Daily Archives: June 26, 2014

The Fermi Paradox

Enrico Fermi (1901–1954)

Our universe is vast beyond imagination. Really, you cannot imagine how vast it is. If you look into the night sky in perfect conditions you might be able to see about 2,500 stars, but that is only 0.000001 of the stars in the Milky Way. There are between 100 and 400 billion stars in our galaxy and about the same number of galaxies in the observable universe. Thus there are about 1023 total stars or 100000000000000000000000 stars in the observable universe. For every grain of sand on earth, there are 10,000 stars out there! If you don’t think that is a lot go to the beach, play in the sand, and look around.

And these numbers are just stars. If our star, the Sun, is typical in having 8 planets then our galaxy alone contains something like 2 trillion planets! Now we don’t know what percentage of those stars are sunlike but if we go with a conservative estimate of 5% and the lower end for the number of stars, 1022 then there are about 500 quintillion or 500 billion billion sun-like stars!

Now if we go with the most recent conservative estimate of how many of those sun-like stars are orbited by Earth-like planets, around 22%, that leaves us with 100 billion billion Earth-like planets! A hundred Earth-like planets for every grain of sand on earth. Now if only 1% of those Earth-like planets orbiting sun-like stars developed life and if only 1% of those planets developed intelligent life then there would be 10 quadrillion, or 10 million billion intelligent civilizations in the observable universe! In our galaxy alone there would be 100,000 intelligent civilizations.

All of which caused the physicist Enrico Fermi to ask, why haven’t we encountered beings from other worlds? 

The fact is that SETI (Search for Extraterrestrial Intelligence) has never picked up a single radio wave or any other form of contact. If you don’t think this is surprising to consider that there are older stars with far older Earth-like planets on which more advanced civilizations could have developed. They could be civilizations that have harnessed all the energy of their planet or the star or their entire galaxy if they were sufficiently advanced.  If so they would have seemingly colonized the entire galaxy. Some scientists have hypothesized that civilizations could create self-replicating machinery that colonizes the entire galaxy in around 4 million years.

Source: Scientific American: “Where Are They”

And if only 1% of intelligent life survives long enough to become a potentially galaxy-colonizing civilization, there would still be 1,000 of those types of civilizations in our galaxy alone. So again, why haven’t we seen or heard for them? Where is everybody? This is the Fermi Paradox.


Here are a few of the explanations proposed to explain the paradox:

1) Higher civilizations are rare. Maybe something dooms them as they advance. Perhaps only a few of them have managed to surpass whatever it is that dooms civilizations and they have not spread out through the galaxy.

2) Higher civilizations don’t exist. We are the only civilization that has avoided destruction so far and may soon destroy ourselves too.

3)  Higher civilizations visited earth before we were here or before we had ways to record the visit.

4) Higher civilizations have colonized the galaxy but not our part of it.

5) Higher civilizations are not interested in colonization.

6) Higher civilizations know better than to broadcast their existence since there are predator civilizations out there.

7) There is one higher predator civilization which has exterminated all other civilizations.

8) Higher civilizations are out there but we don’t know how to perceive them.

9) Higher civilizations are observing us now but don’t want us to know. Perhaps they abide by the “Prime Directive” of Star Trek’s Federation.

10) We are wrong about reality; the universe is not vast in space and time.

I have no idea which if any of these hypotheses is true. What I do know is that our ignorance humbles me. The universe is not only bigger than we can imagine but probably stranger than we can imagine as well. As Xenophanes said long ago, “All we have is but a woven web of guesses.” And for those who cannot tolerate uncertainty and ambiguity, there is always fanatical ideology. As for me I’ll accept uncertainty and reject fanaticism, thereby living with integrity.

Note – This post relies heavily on and was inspired by an article published here at the website “Wait But Why?” http://waitbutwhy.com/2014/05/fermi-paradox.html