For more than 100 years the spaceships of science fiction have whisked among interstellar civilizations on missions of exploration, trade, war, romance, and adventure.
Such tales of sidereal derring-do prompted Enrico Fermi wistfully to ask his colleagues over lunch at Los Alamos one day in 1950, “Where are they?” He meant intelligent aliens. If they’re out there, shouldn’t we see or hear something? Enrico obviously did not take flying saucer sightings seriously, even though they were all the rage in 1950.
When you are as famous a scientist as Fermi, even a casual question is likely to get named after you, and this one is known as Fermi’s Paradox. Astronomer Frank Drake had no specific answer for Fermi, but in 1960 he wrote a formula to describe his lack of knowledge accurately. This is the Drake Equation:
N = R* x fp x ne x fℓ x fi x fc x L
The factors for the number of detectable galactic civilizations are the rate of star formation, the fraction of stars that have planets, the fraction of planets that are habitable, the fraction of those that actually develop life, the fraction that develop intelligent life, the fraction that develop a detectable technical civilization, and the length of time technical civilizations last.
Ever since 1960 scientists and amateurs alike have tried plugging numbers into this formula, and have come up with results as disparate as 1 and 50,000. One recent effort was by Duncan Forgan at the Royal Observatory in Edinburgh. He estimated a number for each variable based on current data. Forgan published his conclusions in the International Journal of Astrobiology.
Forgan’s answer is that there are 361 advanced civilizations in the Milky Way galaxy. His number presupposes that life can arise only under a narrow set of conditions and circumstances. He notes that if life can spread easily from one planet to another (carried perhaps by comets, meteorites, or asteroids) the number skyrockets, since life can exist in conditions where it wouldn’t form spontaneously. If this is the case, the number of civilizations is closer to 38,000.
361? I hear Fermi’s voice saying, “Where are they?” Forgan says that at 361 they are very thinly spaced in a galaxy 100,000 light years across. Even at 38,000, he adds, there is no guarantee of mutual contact. Perhaps not. At the larger number, though, one way contact seems pretty hard to avoid; even at the smaller number it is likely, especially since some of those aliens are bound to have had a substantial head start over humans at civilization. The earth has been flashing brighter than the sun in some frequencies ever since defense radars came on line more than 70 years ago. If ETs are doing the same, we should overhear some of their phone calls. So far there is silence.
Despite Forgan’s rigorous mathematical treatment of his data, he acknowledges the last few variables of Drake’s equation are still, at bottom, filled by guesses. Forgan knows more about the first few variables than I ever will, but it is my instinct that his guesses on the last few are wildly optimistic. So, therefore, is the number 361.
It’s a big universe, and I’ve little doubt that other self-important life-forms out there somewhere have posed the question, “Where are they?” Nonetheless, they may be long long ago in a galaxy far far away.