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Scientific American

Why ET Hasn’t Called

published August 2002 | comments (19)
The lifetime of civilizations in the Drake equation
for estimating extraterrestrial ntelligences
is greatly exaggerated
magazine cover

In science there is arguably no more suppositional formula than that proposed in 1961 by radio astronomer Frank Drake for estimating the number of technological civilizations that reside in our galaxy: N = R fp ne fl fi fc L

In this equation, N is the number of communicative civilizations, R is the rate of formation of suitable stars, fp is the fraction of those stars with planets, ne is the number of Earth-like planets per solar system, fl is the fraction of planets with life, fi is the fraction of planets with intelligent life, fc is the fraction of planets with communicating technology, and L is the lifetime of communicating civilizations.

Although we have a fairly good idea of the rate of stellar formation, a dearth of data for the other components means that calculations are often reduced to the creative speculations of quixotic astronomers. Most SETI (search for extraterrestrial intelligence) scientists are realistic about the limitations of their field; still, I was puzzled to encounter numerous caveats about L, such as this one from SETI Institute astronomer Seth Shostak: “The lack of precision in determining these parameters pales in comparison with our ignorance of L.” Similarly, Mars Society president Robert Zubrin says that “the biggest uncertainty revolves around the value of L; we have very little data to estimate this number, and the value we pick for it strongly influences the results of the calculation.” Estimates of L reflect this uncertainty, ranging from 10 years to 10 million years, with a mean of about 50,000 years.

Using a conservative Drake equation calculation, where L = 50,000 years (and R = 10, fp = 0.5, ne = 0.2, fl = 0.2, fi = 0.2, fc = 0.2), then N = 400 civilizations, or one per 4,300 light-years. Using Zubrin’s optimistic (and modified) Drake equation, where L = 50,000 years, then N = five million galactic civilizations, or one per 185 light-years. (Zubrin’s calculation assumes that 10 percent of all 400 billion stars are suitable G- and K-type stars that are not part of multiples, with almost all having planets, that 10 percent of these contain an active biosphere and that 50 percent of those are as old as Earth.) Estimates of N-range wildly between these figures, from Planetary Society scientist Thomas R. McDonough’s 4,000 to Carl Sagan’s one million.

I find this inconsistency in the estimation of L perplexing because it is the one component in the Drake equation for which we have copious empirical data from the history of civilization on Earth. To compute my own value of L, I compiled the durations of 60 civilizations (years from inception to demise or the present), including Sumeria, Mesopotamia, Babylonia, the eight dynasties of Egypt, the six civilizations of Greece, the Roman Republic and Empire, and others in the ancient world, plus various civilizations since the fall of Rome, such as the nine dynasties (and two republics) of China, four in Africa, three in India, two in Japan, six in Central and South America, and six modern states of Europe and America.

The 60 civilizations in my database endured a total of 25,234 years, so L = 420.6 years. For more modern and technological societies, L became shorter, with the 28 civilizations since the fall of Rome averaging only 304.5 years. Plugging these figures into the Drake equation goes a long way toward explaining why ET has yet to drop by or phone in. Where L = 420.6 years, N = 3.36 civilizations in our galaxy; where L = 304.5 years, N= 2.44 civilizations in our galaxy. No wonder the galactic airways have been so quiet!

I am an unalloyed enthusiast for the SETI program, but history tells us that civilizations may rise and fall in cycles too brief to allow enough to flourish at any one time to traverse (or communicate across) the vast and empty expanses between the stars. We evolved in small hunter-gatherer communities of 100 to 200 individuals; it may be that our species, and perhaps extraterrestrial species as well (assuming evolution operates in a like manner elsewhere), is simply not well equipped to survive for long periods in large populations.

Whatever the quantity of L, and whether N is less than 10 or more than 10 million, we must ensure L does not fall to zero on our planet, the only source of civilization we have known.

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19 Comments to “Why ET Hasn’t Called”

  1. Adrian Says:

    Dear Mr. Shermer,

    Please allow me to correct you (I’m no doubt not the first one, but just in case!)…

    You say:

    “I find this inconsistency in the estimation of L perplexing because it is the one component in the Drake equation for which we have copious empirical data from the history of civilization on Earth.”

