This article concerns intelligence, but it is not concerned with what it is to be intelligent. Rather, it is founded on three properties of intelligence, properties which I feel are universally accepted as such.
Given these properties, what I intend to discuss is probability of discovering intelligence which exists other than on Earth.
In 1961, Dr Frank Drake, now President of the SETI Institute , proposed a formula for estimating the existence of communicating Intelligent Life elsewhere in our galaxy. This is now known as The Drake Equation, and is used by the scientific community as a valid scientific tool. The equation states that
It is important to realise that the The Drake Equation does not formulate the probability that Intelligent Life exists elsewhere. Rather, it acknowledges the fact that the nature of human technology is such that the search for non human intelligent life is necessarily the search for evidence of non human technology. More specifically, it is the search for non human radio emissions which really defines the current approach to extra-terrestrial intelligent life.
Whether we could succeed in this search I will come to later, but first I will add a little to the definitions of the terms which The Drake Equation employs, as well as some speculation as to what the value of these terms may be.
Given the age of the Milky Way, the existence of a time during which the galaxy was too young for star formation to occur, and the current estimate for the number of stars in the galaxy (about 200,000,000,000) R is held to be about 10,000 a year
To date, the existence of eight planets outside our solar system has been confirmed. All are massive (varying from 0.5 to about 10 times the size of Jupiter ), and many are extremely close to the star which they orbit. However, these planetary features are as much to do with the limitations of detection technology as they are to do with the probable general specifics of distant planets. As technology improves, more planets will undoubtedly be detected. Current estimates of the value of Fp range from 1/8 to 1/5.
All life as we know it requires water. In our solar system alone Earth, Mars and some of Jupiter's moons (Europa , Ganymede , and Callisto ) show signs of possessing or previously possessing water. This may indicate that planets (and planetoids) which hold or have held water are relatively common. It should also be remembered that water is not necessarily required for life. It may be that other solvents, such as liquid ammonia or liquid methane, which have been found to occur on much colder planets, could also sustain an evolutionary process.
What is necessary is that any such planet fall within the 'hospitable zone' of a star. That is, the area of possible orbits in which the planet is not so close to the sun that the solvent boils away, and not so far away that the solvent freezes. Of the eight extrasolar planets, one has been found to orbit within this zone for the star which it orbits, and it is reasonable to expect other stars to have similarly positioned planets. Of course, what we need is for these hospitable zone planets to be 'liquid solvent' planets as well. A (again conservative) estimate of this would put 1 in 100 planets into this category.
It is also the case that most stars are smaller than our sun, and consequently will have a much smaller hospitable zone. Even so, sun sized stars make up about 10% of appropriate stars, and the consideration of these alone would set a value for Ne at about 1/1000.
There are currently three proposed projects with the aim of searching for Earth type planets; a Discovery class Project Kepler, the European Space Agency's Project Darwin and NASA's ExNPS.
The probability of life developing, given an appropriately situated planet, is extremely hard to determine. Currently, any decision will be dependent on the degree to which our solar system is considered representative of systems in general. Life on Earth being unique would appear to be statistically remote; 1 : the number of suitable planets (possibly over 1,000,000). At the other end of the scale, it can be argued that given enough time and energy, life will inevitably develop, giving a value for Fl as 1. More inter- and extrasolar understanding of planetary consistency will provide a clearer idea of an appropriate value. As for Earth, it took under a billion years for life to develop, and the Earth itself is only 4.5 billion years old, approximately a third the age of the Milky Way. Cosmologically speaking, life on Earth would seem to have developed extremely rapidly, which would imply a relatively large Fl value.
Again, any evaluation of this factor will depend on the degree to which the case on Earth can be thought of as representative. This influences both our definition of intelligence, and the speed with which we think it may develop. With regard to the latter, similar arguments to those concerning Fl can be employed, giving possible values for Fi from 1 (that it is inevitable) downwards.
Given the complexity of communicative technology relative to other aspects of evolutionary development, and the competitive nature of the evolutionary process as we currently understand it, it seems that at most one form of intelligent life per planet would be able to reach this stage. However, this does not imply that at least one intelligent lifeform would do so. Nor does it mean mean that lifeforms capable of developing such technology would actually develop it. Non human Intelligent life forms may not consider, or may have no need of, developing communicative technology.
It may well be the case that, as a result of the competitive nature of evolution, intelligent, technologically advanced civilisations are inherently unstable. If this is so, then L would be necessarily small. In the case of life on Earth, this value could be as little as 100 years - the time from Marconi's 1901 radio transmission to an end of the millenium nuclear holocaust. Conversly, a cooperatively driven evolutionary process may well result in a value of L in the thousands or even tens of thousands.
The Drake Equation has been used to provide values of N ranging from a somewhat depressing 1 (us) to a spectacularly optimistic 10,000,000. Dr Drake himself feels the value is about 10,000. Given the nature of the parameters, it is clear that almost any reasonable value could be argued for, and that a single discovery could radically alter any value found.
A final question then is why, if perhaps tens of thousands of planets are currently communicating, we have yet to hear from them. The most probable reason for this that the distance such signals may have to travel (the Milky Way has a diameter of approximately 100,000 light years, and we live on the edge of it) is huge in relation to both the value of L, and the length of time that we have been listening. An Intelligent planet like Earth may only emit a hundred year burst, at under the speed of light, which would have to reach other planets within their 'technologically receptive' window (again perhaps extremely small). Given the vast chance that this window will be missed, our lack of success is not so surprising. It may happen eventually, but there is no reason to think it should have happened already, and little more to think it will happen soon.