The Great Filter

The Great Filter is an implication of the failure to observe any extraterrestial life, despite considerable effort (the Fermi paradox). One possible explanation is that there is something, the Great Filter, which acts to reduce the great number of potential sites to the tiny number of intelligent species actually observed (currently just one: ours). It might work either by one or more barriers to the evolution of intelligent life, or a high probability of self-destruction. [cite web |url=http://hanson.gmu.edu/greatfilter.html |title=The Great Filter — Are We Almost Past It? |author= Robin Hanson |year=1998] [cite web |url=http://www.nickbostrom.com/extraterrestrial.pdf |title=Where Are They? Why I hope the search for extraterrestrial life finds nothing |author=Nick Bostrom |publisher=MIT Technology Review |date=May/June 2008 |access_date=2008-05-01] The main counter-intuitive conclusion of this observation is "The easier it was for life to evolve to our stage, the bleaker our future chances probably are."

Main argument

The main argument is based on the observation that we have not yet observed extraterrestial intelligent life, even though we have observed a great number of stars. Therefore, the whole process of starting with a star, and ending with communicating intelligent life, must be unlikely (the Great Filter). This implies that at least one step in this process must be improbable. Hanson listed the likely steps as:

# The right star system (including organics)
# Reproductive something (e.g. RNA)
# Simple (prokaryotic) single-cell life
# Complex (archaeatic & eukaryotic) single-cell life
# Sexual reproduction
# Multi-cell life
# Tool-using animals with big brains
# Colonization explosion

At least one of these steps must be improbable. If it's not 1-7, then the implication is that our future is bleak. If 1-7 are likely, then many civilizations would have developed to the current level of the human race. However, none have made it to step 8, or the galaxy would be full of colonies. So it must be that step 8 is the unlikely one, and the only thing that appears likely to keep us from step 8 is some sort of catastrophe. So by this argument, finding multi-cellular life on Mars (provided it evolved independently) would be very bad news, since it would imply steps 1-6 are easy, and hence only 7 or 8 could be the big problem. (Note that although step 7 has occurred on Earth, it still might be the unlikely step. We cannot judge using ourselves as an example, since if step 7 had not occurred, there would be no-one to ask the question. This is an excellent example of anthropic bias.)

References