Parameterizing Drake
    By Josh Witten | November 3rd 2009 03:37 PM | 1 comment | Print | E-mail | Track Comments
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    In 1960, Frank Drake did something extremely clever. At least it was something the people in my lab would consider clever. He wrote down an equation that would define how frequent detectable, intelligent life is in the galaxy, if only we knew the values of the variables.
    The Drake Equation (simple form, there are a lot of variations and adjustments):
    N=number of detectable civilizations in the galaxy
    R*=rate of star formation
    fp=fraction of those stars with planets (including their moons as part of the planet)
    ne=average number of planets per star with planets
    fl=fraction of planets with life
    fi=fraction of planets with life with intelligent life
    fc=fraction intelligent life with technology that generates a detectable signal
    L=length of time of signal release
    But, he did something more clever than simply writing an calculable equation. It gave us a framework to think about the variables that are important in the search for extraterrestrial, intelligent life. Why do we need such a framework?

    Well, the Milky Way (not to mention the Universe) is a big place. A really big place (a volume of about 1000 billion cubic light years [ly3]* containing 100-400 billion stars). It is not practical to search the entire galaxy. The Drake Equation gives us a framework for making intelligent decisions about how to constrain that search space (which is why SETI loves the Drake Equation) or even bother searching. For example, Carl Sagan's interest in environmental issues was partially driven by his belief that L (essentially the life span of civilizations) was the most important variable in the Drake Equation was very small relative to the age of the galaxy.

    Which brings us to the Fermi Paradox. Assuming that the Earth is not a singular planet, given the age of the universe, why isn't intelligent life everywhere?

    Earth, as seen from not Earth (Apollo 17)Which brings us to science-fiction writer Charlie Stross, who suggests that part of the paradox is that the Earth is not as habitable as we like to think. Read his post for the full argument and discussion of meat puppets, but the take home message is that only about 15% of the current Earth's surface is habitable without special equipment or knowledge. And it gets worse. Over the history of the Earth, due to lack of oxygen and nasty ice ages, those habitable conditions have only existed for about 8% of the Earth's history. The bad news is that those habitable conditions may only exist for another 0.3-1 billion years.

    Stross' ruminations are, essentially, providing a framework for thinking about the parameters for fi, fc, and L. Even on an "ideal" planet like Earth, they may not be very large.

    *Which is, incidently, my new favorite unit.


    The voice of an older man began to question him, and Nunez found himself trying to explain the great world out of which he had fallen, and the sky and mountains and sight and such-like marvels, to these elders who sat in darkness in the Country of the Blind. And they would believe and understand nothing whatever he told them, a thing quite outside his expectation. They would not even understand many of his words. For fourteen generations these people had been blind and cut off from all the seeing world; the names for all the things of sight had faded and changed; the story of the outer world was faded and changed to a child's story; and they had ceased to concern themselves with anything beyond the rocky slopes above their circling wall. Blind men of genius had arisen among them and questioned the shreds of belief and tradition they had brought with them from their seeing days, and had dismissed all these things as idle fancies, and replaced them with new and saner explanations. Much of their imagination had shriveled with their eyes, and they had made for themselves new imaginations with their ever more sensitive ears and finger-tips. Slowly Nunez realized this; that his expectation of wonder and reverence at his origin and his gifts was not to be borne out; and after his poor attempt to explain sight to them had been set aside as the confused version of a new-made being describing the marvels of his incoherent sensations, he subsided, a little dashed, into listening to their instruction. And the eldest of the blind men explained to him life and philosophy and religion, how that the world (meaning their valley) had been first an empty hollow in the rocks, and then had come, first, inanimate things without the gift of touch, and llamas and a few other creatures that had little sense, and then men, and at last angels, whom one could hear singing and making fluttering sounds, but whom no one could touch at all, which puzzled Nunez greatly until he thought of the birds."

    Fragment from “THE COUNTRY OF THE BLIND” by
    H. G.Wells