http://en.wikipedia.org/wiki/Fermi_paradox
Here are some other recent references:
http://science.slashdot.org/article.pl?sid=07/02/19/1515205
In other words, what are the odds that we are alone in this galaxy, and/or in the universe? And if it is not vanishingly unlikely for technical civilizations to develop, why aren't we now able to detect them, if not communicate with them? I've always been intrigued by this question, particularly the issue of whether or not we have any empirical basis on which to make estimates. We only have one example of a planet with life: Earth. We only have one example of a highly intelligent species with a primary adaptation to culturally transmitted technology: humans. We only have one example of the independent, autonomous development of high technology and space travel: Western European nation-states.
I've always been exasperated by other commentators and students of this subject. They usually conclude that all these things (life, intelligence, technical civilization) are extraordinary events, and that we are therefore likely to live in an empty universe. Or they conclude that all these things are common, and are faced with the Paradox. "Drake's Equation" is an effort to quantify all the various factors that are necessary prerequisites for the development of a space-faring civilization that we would expect to be able to detect and communicate with. The first factor of the Drake equation is the incidence of new star system formation, and each subsequent factor an estimate of what fraction of star systems that reached the previous level will achieve the next. I won't do a full Drake equation calculation here, just a simplified version.
My take on the issue is to say that we *do* have a basis for assigning likelihoods to these events, based on a straightforward look at the one example of these things we do have-- our solar system, planet, biosphere, and species.
Planetary Systems Compatible with Life
There doesn't seem to be anything particularly remarkable about our solar system, and there are 3 rocky planets orbiting close to the "Goldilocks zone": Venus, Earth, and Mars. Only happenstance gave Venus and Mars a bad deal for life, Mars clearly had liquid surface water at some point, and may still prove to have conditions compatible with life. Moreover, two of the moons of Jupiter (Callisto and Europa) are likely to have liquid water environments.
As astronomical observation methods improve, more and more extrasolar planets are being discovered, with the first rocky, non-gas giant just being found recently. Most nearby star systems have proven to have some planetary system around them. It appears that planetary systems are common, and that rocky, element-rich planets merely require a second or third generation star, which is also common.
The only reason this factor is relatively small, is that a good number of the 400 billion star systems in our galaxy are either young-generation systems with few elements other than hydrogen and helium, they are in intrinsically inhospitable locations (e.g. near supernovae or radiation sources), or they have short-lived stars.
Likelihood: .2
400,000,000,000 X .2 = 80,000,000,000 systems compatible with life.
Life
Simple single-celled life appeared on Earth very, very quickly after the surface was no longer molten. Now, this suggests that biogenesis is either very, very easy and can be expected to occur fairly quickly (on a geological timescale) in any likely environment, or that life originated very long ago (prior to the origin of our solar system) and that the interstellar dust has sufficient numbers of dormant "spores" to seed any newly-formed solar system. My inclination is to guess that some sort of replicating chemistry is very likely, and that biological evolution very quickly takes that to form full-blown life.
Likelihood: .99
80,000,000,000 X .99 = 79,200,000,000 systems with life
Multi-cellular Life
While protist and bacterial life appeared quickly (3.8 billion years ago), multi-cellular life did not appear until 600 million years ago. So for more than 3 billion years, life happily plugged along in a quite simple state. This suggests to me that it is the shift from bacterial to multicellular life that is the real crapshoot. This kind of timescale says that the emergence of multi-cellular life may or may not occur before stars burn out (in our case, another 3 billion years). It suggests that this is the leap that may literally be a "one in a billion" chance.
Likelihood: .0001
79,200,000,000 X .0001 = 7,920,000 systems with multi-cellular life
Complex animal and plant evolution
Once multi-cellular life emerged, there was very quickly an explosion of evolutionary radiation into myriad body plans and ecological niches "the Cambrian Explosion". Thereafter, even after multiple mass extinctions, life re-radiated into all the emptied ecological niches. So it seems that once the capacity for inter-cell communication and cooperation evolves, global colonization by multitudes of plant and animal species follows quickly. And, once this has occurred, life is very resistant to subsequent total destruction.
Likelihood: .99
7,920,000 X .99 = 7,840,800 systems with complex, macroscopic, heterotrophs and autotrophs
Social Species, Tool Usage, Symbolic Communication
Once complex life emerged, there were several eras of evolutionary radiation punctuated by massive extinction events. Within these eras, long periods of relative stasis were common. Without an external catastrophe, it is quite plausible that the age of dinosaurs would never have ended, and that therefore the age of mammals would never have begun. But for Earth, most of these events are hypothesized to have essentially typical stochastic causes: gamma ray bursts from nearby supernovae, ice age onset due to tectonic plate migration, asteroid and/or cometary impacts, volcanic events. All these things occur rarely on a local timescale, but become nearly inevitable on a geological timescale. So while a given planet could just happen to avoid all these random disasters, occasional catastrophes are actually likely.
With sufficient evolutionary turnover triggered by catastrophe, it seems plausible that any planet with animal life will eventually produce some social species, that some of these will use tools, and that some of those will develop some form of symbolic communication. The current animal world has numerous examples of species with some or all of these characteristics in different measures: chimps, bonobos, bees, dolphins, whales, crows, parrots, wolves, raccoons, octopi. If humans hadn't existed, several other species might plausibly end up filling a similar niche.
Likelihood: .9
7,840,800 X .9 = 7,056,720 systems with semi-sentient life
Agriculture, High Technology
In human history, there have again been long periods of stasis (e.g. 1.8 million years of Homo erectus) where we were a technology-using, verbally communicating, social species, but we did not make any progress from the savannah to the stars. (or even the Quicky-mart). So this is another area where we appear to be looking at something very unlikely, something that depends on an idiosyncratic set of circumstances driven by environmental turmoil.
This is also the point at which we are no longer talking about geologic/astronomic timescales and we begin talking about tiny, history-sized timescales. How long, in geological time do technical civilizations last? Do potential space-faring species frequently destroy themselves? Do they upload themselves into virtual worlds powered by Dyson spheres around faded stars?
Likelihood: .0001
7,056,720 X .0001 = 705 potentially space-faring species in our galaxy in its history.
Conclusion
If these premises are correct, then when we begin exploring space we will find results very different from the traditional "galactic civilizations" vs. "lifeless nothingness" dichotomy that is usally taken as a given. We will find that roughly 1 of every 8 star systems has life, and we will find *millions* of star systems teeming with plant and animal life, some of them semi-sentient. If we are very lucky, every 5,000 light years or so we *might* find archaeological remains of a long-dead alien civilization. But only if we are extraordinarily lucky, and our own survival odds are better than we think, will we encounter another space-faring sentient civilization.
