If the Universe is teeming with planets that could foster Life, then why is it we are yet to meet Aliens, pricks the Fermi paradox. The implication is, the appearance of life in the Universe is not such an easy thing after all. Yes, supports the Great Filter theory proposed originally by the cosmologist Brandon Carter in 1980 (according to Paul Davies in his recent book, Eerie Silence) and later in 1998 by economist Robert Hanson (his blog).
The Wikipedia page for the Great Filter theory is brief enough to tempt me to inject some web content -- based on the Eerie Silence. Of course, I can't beat the original paper by Hanson -- written first on the web and very much accessible -- for content and clarity. The intent of this note is to give a summary to get you interested in reading that original.
Here is a schematic of key events on Earth that we should comprehend before moving on to the arguments of the Great Filter.
The Great Filter is a series of improbable events that should occur, if at all, in succession to evolve intelligent life (like us) in a planet. In short, The Great Filter is what prevents dead stuff like stars and planets around them, to evolve intelligent life in them. The collection of improbable events that are necessary conditions for intelligent life to happen are so improbable and equally unlikely that, our Earth remains as a statistical outlier to the norm -- that intelligent life is rare in the Universe.
Now for the arguments.
If the expected time for intelligence to evolve in a habitable planet is much shorter than the average life span of a right star system that supports such planets, then one should see more such events happening around the Universe. That is, since we keep finding more exo-planets, we should expect the Universe teeming with intelligent life. However, it would be hard to explain then why Erath took close to a billion years (long time when compared to its total habitable lifetime) to consummate life. We should conclude, although it is easy to spot intelligent life elsewhere in the Universe, for some peculiar reason it took an atypical while to evolve in Earth.
On the other hand, if the time of evolution of intelligent life in a planet is much longer than the average lifetime of the (typical) supporting star, then one should expect it should have taken the entire habitable time of the planet for this improbable event to have occurred. Checking with Earth where intelligent life did evolve, it took about 4 billion years -- almost the entire habitable window (which is about 5 billion years) -- for Earth to evolve its intelligent life. This observation does agree with the aforesaid argument.
Carter and Hanson, the proposers of The Great Filter theory, quantified these arguments. Assume that there are several equally hard improbable steps or events that need to occur in succession to result eventually in intelligent life. The hardness or improbability of each event is such that it would take more than the lifetime of the typical star for these events to individually happen once. Such a sequence of improbable events is The Great Filter. Let us say there are N such events and intelligent life did arise in a planet surmounting this filter. Then (Carter and Hanson show using probability theory that) the expected time between the occurrence of each of the N steps is equal and is equal to 1/N-th of the total time of the habitable window, with one more 1/N-th of time left before the habitability window ends.
Suppose if we apply this theory to Earth, what do we get? From our best estimates about the lifetime of our Sun, then it is believed about 800 million years are left from now, before the Sun sizzles our Earth -- too hot for current intelligent life to survive. This means, if we divide the total habitable lifetime of Earth, which is about 5 billion years, by 800 million years, we get N = 6. This means Earth should have encountered at least six improbable events that lead to intelligent life (like us).
Here is what Hanson and Carter are suggesting; six equally unlikely improbable events happening in series could only result in intelligence species on Earth.
The check list of improbable events proposed by Hanson for intelligent life to evolve were
- The right star system (including organics & potentially habitable planets)
- Reproductive molecules (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
- Where we are now
- Colonization explosion
Focusing on Earth, suppose we begin at a time when simple life forms were possible, i.e. around 3.6 billion years ago, evolution of intelligent life from that stage would require indeed (as we found before) at least six equally unlikely improbable steps (items 3 to 8 in the above list).
One snag is right at the start. Contrary to the 800 million years time window that we allow for life to originate on Earth -- i.e. the first improbable event to occur -- it took only about 200 to 300 million years for Earth to throw up (simple) life.
A counter argument given by Hanson was that we cannot be quite sure that life actually originated on Earth. May be life originated on Mars even earlier -- backtracking to 4 billion years and thus covering for the time lag -- and then got transported to Earth through meteorites (remember ALH84001?) around 3.6 billion years ago. This would explain the sudden popping up of life much 'earlier' as the first improbable event after the habitable window of Earth is opened around 3.8 billion years ago.
This kind of argument is difficult to refute conclusively.
With this Great Filter backdrop, intelligent life on Earth may seem just a Cosmic fluke.
So there is every chance that elsewhere in the Universe in some planets we could expect simple lifeforms as a freak improbable event. But to expect the series of six improbable events to happen within the lifetime of the planet or the parent star, to yield intelligent lifeforms, that is pretty remote. Expecting Alien lifeforms elsewhere in the Universe is just about useless.
That is one side of the proposal. There are some percolation holes in the Great Filter and go-arounds for Fermi paradox. Should require another note to write them up. We can still hope to see Aliens outside of Sigourney Weaver starers.