Drake’s Equation and the Great Filter

Astronomer and scientist Frank Drake devised what is now known as the Drake Equation to answer the question of how much extraterrestrial life exists in the universe. In this equation, Drake included a variety of factors that determine if detectable intelligent life can exist (click on image at right for exact explanation of variables). In 1961, Drake and his colleagues estimated the values of these factors, and their result was that there should be approximately ten planets in the galaxy with intelligent life. This gave much encouragement and support to programs such as SETI, but so far NASA has been unable to find either an Earth-like planet or life beyond Earth. Today, the values in his equation have been revised, yielding a new result of 2. Even so, the Drake Equation is mostly based on conjecture, and though it is the best attempt to explain the apparent absence of extraterrestrial life it is far from reliable. Also, Drake’s equation only applies to detectable extraterrestrial life—for all we know, there could be plenty of life out there: as long as it isn’t constantly broadcasting on the handful of frequencies we monitor, we would have no way of finding it.

Regardless, the question remains: Is there extraterrestrial life? And if there is, why haven’t we seen it? Futurist Robin Hanson suggests that there is a “Great Filter,” or a factor preventing extraterrestrial life from flourishing. Hanson examined the evolution of life on Earth, and his conclusion (which is mostly accepted in the scientific community) is that the Great Filter exists in one of the following eight steps toward complex life:

1) The right star system (including organics)

2) Reproductive something (e.g. RNA)

3) Simple (prokaryotic) single-cell life

4) Complex (archaeatic & eukaryotic) single-cell life

5) Sexual reproduction

6) Multi-cell life

7) Tool-using animals with big brains

8) Colonization explosion

Today, we are still unsure of which one of these is the Great Filter. However, many scientists and philosophers have examined the Great Filter, and various different conclusions have been drawn. My favorite analysis of the subject is the futurist Nick Bostrom’s (the link to his paper can be found here). I would like to take a moment to summarize Bostrom’s analysis and conclusion and why I find it to be the most satisfactory.

Bostrom explains that human beings are between steps 7 and 8. Therefore, the Great Filter could be either behind us (in steps 1-7) or ahead of us (step 8). However, most of science tells us that steps 2-7 follow naturally from each other, since once simple life begins (at least according to biologists) it will eventually evolve into complex life. This leaves us with two options: either the Great Filter is the initial synthesis of life, or it is space colonization and expansion. There is some evidence that suggests it is the former: though organic compounds have been spontaneously created in a simulated “natural” environment, no cellular life has ever been synthesized. If it is the latter, though, our future looks quite bleak. This would mean that some factor that we will encounter in our near future will either cripple us or destroy us entirely (Nuclear war? Overpopulation? Epidemic?) For this reason, says Bostrom, we should rejoice at not seeing any simple extraterrestrial life, since that would imply that the Great Filter is ahead of us.

Looking at the problem from a purely scientific point of view, it seems to be common sense that some extraterrestrial life must exist. The galaxy is not the universe—there are billions of galaxies, and so many stars that we can never search them all. Though the Drake Equation says life is unlikely, the universe is so big it is bound to be somewhere. For now, though, we are going to have to be content with pointing radio antennas at the sky, since we have no other way of searching for life. Perhaps this is for the best—I doubt human beings would react well to finding extraterrestrial life. But that’s a discussion for another day.