Nearly everyone interested in SETI has heard of the Drake Equation, but views of its utility vary widely. The equation is a useful way to estimate the number of technological civilizations in our galaxy and the chances of detecting an extraterrestrial message. Others view it as a wasted effort, given the huge range of conjectures involved in its components. There is a middle ground to use it to update and assess the reliability of relevant data that we have and the ways to improve upon the uncertainties. It can certainly be an effective tool for stimulating curiosity on this subject. Frank Drake presented the Equation in 1961 at what may have been the first formal SETI conference with the intent to stimulate discussion and evaluate proposed research, not to arrive at a true estimate of the number of intelligent ETs whose signals we could detect by multiplying its components.
The Drake Equation is:
N = R* x fp x ne x fl x fi x fc x L where:
* N is the number of intelligent civilizations in the Milky Way Galaxy, who, for this exercise, emit radio, light or other transmissions that are detectable from afar.
* R* is the rate of star formation per year for the galaxy.
* fp is the fraction of those stars with planets.
* ne is the average number of planets capable of supporting life (think of e for “earth-like” or “ecologically fit”).
* fl is the fraction of those that actually develop life.
* fi is the fraction of those where life becomes intelligent.
* fc is the fraction of emitting detectable signals into space.
* L is the lifetime of a communicating civilization.
N is usually considered the number of communicating civilizations in just our Milky Way galaxy. Keep in mind that there are hundreds of billions of other galaxies in the visible universe. The factor R* is estimated to be about 1 by some astronomers, based on the current rate of star formation in our entire galaxy. But the rate of star formation was once higher, and dividing the number of stars in the galaxy (200-400 billion) by its estimated age (roughly 10 billion years) would yield an R* closer to 10 per year, the number I will choose to plug in.
It’s probably safe to plug in the estimate of fp = 0.5. (number of stars with planets is 50% or higher)
Richard’s personal tally for all of the Drake factors is 0.0625. L is the lifetime of communication civilization. By our definition of a technological (transmitting) civilization, we have been around for about 100 years. How sanguine are you about the future of homo sapiens? He personally can’t come up with a more precise conjecture than between 500 and 5 million years.
Richard’s current guess for N (number of intelligent civilizations) ranges from 31 to 310,000.