    You seem to have misunderstood how L is defined. Since the goal of the equation is to obtain an estimate of the number of civilizations that could conceivably send signals to us (and hence make a guess concerning SETI’s chances of success), it follows that L is the average lifetime of a *radio-capable* civilization (otherwise we wouldn’t be able to detect them via their signals!).

    We are, of course, the only example of such a civilization that we know of – and we have just begun to send signals perhaps 80 years ago or so! We also can hardly predict with any certainty how long we’re going to last (maybe not much longer!), hence the difficulty assigning a value for L. The question people are asking themselves is: “how long are we going to last?” The answer to this would give us that one, precious (but still woefully inadequate) data point.
    In other words, today we don’t even have a SINGLE reliable data point to extrapolate from.

    Adrian Schubert

  2. Dr.Q Says:

    visits of space ships are seen in many ancient paintings.
    never the less, I have not found anywhere ancient report of alien abductions. The earliest I have found are those of aroun the 1950s.
    I throughly believe if such civilisations actually do exist, they would allready have made contact with us unless, they cant really travel trough space.

  3. Skeptigator Says:

    I was actually thinking along the same lines as Adrian however I think where Mr. Shermer might have been going with his estimates is that if you were to generalize the longevity of a complex civilization (such as our own) you only need to look at Earth’s history to get a rough estimate of how long we may last.

    What I find interesting is the isolation of any given unknown variable of this component as any more compelling/perplexing as another. For example, we may be able to estimate that complex civilizations last 300 years on Earth, the introduction of the variable fc asks that even bigger question, what percentage of those civilizations that will rise and fall on other worlds are actually radio-capable.

    If you take Mr. Shermer’s numbers you could look say that in the instance of our singular planet, we have had 1/60th of our planet’s civilizations and to take this even more extreme only about 1/4 (80 years) of the time of that 1/60th civilization.

    That’s a fun little thought experiment, but in either case it illustrates that the possibility of two civilizations capable of communicating with each other and existing at nearly the exact same fraction of a tick in the Universe’s life and that they should be reasonably situated near each other in that same Universe is extremely unlikely.

  4. Skeptigator Says:

    wow, my English must be failing me, let me rephrase,

    If you take Mr. Shermer’s numbers you could say that in the instance of our own planet, we have had 1/60th of our planet’s civilizations radio-capable and to take this to even further extreme only about 1/4 (80 years) of the time during that 1/60th civilization’s existence was capable of being able to communicate.

  5. Sungod Says:

    I disagree with two of Mr Shermer points:

    He indicates that the only well known term is
    the rate of star formation. This is not quite true-
    there is some disagreement over what mass stars
    can support ‘civilizations’ so we don’t know how
    many forming stars to count. There is probably
    less disagreement over what fraction of stars have
    planets than what types of stars are suitable.

    My second ‘bone to pick’ is that he selects a
    ‘conservative’ estimate of values for the Drake
    Equation, but it is by no means representative of
    the pessimists’ estimate. Many scientists
    (particularly in the life sciences) put the odds of
    life arising at closer to one-in-a-million than
    the 20% cited. Also, ‘intelligent’ life .. or even
    multi-cellular life may be extremely rare. These
    ‘pessimists’ put the estimated number of communicative
    civilizations at far less than zero (even with an
    optimistic view of life expectancies). For them it
    is obvious why ET doesn’t call home – he was never

  6. Mark A. Craig Says:

    I have an alternative theory why ET hasn’t come calling: ET had his own “Peak Oil” catastrophe and then fell into a permanent Dark Age.

    ET, upon discovering his own planet’s vast stores of organic chemical potential energy, did the same thing we’ve done and gorged upon them in an orgiastic refutation of the existence of tomorrows; ET was paying no attention to the Big Picture… until it was too late. ET made no successful effort to establish a sustainable presence in space or upon other planets, rather choosing to squander those reserves upon expensive energy baubles like street lights, neon signs, personal mechanized transportation for everyone, and constant streams of “entertainment”.

    When the inevitable Peak Oil event occurred, ET found himself then lacking the enormous energy resources required to lift himself off of his rock, unable to belatedly establish that sustainable presence in space.

    ET got stuck on his little rock, sullen and depressed in his shortsighted stupidity, unable to dig himself back out of the deep planetary grave he had dug for himself, and eventually withered away.

