The Great Filter

When water was discovered on Mars, people got very excited. Where there is water, there may be life. Scientists are planning new missions to study the planet up close. NASA’s next Mars rover is scheduled to arrive in 2010. In the decade following, a Mars Sample Return mission might be launched, which would use robotic systems to collect samples of Martian rocks, soils, and atmosphere, and return them to Earth. We could then analyze the sample to see if it contains any traces of life, whether extinct or still active. Such a discovery would be of tremendous scientific significance. What could be more fascinating than discovering life that had evolved entirely independently of life here on Earth? Many people would also find it heartening to learn that we are not entirely alone in this vast cold cosmos.

But I hope that our Mars probes will discover nothing. It would be good news if we find Mars to be completely sterile. Dead rocks and lifeless sands would lift my spirit.

Conversely, if we discovered traces of some simple extinct life form—some bacteria, some algae—it would be bad news. If we found fossils of something more advanced, perhaps something looking like the remnants of a trilobite or even the skeleton of a small mammal, it would be very bad news. The more complex the life we found, the more depressing the news of its existence would be. Scientifically interesting, certainly, but a bad omen for the future of the human race.

How do I arrive at this conclusion? I begin by reflecting on a well‐known fact. UFO‐ spotters, Raelian cultists, and self‐certified alien abductees notwithstanding, humans have, to date, seen no sign of any extraterrestrial intelligent civilization. We have not received any visitors from space, nor have our radio telescopes detected any signals transmitted by any extraterrestrial civilization. The Search for Extra‐Terrestrial Intelligent Life (SETI) has been going for nearly fifty years, employing increasingly powerful telescopes and data mining techniques, and has so far consistently 1 corroborated the null hypothesis. As best we have been able to determine, the night sky is empty and silent—the question “Where are they?” thus being at least as pertinent today as it was when Enrico Fermi first posed it during a lunch discussion with some of his physicist colleagues back in 1950. 

Here is another fact: There are on the order of 100 billion stars in our galaxy alone, and the observable universe contains on the order of 100 billion galaxies. In the last couple of decades, we have learnt that many of these stars have planets circling around them. By now, several hundred exoplanets we have discovered. Most of these are gigantic, but this is due to a selection effect: It is very difficult to detect smaller exoplanets with current observation methods. (In most cases, the planets cannot be directly observed. Their existence is inferred from their gravitational influence on their parent sun, which wobbles slightly when pulled towards a large orbiting planet; or alternatively by a slight fluctuation in their sun’s perceived luminosity which occurs when it is partially eclipsed by the exoplanet.) We have every reason to believe that the observable universe contains vast numbers of solar systems, including many that have planets that are Earth‐like at least in the sense of having a mass and temperature similar to those of our own orb. We also know that many of these solar systems are much older than ours. 

From these two facts it follows that there exists a “Great Filter”. The Great Filter can be thought of as a probability barrier. It consists of one of more highly improbable evolutionary transitions or steps whose occurrence is required in order for an Earth‐like planet to produce an intelligent civilization of a type that would be visible to us with our current observation technology. You start with billions and billions of potential germination points for life, and you end up with a sum total of zero extraterrestrial civilizations that we can observe. The Great Filter must therefore be powerful enough— which is to say, the critical steps must be improbable enough—that even with many billions rolls of the dice, one ends up with nothing: no aliens, no spacecraft, no signals, at least none that we can detect in our neck of the woods. Now, an important question for us is, just where might this Great Filter be located? There are two basic possibilities: It might be behind us, somewhere in our distant past. Or it might be ahead of us, somewhere in the millennia or decades to come. Let us ponder these possibilities in turn.

by Nick Bostrom, Future of Humanity Institute, Oxford University |  Read more (pdf):

Image: NASA, ESA, and the Hubble Heritage Team [STScI/AURA]

[ed. From 2008 but still relevant as we seem to be acquiring more objects of Great Filtering potential. Various possiblities include things like nuclear holocaust, bioterrorism, cyberterrorism, uncontrollable AI, climate change, etc. (for a scary list, see: Global Catastrophic Risk). Also, for another completely different possibility, see: The Dark Forest (Duck Soup). Recommended.

By the way, the 9 steps involved in the Great Filter include:

With no evidence of intelligent life other than ourselves, it appears that the process of starting with a star and ending with "advanced explosive lasting life" must be unlikely. This implies that at least one step in this process must be improbable. Hanson's list, while incomplete, describes the following nine steps in an "evolutionary path" that results in the colonization of the observable universe (as proposed by Robin Hanson):

1. The right star system (including organics and potentially habitable planets)
2. Reproductive molecules (e.g. RNA)
3. Simple (prokaryotic) single-cell life
4. Complex (eukaryotic) single-cell life
5. Sexual reproduction
6. Multi-cell life
7. Tool-using animals with intelligence
8. A civilization advancing toward the potential for a colonization explosion (where we are now)
9. Colonization explosion

According to the Great Filter hypothesis at least one of these steps—if the list were complete—must be improbable. If it's not an early step (i.e., in our past), then the implication is that the improbable step lies in our future and our prospects of reaching step 9 (interstellar colonization) are still bleak. If the past steps are likely, then many civilizations would have developed to the current level of the human species. However, none appear to have made it to step 9, or the Milky Way would be full of colonies. So perhaps step 9 is the unlikely one, and the only things that appear likely to keep us from step 9 are some sort of catastrophe, an underestimation of the impact of procrastination as technology increasingly unburdens existence or resource exhaustion leading to the impossibility of making the step due to consumption of the available resources (for example highly constrained energy resources). (Wikipedia).]