The Fermi Paradox - Part 1

This post is intended for those unfamiliar with the Fermi paradox. If you've already read a thing or two about it, I invite you to skip this one and wait until Part 2.

This is a companion post to Episode 2 of the Wow! Signal Podcast.  You can go over there and get the audio version with cool music.

For the purpose of this post, we are going to take as a given the null hypothesis about alien visitors to Earth - that this has never happened.  This is what the controversial astrophysicist Michael Hart called Fact A.

It turns out that accepting Fact A presents an interesting dilemma; first recognized by the distinguished physicist Enrico Fermi in 1950.  The unsolved problem is, that we would expect alien visitors on our planet, given a set of very reasonable assumptions. It’s really not such an extraordinary claim, after all. Assuming Fact A, this presents us with a puzzle. Either one or more assumptions have to go, or something really weird is going on.

Douglas Adams once famously wrote:

"Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space."

But space is also hugely old The entire history of Homo Sapiens is only about one twenty-thousandth the age of the Earth, and the age of the Earth is less than a third the age of the universe. What would seem to be likely is that at least local to our galaxy, time eventually overwhelms space with respect to galactic colonization.


Our galaxy is more than 10 billion years old, but is roughly 100,000 light years across. There are hundreds of billions of stars in the galaxy, and maybe trillions of planets. It only takes one civilization to arise in all this vastness, with the resources, technology and persistence to initiate and maintain the process of galactic colonization, and they should be be done by now. You don’t have to be able travel faster than the speed of light, or anywhere near it.

You can illustrate this by taking a glass of water and putting one drop of food coloring in it. If you wait a while, the entire glass of water takes on the color of the food coloring. This food-coloring-in-water analogy is called a diffusion model. The galactic process of colonization by diffusion would require colonists - either robotic, biological, or some of both - to maintain their momentum and motivation and to make use of local materials and energy, sending out new colonies in all directions. It would take somewhere between about 1 million and 100 million years to colonize the galaxy this way. Even 100 million years is just 1% of the age of the galaxy. Even if you assume it took several billion years for intelligent life to evolve anywhere, it’s still only 2-3% of the available time to colonize.

Most of us don’t think Earth is all that special, or that you would need a planet just like Earth to evolve life that can develop and use technology.  We don’t know this for sure, but it seems reasonable, vs. the assumption that our planet and the pushy primates who dominate it are something really special on at least a galactic scale. Therefore, we would expect that we are not the first civilization, and probably not the most capable, intelligent beings.

So, how do we account for Fact A? It turns out that there are a large number of proposed solutions to this apparent paradox. Some of the most popular ones include:

Fact A is mistaken - although we have already crossed that off our list for the purpose of this discussion, it is an explanation we will entertain in future entries.

Advanced civilizations are so highly likely to self destruct that they don’t live long enough to colonize.

Advanced civilizations retreat into a virtual world and and lose interest in the external universe.

We really are the first technological civilization in the galaxy.

We are the subject of a quarantine or a part of a galactic zoo.

Interstellar travel is just so arduous and expensive that no one attempts it.

… and so on. Physicist Stephen Webb documented fifty proposed solutions in his book “Where is Everybody.” However, most of these options assume that either advanced civilizations colonize or they don’t, and if they do, they keep doing it indefinitely, presumably under some sort of central driving force or control determined by where they originally came from. Does that really make the most sense?

Scientist and SF Author Geoffrey Landis

Geoffrey Landis had some thoughts about this, and in the early 1990s wrote them down in a much-cited paper for the Journal of the British Interplanetary Society: The Fermi Paradox: An Approach Based on Percolation Theory. Percolation is analogous to water trickling down through a sand or gravel medium. If the medium is loosely packed, then water will flow fairly freely through the rock, and everything below it will get wet. If the medium is tightly packed and more solid, then water will flow more slowly and only into certain areas, and at least of the some of the area below will remain dry. It turns out there is a critical point where the medium is just loose enough, and almost all the water gets through and the area below gets soaked. Just a little more tightly packed than that, and there will be a few small dry areas below the medium. This is kind of like colonization. In the percolation model, if the probability that a civilization will colonize the next star systems is less than a certain critical level, there can be large voids of spaces not colonized.  This can be interpreted as each colony, once it is established deciding independently if it wants to colonize more star systems or stay put.

So, looking at with a simple model, one explanation of the Fermi Paradox is a combination of the difficulty of interstellar travel coupled with a limited drive to colonize that will vary from culture to culture. Percolation theory tells us that if the probability of colonization is low enough, there will be voids surrounded by non-colonizing societies, and these voids could be quite large.

Now,the actual complexities of galactic colonization are greater than we can readily model with simple analytical tools, whether it be the diffusion equation or the random graph theory used to make sense of percolation simulations. Some of the complexities involve the cultural and biological components of the colonization drive and how it may wax and wane with time; contact, competition and even conflict between colonizing cultures, the rise and fall of advanced civilizations, and the apparent likelihood that advanced technologies would offer mind uploading, or that highly advanced sophonts could be pure information beings. As Paul Davies discussed in his book The Eerie Silence, these beings may have mastered something beyond information that we don’t even have a concept for. They might travel the universe on beams of neutrinos, or something even more exotic, with little need for spaceships or physical colonies.

One thing I think is likely in all this multi-dimensional speculation: there are complexities to the Fermi paradox that we haven’t thought of yet, and perhaps even can’t think of.

In future posts, I will examine some of the other creative solutions that have been offered for the Fermi Paradox, as well as the null hypothesis that we really are alone in the galaxy. What do you think?  Please leave a comment below.

The Dream of an Open Channel by PaulCarr is licensed under a Creative Commons Attribution- ShareAlike 3.0 Unported License.