  7. Dave Blythe Says:


    I don’t see the need to limit L to the span of any single civilization. Using our own case as representative, all future civilizations will be capable of radio communication. They will also know how to use it and what frequencies will transmit best across intergalactic space.

    The better figure for L is the projected remaining time for our species to exist. I seem to recall that an average species on our planet has lasted in the range of 2 to 4 million years from inception to extinction. In any event my estimates for L would be much longer than the time for any one transient civilization.

    Further, unless SETI can pick up low energy local radio signals from distant sources there may be nothing to report. Another multiplier in the Drake Equation could be the fraction of civilizations actively trying to communicate with distant planets through focused high energy radio signals. This may be a very low number. We may all be listening quietly.

  8. Cliff Says:

    Dr. Q,

    You say, “I throughly believe if such civilisations actually do exist, they would allready have made contact with us”

    Contact? Contact? Where to begin…

    What do you call this?

  9. cliff Says:

    Hello Michael,

    I work with Joe Firmage and recently discovered your reference in the second edition of your book to his writings on the questions surrounding UFOs. After pointing that out to him, he suggested I send you a request to discuss these questions on your program and/or blog. Open invitation for useful discussion for the sake of useful discussion.

    Very best, Cliff Lyon

  10. DaveP Says:

    Crop circles are your proof of planetary visitors??? Are you serious?!?

    Firstly, and most importantly, it is well established that humans are VERY capable of being 100% responsible for every crop circle ever discovered.

    Secondly, would YOU travel several thousand light years just to draw some geometric pictures in the fields of a few farms as your ONLY form of communication to the local inhabitants? I seriously doubt it.

    Even with the endless possibilities of what an alien culture might produce for an introduction ritual, using agricultural vandalism as your only way of saying “Hello” seems unlikely.

    As appealing and romantic as the thought of ‘crop circles being caused by aliens’ might be, when you compare human capabilities (both culturally & technologically) verses what the likely motivations of a race capable of visiting planet Earth would be – crop circles don’t appear to present much evidence for contact with aliens.

  11. cliff Says:

    Ho dear DaveP,

    It may be well established (in your mind) that “humans are VERY capable of being 100% responsible for every crop circle ever discovered.”

    But you would be wrong. You intensity of you confidence does not seem, I believe, correlate to the intensity of your own investigation.

    Not only has never been demonstrated (incl the BBC thing in New Zealand) that humans ARE capable of doing this, but there are no serious, respected scientists who will endorse your claim.

    Lastly, the suggestion that you might know the mind of an with the intelligence to get here, is well, arrogant…though very humanesque.

    Since you suggested a comparison to human culture, perhaps you could point me to an artist of any age whose work/designs resemble the designs in the crop circles.

    This is my field and I can assure you, know one in the art world that I know has ever seen anything close to the kind of beauty of the crop circle designs from a human artist.

    Off the top of your head, do you know how many crop circles appeared just this year?

  12. ryan cuggy Says:

    Maybe someone pointed this out in a comment I missed, but this supposes that an aliens would have the same motivators as homo sapiens

    Maybe rape never evolved? Maybe they’re herbivorous? any number of differences from human animals would produce very different civilizations. To assume our civilization is standard seems very silly to me.

  13. Wesjo Says:

    The equation is inadequate to begin with. There should be a factor relating to the mean time between catastrophic collision events, the mean time for life (of perhaps unknown types) to develop, etc.

  14. Paul Gracey Says:

    Another possible parameter to the equation could be the length of time until local planetary communication methods invented by the civilisation improve to the point where they are indistinguishable from background radiation. A point we are rapidly reaching with the spread spectrum, burst technologies we use today. We are already at this point a mere century after the extremely noisy, but coherent methods of the early days of spark gap transmitters.

    What we need to hope for is that our civilisation will last long enough to begin sending a simple signal at a sufficient power level to attract attention from ET wherever he may be. Hopefully it wont have to be a distress signal.

  15. Kenn Says:

    You won’t agree with his anthropic principle, but Dr. Hugh Ross’ contribution to parameters is worth the read…

    Examples (First 3 of 66)

    1 – galaxy size
    if too large: infusion of gas and stars would disturb sun’s orbit and ignite too many galactic eruptions.
    if too small: insufficient infusion of gas to sustain star formation for long enough time.

