It seems that this may be the year when average people start looking to the skies to see whether we are alone in the universe.. For those who have never been interested in science fiction and whose only conception of aliens has been jokes about little green men, crop circles, and that story your crazy uncle told you about what happened when he was alone on that road one night, this can all seem a little “kooky”. But believe it or not, a lot of scientists and serious people have spent a lot of time contemplating whether we are really alone in the universe, and billions of dollars has been invested in the possibility. For those who are unfamiliar with this area of scientific speculation, there are two fundamental ideas you should be familiar with: the Drake Equation and the Fermi Paradox.
The Drake Equation is an attempt to figure out how many intelligent species are out there. It looks like this (don’t worry that it looks a little complicated – I’ll simplify it after):
N = Rstar x fp x ne x fl x fi x fc x L
- Rstar is the average rate of star formation in our galaxy
- fp is the fraction of those stars that have planets
- ne is the average number of planets that can potentially support life per star that has planets
- fl is the fraction of planets that could support life that actually develop life at some point
- fi is the fraction of planets with life that actually go on to develop intelligent life (civilizations)
- fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
- L is the length of time for which such civilizations release detectable signals into space
If you put it in simpler terms, it’s basically this:
Number of alien civilizations in the galaxy =
[number of stars in the galaxy] x [average number of planets per star] x
[average chance that a planet will develop life] x [average chance that that life will create a civilization]
This ignores the parts about the idea that alien civilizations could have colonized the galaxy a billion years ago and we’d never know about it if they all died out, but let’s keep it simple.
Obviously we don’t know all the numbers to plug into this formula. But even if you imagine that the last few parts of this equation are really, really low, like 0.001% chance of each, we now know that the first two [100,000,000,000 stars]x[quite possibly an average of 1.6 planets per star] are so high that the answer is pretty much guaranteed to be “more than one” (and this is just for our galaxy – multiply that by another 125,000,000,000 galaxies and it becomes basically a mathematical certainty that there’s life elsewhere in the universe).
So the Fermi Paradox responds, “so where the hell are they”? If there is pretty much a mathematical guarantee that someone should be out there, why has there been no confirmed contact with any aliens? This Fermi Paradox has generated a number of possible responses. One is the “Zoo Hypothesis” – the idea that these alien civilizations actively don’t want us to see them – they’re observing us from afar, either for nefarious purposes (like harvesting us once we’ve done the dirty work of extracting all Earth’s minerals or something) or for benign purposes (like waiting for us to develop on our own to a certain point (you may be familiar with the Star Trek version of this idea, the Prime Directive, in which it’s forbidden to interfere with civilizations that haven’t figured out to travel faster than light)). The other, scarier hypothesis, is the “Great Filter” – the idea that maybe plenty of civilizations arise but that something – nuclear war, rogue AI, Reapers, take your pick – inevitably prevents civilizations from staying together for long enough to go out and colonize the galaxy.
Recently, futurist Robin Hanson and a team of colleagues proposed a new answer to this question in a paper (https://arxiv.org/abs/2102.01522) – basically that there could be numerous civilizations currently expanding throughout the galaxy but that there is no reason that we should have any sign of them yet because space is huge and it just takes millennia and millennia to make even the tiniest dent in the galactic map, and that even things like radio signals don’t move that fast either in the grand scheme of things – and those radio signals would form just thin shells around the expanding sphere of colonized worlds.
Likely, no one reading this blog will ever hear of any definitive evidence of extraterrestrial life, either that which has visited Earth or that circling distant suns. But as has often been the case, the idle speculation of today becomes the testable hypothesis of tomorrow, and the sound science of centuries to come.