing evidence by asserting evidence that is itself missing (UFOs and UAP, Burden of proof).