Nick Bostrom’s “Great Filter” and the Fermi Paradox

This Fall DARPA invited me to speak at their annual gathering “Wait, What! A Future Technology Forum.” I thought that I was invited to speak about my research on tipping points in biological populations, but as it turns out they wanted me to serve on a panel entitled “Are we alone, and have we been?” together with Lucianne Walkowicz, an astrobiologist, and Mark Norell, a paleontologist. Needless to say, this was pretty far from my area of expertise, but it gave me an exciting opportunity to read about issues that all of us are curious about.

One of the most exciting ideas that I have come across over the last ten years originated in a wonderful essay by Nick Bostrom in 2008 entitled “Where are they? Why I hope the search for extraterrestrial life finds nothing” (pdf). I strongly encourage you to read the article if you are not familiar with the argument, as I will only give a brief sketch here. The starting point is the Fermi paradox, in which the famous physicist Enrico Fermi asked his lunch partners “Where is everybody?” Fermi was referring to the apparent contradiction between estimates of the likely number of intelligent lifeforms throughout the galaxy and the absence of any evidence that they exist.

One resolution of this paradox discussed by Bostrom and others is The Great Filter, which is whatever stops a huge collection of lifeless planets leading to life that expands across the galaxy. Bostrom’s argument is that there must be something that is exceedingly unlikely in this process. Perhaps the extremely difficult step is evolving the first self-replicators, or maybe it is evolving cell-based life, or possibly it is evolving multi-cellularity. On the other hand, it is also possible that essentially all technologically advanced life wipes itself out by powerful nuclear or biological weapons (or by advanced AI, a concern recently expressed by my colleagues Max Tegmark and Frank Wilczek).

This gets us to the crux of the argument made by Nick Bostrom in his 2008 article in Technology Review. The Great Filter is something that is either in our past or in our future. If we go to Mars and find life that evolved independently from us, then this makes it more likely that the Great Filter is ahead of us. Finding life on other planets therefore bodes poorly for humanity’s future. Indeed, the more advanced the life is that we find the worse the news is for us.

There is another “solution” to the Fermi paradox that is more hopeful for our future survival but perhaps more depressing regarding the quality of that future. Many people take it for granted that if we wait long enough then we will develop the technology to reach neighboring star. For example, Marc Millis has calculated based on the world energy budget that we will not be able to send an interstellar probe (much less human colony) to another star for 500 years. Even this depressing estimate is perhaps overly optimistic, as it assumes that the world energy allowance increases indefinitely by about 2% per year. However, recent evidence suggests that worldwide human expenditures will actually stop growing in the next fifty years. Indeed, over the last 30 years total energy consumption per person has been constant in both the United States and Europe. If humanity’s energy budget stops growing then perhaps we will never send a colony to another star, and the resolution of Fermi’s paradox may simply be that traveling between the stars is prohibitively costly.

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