The Drake Equation

Science, Skepticism, Webcomics February 17th, 2008

The Drake Equation, according to the world’s worst encyclopedia is

… a famous result in the speculative fields of exobiology and the search for extraterrestrial intelligence (SETI).

This equation was devised by Dr. Frank Drake (now Professor Emeritus of Astronomy and Astrophysics at the University of California, Santa Cruz) in 1960, in an attempt to estimate the number of extraterrestrial civilizations in our galaxy with which we might come in contact. The main purpose of the equation is to allow scientists to quantify the uncertainty of the factors which determine the number of such extraterrestrial civilizations.

Or at least, that’s what the article said at the precise moment I accessed the page. Who knows what it will say when you do the same?

The Drake Equation is given by the following formula:

N = R^* \times f_p \times n_e \times f_l \times f_i \times f_c \times L

where

N is the number of civilizations in our galaxy with which we might hope to be able to communicate

and

R^{\ast} is the average rate of star formation in our galaxy
f_p is the fraction of those stars that have planets
n_e is the average number of planets that can potentially support life per star that has planets
f_l is the fraction of the above that actually go on to develop life at some point
f_i is the fraction of the above that actually go on to develop intelligent life
f_c is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L is the length of time such civilizations release detectable signals into space.

The Drake Equation has been criticized by many scientists as meaningless. Michael Crichton in his speech “Aliens cause global warming” says of the Drake Equation:

This serious-looking equation gave SETI [the Search for Extraterrestrial Intelligence, founded by Frank Drake] a serious footing as a legitimate intellectual inquiry. The problem, of course, is that none of the terms can be known, and most cannot even be estimated. The only way to work the equation is to fill in with guesses. And guesses-just so we’re clear – are merely expressions of prejudice. Nor can there be “informed guesses.” If you need to state how many planets with life choose to communicate, there is simply no way to make an informed guess. It’s simply prejudice.

As a result, the Drake equation can have any value from “billions and billions” to zero. An expression that can mean anything means nothing. Speaking precisely, the Drake equation is literally meaningless, and has nothing to do with science [my emphasis]. I take the hard view that science involves the creation of testable hypotheses. The Drake equation cannot be tested and therefore SETI is not science. SETI is unquestionably a religion. Faith is defined as the firm belief in something for which there is no proof. The belief that the Koran is the word of God is a matter of faith. The belief that God created the universe in seven days is a matter of faith. The belief that there are other life forms in the universe is a matter of faith. There is not a single shred of evidence for any other life forms, and in forty years of searching, none has been discovered. There is absolutely no evidentiary reason to maintain this belief. SETI is a religion

Randall Munroe, who draws the consistently brilliant webcomic XKCD, has recently discovered that the Drake Equation is missing a term:

XKCD - The Drake Equation

6 Comments

  1. John Creighton
    02/17/2008 at 11:12 am

    I never took the Drake equation as science but rather I took it as philosophy. Bias or not, if we consider the scale of the universe and how small we are in it, then the possibility of life seems probable and I do not care weather you call the feeling science, philosophy prejudice or bias.

    A big part of science is wonder. Without it what drives us to investigate the universe beyond what is know? The prospect of making contact with another civilization to me would seem profound. The idea that we are alone in such a big universe begs the question of why? Why was all this created just for us? Are we that important that special given that we will only be here for an instant in the lifetime of the universe. To me the quote in the move contact, “If we are alone it would be an awful waste of space” may seem simple but I consider it deep.

  2. The Drake Equation does raise an important question in the philosophy of science – if I parametrize my beliefs into an untestable equation containing unknowns which cannot be measured – is it still science or is it disguised religion?

  3. John Creighton
    08/28/2009 at 6:37 pm

    We can possibly estimate all the parameters except for f_i, f_c and L.

  4. Which means its still meaningless.

  5. John Creighton
    08/29/2009 at 4:44 am

    I still think question of if A then B can be interesting even though we may not know if A is true. Besides, you can take off the terms we don’t know or can’t determine and it still answers interesting questions. For instance without (f_i, f_c and L.) it will still give us the number of planets which support life.

    Perhaps f_c isn’t completly un-estimatable. Wireless comunication seems to be a natural evolutionary of technology. Surely at some point in technological evolution people will develop television. Of course, I’m not sure it will ever be possible to detect such signals at any sufficient distance. Also as technology becomes more advanced people are able to transmit more information with less power so perhaps the dectection problem is extreemly difficult.

    I wonder what it would take for a signal to be detectable and how an alien race would know how to look for it or even what it means. Consider some of the problems of detection. I presume all frequencies won’t travel at the same speed so I image the original waveform would be quite difficult to reconstruct even if it is highly noise resistant. (How will the detector know how much phase delay there is for each frequency component or how long the word length is)

    As for f_i, who knows……..maybe a new field in mathematics will develop called evolutionary statistics. How many possible gene combinations are there (given a finite genome length)? How likely is each combination to survive? Or perhaps put another way, what is the average time to extinction for a given species? How long on average would it take to develop advanced technology (social evolutionary statistics). Who knows what kinds of advance problems we might solve in the future. You may think the question is uninteresting. I think it is interesting. Besides isn’t what is interesting somewhat subjective?

  6. John,

    The formula is based upon guesses of each parameter with such wide possible values that it can mean anything at all. That’s isn’t science, its faith in mathematical form, because no estimate can be shown to be wrong and therefore unfalsifiable.

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