    2 – galaxy type
    if too elliptical: star formation would cease before sufficient heavy element build-up for life chemistry.
    if too irregular: radiation exposure on occasion would be too severe and heavy elements for life chemistry would not be available.

    3 – galaxy location
    if too close to a rich galaxy cluster: galaxy would be gravitationally disrupted
    if too close to very large galaxy(ies): galaxy would be gravitationally disrupted.

  16. DaveP Says:

    I agree that I do indeed lack the skills to put forth a good argument, but you must also concede that offering links to as examples of proof do not lend much credibility to your position either.

    Here is what I have been reading and accept to be the best and most plausible explanations to date.

    I don’t doubt the possibility of extra terrestrial life forms at all, I simply think that crop circles, alone, are insufficient as proof of their arrival and the supporters of the alien cause hypothesis have yet to provide a compelling reason that leads me to believe that crop circles are caused by anything but human or some other natural phenomena.

    I wish you luck in your quest to prove your assertions, but your arguments, as presented, don’t even tempt me, a layperson, to be interested in what you have to say.


  17. Bart Stewart Says:

    It’s slightly off the topic of the current article, but I wanted to salute Michael Shermer for his valiant defense of rationality and skepticism on the Larry King show the other night, going on as the only skeptic (as usual) against three or four frantic UFO promoters. Shermer was given about a minute (as usual) to respond to the onslaught of emotional UFO promos.

    This show dealt with the recent Texas UFO case, of course, which I grant may rank as one of the sturdier cases, if only because the government is now admitting that they scrambled fighter jets after first denying it. Still, we should remember that UFO history is littered with the wreckage of cases that seemed quite sturdy at first. Roswell comes to mind, and of course that one really was a government cover-up, just not of UFOs. The Texas case is very recent, and it often takes time for these things to be sorted out. Bear in mind, at one time people were freaking out over George Adamski.

    Shermer pointed out on the King show that there are residual cases in all fields of inquiry that are not quickly resolved. With something as fleeting as a UFO case, we may never know what those lights in the sky were.

    I always thought it was interesting that UFOs tend to be seen more by one culture than another. It really is a white thing, more or less, isn’t it? The overwhelming percentage of the UFO witnesses, abductees, etc., tend to be caucasian. And the crop circles, mentioned earlier, seem to have a cultural center of gravity, too. They are heavily British, aren’t they?

  18. Old sliderule geek Says:

    Pardon the correction, but

    L does not equal the lifetime of just any old civilization, rather:

    L = the amount of time a given planet will be transmitting intelligible symbolic language signals in the radio or other spectrums.

    This is a highly debateable number. From our own experience we can set a lower bound of 100 years, but what is the upper bound? If our civilzation returns to a (radio) dark age due to a collaspe caused by overpopulation and/or climate change then our experience will tell us that 100 years is an unfortunate, but realistic estimate leading to a very pessimistic number for N.

    However, what if we are heading towards an “omega point” as some have predicted, where we are overtaken by the intelligence of our machines and a supercivilization unhindered by mere biological lifespans and evolutionary patterns emerges (e.g. Carl Sagan’s “billion year old supercivilizations). Then who knows where the upper bound is or for that matter will it matter to us human beings in the end?

    Then again, it is true that we have radio technology, but is that really a sign of intelligence on the cosmic scale. Pre-homo sapiens hominid species made and used tools, but did not have the intelligence our creative abilities to establish a culture that went from the sharpened rock scrapers to the bow and arrow and from hunter gathering to agriculture and an industrial revolution. To survive beyond the 5000 or so years of written languge civilization we must invent sustainable ways of using technology. We have not and may not ever do this, based on current experience.

    To support the late Carl Sagan’s highly optimistic conclusions we need a value of L based on a radio technology civilization that concurrently has a sustainable means of supporting itself. We have no experience base for that. Therefore at this time N, in my opinion, has an indeterminate value.

    Still, best of luck to all SETI researchers.

  19. Josh Says:

    I think it’s better to consider the lifetime of a civilization capable of intergalactic communication, which at this point is immeasurable for us. Who cares how long rome existed, they couldn’t communicate to faraway places.