Shermer and the Drake Equation


To how many technological civilisations is our galaxy home at this moment? It would be nice to know, so we could estimate our chances of ever coming into contact with somebody out there. In 1961, astronomer Francis Drake suggested a number of parameters relevant to this issue, and summarised them in an equation that bears his name to this day. One of the parameters is the mean life-span of a technological civilisation.

In issue 2008:2 of Skeptic Magazine that reached me today, Michael Shermer has an interesting paper where he states that of Drake’s parameters, the mean life-span is actually one of the few that can be given an estimate from empirical evidence. Shermer calculates the mean length of historical civilisations on Earth and arrives at a figure of 420.5 years. This is in my opinion all backward. Shermer has mixed up the uses of the word “civilisation”. Says he:

“… I compiled the lengths of 60 civilisations (the number of years from inception to demise), including: Sumeria, Mesopotamia, Babylonia, the eight dynasties of Egypt, the six civilisations of Greece, the Roman Republic and Empire, and others in the ancient world, plus various civilisations since the fall of Rome, including 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.”

What Shermer has collected is the lengths of political blocks in the chronologies of areas with continuous complex societies. As he starts his list, “Sumeria, Mesopotamia, Babylonia”, he is simply dealing with phases in an unbroken sequence of civilisation that continues to this day in Mesopotamia. Likewise with the dynasties of Egypt and China. They weren’t independent new starts from a repeatedly cleaned slate, they were simply phases in the lives of cultures that are still with us today. (Strangely, Shermer quotes Thomas R. McDonough of the Planetary Society on this very point in an endnote, but makes no mention of it in his text.)

Extraterrestrials working on the Drake equation won’t be interested in the political details of small parts of Earth’s surface over time. They want to know the likelihood of being able to catch a transmission from somewhere in our solar system. So in fact, Earth’s world history offers us only a single data point to judge what Drake’s mean life-span might be like. If by “civilisation” we mean an agricultural society with cities, then we’re at about 11,000 years and counting. If instead, and more reasonably, we mean a society with radio broadcast technology, then we’re less than 107 years into our window of interstellar visibility, counting from the first trans-Atlantic transmission.

Anyway, the search for extraterrestrial intelligence by means of radio astronomy appears quixotic to me. Let’s say somebody in a far-off solar system is transmitting in our direction. Would we even be able to separate such a little ghost of a whisper from the roar of that person’s sun?

[More blog entries about , , , ; , .]


16 thoughts on “Shermer and the Drake Equation

  1. I agree with you, and think Shermer is way off base here. From an exterior point of view, the total number of human civilizations that have been erased is 0. They don’t vanish or destroy themselves completely; they’re conquered and assimilated. For instance, the Aztec civilization being destroyed by the Europeans isn’t so much the real destruction of a potential future space communicator as it was the violent rapid change in a population of potential future space communicators. From the perspective of a space alien, it could very plausibly have been interpreted as a step upward in terms of the total liklihood of the species eventually broadcasting.


  2. One of the problems with the original SETI approach was the idea that advanced civilizations would be detectable through radio waves. This is a very inefficient means of communication and would probably only be used in a brief burst at the outset of the development of technological civilization. Considering life on earth existed for something like 4 billion years before humans evolved and that radio communication is already being phased out (less than 150 years after its beginning) in favor of cable and other means of communication we are talking about an incredibly tiny window of opportunity even if we consider that every sun in the galaxy contains a planet with life.


  3. The life-span of a civilization makes sense here only in terms of the civilization communicating its existence to the rest of the universe. The life-span would run from ‘first light’ until ‘darkfall’.

    Of all the manmade electromagnetic radiation produced, almost all of it is dispersed over so wide a beamwidth and so wide a bandwidth as to be undetectable even one light year away by technology equivalent to what we have in use today. (Cf. for the best equipment we have fielded.)

    Powerful television signals are spatially and spectrally scattered: a TV receiver on the Moon could never detect any station on Earth. The 1 MW uplink of SPASUR is spectrally narrow (a tone) but the beamwidth covers the entire sky, which is half the universe.

    The only signals from Earth that would be detectable light years away would be microwave uplink carriers used in deep space communication and in planetary radar.

    Thus Earth’s ‘first light’ was in 1958 when the 26m antenna at Goldstone, California, first calibrated its 960 MHz uplink transmission, making the age of civilization’s a mere 50 years today.

    Everybody knows about The Wow Signal, and most know about the wow signals that followed. If we ever did detect an artificial signal from out there, we would almost certainly not see the main lobe but rather some sidelobe, where coherence is degraded, and intelligence unrecoverable. Also, every deep space carrier we produce is always sweeping spatially: an antenna radiating at zenith for calibration is still sweeping the celestial sphere owing to Earth’s rotation and orbital motion. If anybody out there has ever seen one of our signals, it is unlikely they would ever see it again.

    We may actually have detected distant civilizations in those wow signals, but what good is that? What if the average life-span, from first light to darkfall is 100 years?

    We have lots of problems on our planet, but our explosive world population growth trumps everything else in importance. When staying alive becomes Job One for most people on the planet, scientific exploration of the universe will be seen as a pointless waste of resources. Of course if course we have a nuclear war, then it’s all over.

