Given that so many people casually toss around the term "the Drake equation" (Google search: 427,000 hits), its creator, Frank Drake, might consider trademarking the term. I made this suggestion to Frank at an astrobiology conference in 2006.

Drake is the astronomer who, in 1960, conceived and executed Project Ozma, known as the first search for radio signals of extraterrestrial intelligent origin. In 1961, for a gathering of scientists with an interest in the search for extraterrestrial intelligence (SETI), Drake concocted the so-called Drake equation as a tool for thinking and talking about SETI scientifically.

According to that authoritative source Wikipedia, the purpose of the Drake equation "is to allow scientists to quantify the uncertainty of the factors that determine the number of such extraterrestrial civilizations." The Drake equation has been so widely popularized as a rationale for SETI that I'd guess few people outside the small group of scientists actively involved in the search think much about what the equation actually is, and means.

"What is the equation?" Drake himself has asked. "It encapsulates our understanding of the evolution of our galaxy and of our solar system." (See: http://www.seattlepi.com/local/405707_drake30.html?source=mypi.

For a decent history of the Drake equation, see:
http://www.planetary.org/explore/topics/search_for_life/seti/drake_equation.html.)

That's an honest answer. Problem is, we know very little for certain about the evolution of our galaxy and our solar system. The SETI community does not discourage the oversimplification of its scientific rationale to fit into Drake's template, which is not necessarily a good thing.

In mathematics, an equation is "an expression or a proposition, often algebraic, asserting the equality of two quantities." (Source: dictionary.com.) Drake wrote his "equation" to look like a mathematical expression: N= R* fp ne fl fi fc L.

But the Drake "equation" is not an equation.  It's a tool for "guesstimating" how many planets in the Milky Way galaxy might host intelligent life. This guesstimating process rests on a number of assumptions, so its product, the number of communicating civilizations in our galaxy, is by no means a precise quantity, with possible answers ranging from zero to millions or more. Not to mention we're only talking about the Milky Way.

N is the number of civilizations in our galaxy in which communication might be possible. In order to produce a value for the N, one must know the values of the other factors in the equation. R* is the average rate of star formation per year in our Milky Way galaxy, fp is the fraction of those stars that have planets, ne is the average number of planets that can potentially support life per star that has planets, f is the fraction of the preceding value that actually go on to develop life at some point, fi is the fraction of the above that actually go on to develop intelligent life, fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space, and L is the length of time such civilizations release detectable signals into space.

None of these factors has a fixed value. Each factor represents a range -in some cases, a very wide range of estimates based on current scientific knowledge, which is changing daily. Perhaps there's some agreement on the value of R*. But, while astronomers have already identified several hundred extrasolar planets, given the vastness of the galaxy they have barely begun the task of determining the value of fp. After fp, determining values is barely more than a matter of guessing.

So why don't we call the Drake "equation" what it is, not what it isn't? We could call it a hermeneutic tool, something useful for interpreting or explaining SETI. We could call it a heuristic tool, something useful for "stimulating interest as a means of furthering investigation." (Dictionary.com, again.)

What I would like to call it is an allegorical tool.

An allegory is "a representation of an abstract or spiritual meaning through concrete or material forms, a figurative treatment of one subject under the guise of another” (dictionary.com). I like to think of SETI as the scientific community's approach to the human search for meaning, connection, and community, extending this search from the terrestrial to the cosmic scale.

Kurt Vonnegut wrote about this search for meaning in his 1959 novel, "The Sirens of Titan":

"Everyone now knows how to find the meaning of life within himself.

But mankind wasn't always so lucky. Less than a century ago men and women did not have easy access to the puzzle boxes within them.

They could not name even one of the fifty-three portals to the soul.

Gimcrack religions were big business.

Mankind, ignorant of the truths that lie within every human being, looked outward -pushed ever outward. What mankind hoped to learn in its outward push was who was actually in charge of all creation, and what all creation was all about.

Mankind flung its advance agents ever outward, ever outward. Eventually it flung them out into space, into the colorless, tasteless, weightless sea of outwardness without end.

It flung them like stones.

These unhappy agents found what had already been found in abundance on Earth a nightmare of meaninglessness without end. The bounties of space, of infinite outwardness, were three: empty heroics, low comedy, and pointless death.”

An assumption underlying the rationale for SETI is that finding a signal will be meaningful. That remains to be seen.

In a future blog posting, I'll consider how and why the scientific rationale for astrobiology - the study of the origin, evolution, distribution, and future of life in the universe - is different from, and more solidly scientific than the rationale for SETI.