N = R x Fp x Ne x Fl x Fi x Fc x L --- Do The Math To Find Little Green Men
The Drake Equation
A listener, Alexander, has asked the question of whether or not there is a calculation to determine the existence or number of possible alien civilizations. The question is timely, as the organized, scientific search for alien life has never seen a more optimistic time than right now. 1995 brought the discovery of the first planet orbiting a star other than our own sun, and since then the number of extra-solar worlds found by astronomers has soared past a hundred.
Several recent discoveries � vast water ice reserves on Mars, liquid water oceans beneath the surfaces of Jupiter�s moons Europa and Callisto, as well as deep space gas nebulae containing pre-biotic sugars and other chemicals � encourage the notion that life as we know it may be common in the universe. However, we have yet to receive any sign, ambiguous or straightforward, that other civilizations and intelligences populate the cosmos. Many new technologies are emerging that will help us in this search, and they should be on the scene within a couple decades.
When conducting a rational search for alien life, it is apparent that a starting place must be chosen and that it must not be arbitrary. In other words, as Carl Sagan famously put it, there are billions and billions of stars out there whose radio frequencies we can choose to listen to � how do we find the logical place to start? The Drake Equation is the first mathematical description of this idea to be devised.
Dr. Frank Drake wrote down his equation in 1961, when he was a researcher at the National Radio Astronomy Observatory. It must be remembered that this equation does not have a specific answer, as there are no fixed values we know to plug into it. Nor is it a �factual� equation such as those describing physical laws or geometric properties. Rather, it is a tool for quickly making an approximation � an educated guess, really � of how many intelligent civilizations may have arisen.
Stated as a formula, the Drake Equation is
N = R x Fp x Ne x Fl x Fi x Fc x L
N is the number of civilizations within our galaxy that emit radio and other waves that we would be able to detect. R is the rate for the formation of stars that would be able to sustain planets with life. Fp is the fraction of such stars that have planets orbiting them. Ne is the number of planets in each solar system that would be able to support life, orbiting in their star�s habitable zone. Fl is the fraction of planets capable of sustaining life that actually do develop life. Fi is the portion of life-bearing planets on which intelligence develops. Fc is that fraction of planets with intelligent life that develop civilizations whose emissions we can detect with current or future technology. Finally, L is the length of time that signals are emitted by each alien civilization, respectively. Another way of describing the function of L is saying that it represents how long a civilization lasts. For example, the earth has been releasing signals capable of being recognized as intelligent in origin � namely, radio and television waves � for just over a century, with radio coming about 30 years before television. If humanity were destroyed tomorrow, along with our constant broadcasting, then we could say in a Drake Equation for our own planet that L�s value = 100 years.
So we see, the Drake Equation is really a form of narrowing down the search. For more on this subject on the air, stay tuned as This Week In Science hopes in the near future to be interviewing Seth Shostak, a member of Project Phoenix, the multi-spectrum radio scan being conducted under the auspices of SETI (the Search for Extraterrestrial Intelligence).
current science news posted by ted at 6/10/2002 05:36:00 PM
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