    (Nice picture of Parkes, by the way.)

    (Note: Microwave communication is very effective for planets with atmospheres. The bands we use in deep space communications were selected for atmospheric transparency. Light from a ground station is subject to ‘scintillation’, or ‘twinkling’, and thus will not be used for ground communications in deep space.)


  4. Yes, you are right, Shermer is mixing up the terms of civilizations. Regarding your comment about the radio “roar of the person’s Sun”, it is assumed that this signal will be in the part of the radio spectrum where there isn’t much noise from stars or from our galaxy. In fact, it is expected to be in the area of the spectrum what astronomers dub the “water hole” – it has nothing to do with water – and it sits between the emission lines of atomic hydrogen (H) and hydroxyl molecule (OH) (yes, H+OH=H20). Of course, the huge assumption is that the alien civilization is making it easier for us to detect their signal and, more importantly, that their strategic thinking is similar to ours.


  5. I enjoyed reading you post.

    I’m a big fan of Shermer but I agree that he’s not 100% right on every subject. On this one, however, I think he is right. Judging the life length of civilizations is difficult because we only have one to judge by – human. But making an educated guess is what Drake’s equation is all about and you have to start somewhere so looking at civilization divisions is not such a bad place to begin.

    As to “separate such a little ghost of a whisper” your right about that. There are techniques and technologies that make this possible even if its not very probable and the techniques and technologies are available to the average citizen. I have my own SETI station up and running and so I know its possible. See it at http://www.SETI.Net. BTW my station is 1/4 of all the radio SETI stations operating on planet Earth. That’s right there are *only* four radio and only one or two optical stations running.

    We humans talk alot about SETI but we do very little about it.

    Regards……. Jim@SETI.Net


  6. I’m always fascinated by the SETI posts. As an amateur radio licensee I know about polarization of radio signals and so true, most of our TV and broadcast radio goes out in vertical polarization.

    The uplinks, etc. in the microwave bands are more likely to be detected but it’s true, we’re starting to shove a lot more over optical fiber than ever before. Look at the increase of bandwidth to China and India for example.


  7. Great post, great comments. Thanks 6Etc. Very illuminating,very informative. And bless you all, no one has mentioned I Love Lucy. rb


  8. Dr. Jill Tarter explained that current SETI attempts were designed on the assumption that other civilizations might run a deliberate beacon – a signal designed to be detected by relatively primitive civilizations like ours. She clearly doesn’t think current SETI efforts have much chance to detect The ‘leakage’ signals from intra-civilization communication, like our radio, TV, etc. So applying the Drake equation to current SETI efforts actually requires an estimate of how long a civilization is likely to be deliberately transmitting in a way that more primitive civilizations are intended to detect. Since we have not seriously attempted to do any such thing, I don’t think the history of our civilization is much use in making this estimate.


  9. Quoth Jim,

    making an educated guess is what Drake’s equation is all about and you have to start somewhere so looking at civilization divisions is not such a bad place to begin.

    Err… The main message of my blog entry might be summarised, “making an educated guess is what Drake’s equation is all about and you have to start somewhere, but looking at civilization divisions is a really bad place to begin”.


  10. llewelly: re the assumption that other civilizations are deliberately running a beacon.

    We can estimate that simply from our own behaviour. The probability is likely to be very close to zero – after all, we aren’t running a beacon.

    If other civilizations adopt our strategy, then everyone is listening but nobody is talking.


  11. 6EQUJ5: Good comment. You provided a lot of the information I was going to ask for. Questions. I don’t know about the WOW signal, what is it? Some kind of false positive? How far out can microwave carrier signals be detected? And can they be differentiated from natural radiation?

    Yeah, Shermer blew it this time. Normally I like his work.

    Personally, I think we are effectively alone. That is THEY are out there, but the distances in space and time are too great for us to detect there existence or they ours. This is one time where I would love to be proven wrong.


  12. I like this post a lot, including the comments.

    I agree that Shermer’s assessment is not an example of his best work. Martin’s objections seem spot on to me. Flags may change abruptly, but cultures evolve continually or constantly (though admittedly, the changes can be dramatic at times).

    The point is well taken, if missed by Shermer, that civilizations aren’t so clearly delineated discrete entities with clear beginnings and ends. Even in catastrophic culture clashes, there is mixture of cultures, not clean, absolute replacement of one culture for another. The categorical descriptions of civilizations is more a function of our decisions to divide rather than any naturally ocurring divisions that merely need to be observed.

    This entire exercise underscores the role that our assumptions play when we use tools like the Drake equation. While I doubt the utility of the Drake equation to produce a valid probability at the end (and after all, we really want actual evidence, not merely a guess about a probability. That is such a lame consolation prize.), I do see utility in using it to develop a model for what factors play a role in producing a technological species. In a sense, it is a means to self-discovery.


  13. There’s one other problem with measuring how long a civilisation transmits radio: the span of human transmissions in easily-decoded AM (audio or video) is turning out to be about a hundred years. Everything is going to digital, which is difficult to tell from white noise unless you know exactly how it’s encoded and (worse) multiplexed.

    So rather than the radio window – what’s the AM window?


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s