Number of Technologically Advanced Alien Civilisations
By Quamrul Haider
The evolution of intelligent life on Earth seems to suggest that given sufficient time and hospitable conditions, extraterrestrial life might evolve on Earth-like planets orbiting around other stars. How must we ascertain whether such planets with intelligent civilisation exist? If they do exist, how do we determine their number?
Because of the tremendous distances that separate us from other planetary systems and in view of the limitation that prohibit faster-than-light travel, it is exceedingly unlikely that we will ever be able to visit the planets, seeking out life forms that exist there. In 1961, American astronomer Frank Drake tackled this problem by proposing an equation that enables us to estimate the number of technologically advanced extraterrestrial civilisation that may exist today in our galaxy, the Milky Way.
According to the equation, the number of intelligent civilisations currently in our galaxy is the product of seven factors. The first two factors are number of stars born each year and the fraction of stars with planetary systems. The third factor is the number of planets in each system that are in the habitable zone where temperatures are mild enough for liquid water to exist on their surface. The next two factors are the fraction of planets on which life actually develops and the fraction of those planets on which intelligent life has developed. The last two factors are the fraction of planets on which a technological civilisation has evolved to the point at which interstellar communications would be possible and the average lifetime of such a civilisation.
Unfortunately, we dont have firm values for most of the factors in the equation. Hence, prediction of the Drake Equation remains a rough estimate of the number of high-tech civilisations in our galaxy. Nevertheless, the equation is used as a framework for the discussion of alien intelligent life, as well as a means of assessing, as objectively as possible, the chances of making contact with them.
The first three factors are, in principle, quantities that can be known with some degree of certainty from observations. Based on the number of stars and the age of our galaxy, it is estimated that seven to eight stars are formed per year. A conservative estimate for this factor would be one per year. We know that the percentage of stars hosting planets is about 100. Therefore, the second factor in Drakes equation is one. We also know that about 10 to 20 percent of those planets are in the host stars habitable zone. Let us be realistic and choose the lower value for this factor.
The fourth factor in the equation is really a speculative term. We have no way of estimating how likely it is that life should begin, given the right conditions. From the seeming naturalness of its development on Earth, it could be argued that life would always begin if given the chance. Thus, an optimistic value for this factor would be one.
The next two factors, the fraction of those life forms that evolve into intelligent species and the fraction of those species that develop adequate technology advanced enough for interstellar communication, are hopelessly uncertain. Since we do not know if intelligence is a natural or a necessary consequence of biological evolution, logical arguments can be made for both the factors to be one.
We now face a critical question: How long can technological civilisations last? Ours has been sufficiently advanced to send or receive interstellar radio signals for about 50 years. However, there are any number of ways that our fate could be hastened. The moon could take a hit throwing us off balance, a super bug could wipe us out, runaway greenhouse effect could transform Earth into todays Venus, or we could nuke ourselves into oblivion. If any of these events obliterates us in the next 50 years, our lifetime will be 100 years. With this short lifetime, Drake Equation predicts that currently there are only 10 intelligent extraterrestrial civilisations in our galaxy. If these civilisations are equally spaced, they could be at an average distance of 20,000 light years apart. The time it would take for communications to travel between civilisations would be very much longer than our lifetimes, and there would be no hope of establishing a dialogue with anyone out there.
If it is possible that every advanced civilisation successfully manages its problems, it should survive as long as the parent star. In that case, allowing 10 billion years for the average lifetime, we should expect one billion technologically advanced alien civilisations. The distance between the civilisations would be approximately 30 light years.
On a cosmic scale, 30 light years is a short distance, less than the distance our radio signals have travelled since the early days of radio communication. We should, therefore, expect to hear from somebody out in the heavens very soon, provided our radio telescopes are tuned to the frequency with which the smart aliens are trying to communicate with us. So far, we have heard nothing but a dead silence.
The writer is a Professor of Physics at Fordham University, New York.
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