Enrico Fermi, considering the billions of stars out there in the galaxy, noted to a number of physicists back in 1950 that the real mystery is why we haven’t seen any aliens yet. Even given a very low percentage of planets that have intelligent life which produces technology, the colonizing wavefront of at least one of these civilizations should have reached planet Earth. The Fermi paradox could thus be succinctly stated “Where is everybody?”
There are several postulates as to why they haven’t made it here yet, and it helps to review the Drake equation, from wikipedia:
The Drake equation states that:
- N is the number of civilizations in our galaxy with which we might hope to be able to communicate;
- R* 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 the above that actually go on to develop life at some point
- fi is the fraction of the above that actually go on to develop intelligent life
- 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 such civilizations release detectable signals into space.
For most of the numbers we have started to get a better and better guesstimate. As we move from right to left, starting from the equal sign, our certainty about each variable decreases, until we get to “L”
We know almost nothing about L, except what we have observed from our planet. Some despair that L may be very low, because civilizations tend to self extinguish. Or it may be the case that even if they develop the technology, they might be uninterested in using that technology to communicate with us. Imagine, for example that the Earth had cloud cover that never allowed inhabitants to see the night sky. Civilizations that arise on such a planet might be very inward looking, and never even consider what’s beyond. But it seems most likely that that would be rare.
As we find more and more suns that have planets, and some with ones that appear to be earth-like in some ways, the probability is that civilizations tend to self-extinguish once they develop technology cannot be ignored.
Why might that be? Why would L be very small, nearly uniformly?
There are several lines of thought here. All should develop non-idiosyncratic causes, because those could happen in any one given civilization, but not in others.
My favorites are:
1. Technology is developed by competitive species; those tend to be inherently war-like, to some degree. Once technology such as nuclear bombs become widespread, it is nearly inevitable that that civilization will wipe itself out.
2. There is a technology which is discovered, and its dangers are not realized until it has destroyed that civilization. Perhaps genetic engineering or nano-technology has some elementary danger, similar to the grey goo hypothesis.
Some other articles: