Why the Drake Equation Is Useless
If you like science fiction, you're probably familiar with the Drake equation. This famous one-line formula solves for the number of intelligent alien civilizations within our galaxy with whom we might be able to communicate. Supporters of the search for extraterrestrial life (SETI) often refer to the expression to bolster their case.
There's just one BIG problem with the Drake equation. It's completely useless! In fact, I believe it may actually misrepresent the search for ET and limit our ideas about it. Here's why.
N = R . fp . ne . fi . fl . fc . L
That's the entire Drake Equation. As far as formulae go, it's very simple. We want know N, the number of civilizations we might hope to detect by telescope. The problem is in the parameters all multiplied together -- all those f's and n's and so forth. Out of those seven variables, we know the exact value of none of them.
We have a very rough estimate of the first variable and a foothold on the second; the remaining five are posed in such a way that they are essentially impossible to measure.
R is the rate of star formation in the galaxy. Telescopes can tell us about star formation. The problem is that there are many different types of stars which form at different rates and evolve into further types throughout their lifetime. An expanding giant star may swallow or melt planets while a dying dwarf star may slowly freeze them. Formation rates vary across the galaxy and through time as well, but an educated estimate of R is possible.
The fraction of stars with planets, fp, is another number we might vaguely guess at. We currently discover hundreds of exoplanets every year, but most of our detections require a planet with a lucky orbit passing directly between us and the star. This condition and limitations on other exoplanet hunting strategies mean that we can only see some fraction of all existing planets. The smaller or further away a planet is from its star, the less likely we are to detect it.
These first two variables may be roughly estimated with decades of painstaking astronomical research. Now we are stuck with five truly useless numbers.
How can we measure the "habitability," ne, of a planet? Is a planet always habitable, or does it cycle through periods of brutal sterility and lush calm over millennia, millions of years or eons? A planet with extreme conditions might be inhospitable to new life, but life that began in better times might adapt and eventually flourish. Can life thrive in boiling hot water? We used to think not, but we were wrong. We now know that life can survive at far below freezing temperatures too. Is an atmosphere necessary and if so, what sort?
The main question in discussions of habitability is: "How many exoplanets have water?" This question dodges a vastly larger one: Does life even require water, or can life totally different from us form without it? We have no idea what the limitations of life are. In lieu of any scientific criteria for this answer, we just guess: conditions might need to be like those on Earth. We are fundamentally limited in any attempt at scientific study by having one single example of life to observe.
This leads directly to the next parameter, fl. This is the fraction of hospitable planets which go on to develop life. There is no way to measure this without directly visiting many exoplanets, which is practically impossible.
fi and fc are, respectively, the likelihoods of any existing life forms developing intelligence and that intelligent life in turn releasing high-powered electromagnetic radiation into space. Suppose something we recognize as living does evolve. How likely is "intelligent" life? Well, it depends on how you define intelligence. The Drake equation reduces intelligence to "it can produce strong EM waves." Now you need to know how many of these "intelligent" life forms will choose to beam things around with those waves. This is not a question addressable by science.
The final parameter L is the average number of years a civilization will survive. This number too is formulated purely with science fiction and not science fact.
The worst thing about the Drake equation is that it gives us a false idea of grasping the problem we are trying to solve. A mathematical equation connotes some scientific study or understanding of a subject. But this is misleading: SETI is simply NOT a scientific endeavor. It's entirely a leap of faith, albeit a leap that uses tools devised by science. It's like searching for paranormal activity with an electronic sound recorder.
Further, I think the Drake equation takes some of the wonder and imagination out of the idea of alien life. Contemplating the vast number of places life could develop and the myriad forms it might take is a beautiful exercise. It shouldn't be constrained by grandiose limiting assumptions and the search for useless percentages. Life need not be just like us or be found by our most obvious searches. Extraterrestrial life today is the realm of science fiction and imagination, of dreams and endless possibility.
The popular web comic XKCD satirizes Drake's equation by adding a new term: Bs, "the amount of BS you are willing to buy from Frank Drake."