SETI as a Process of Elimination
1 August 2011
Seth Shostak, one of the most eminent SETI researchers, has suggested (in his lectures for The Teaching Company) that the Principle of Mediocrity can be extended beyond the idea that there are no privileged perspectives and therefore nothing unusual or exceptional about our solar system, our planet, or even life on earth, to embrace the idea that there is nothing unusual or exceptional about intelligent life, civilization, and the emergence of the industrial technology that makes SETI possible. I should also note in this context that this extension of the Principle of Mediocrity is thoroughly consonant with the argument that I made in The Continuity of Civilization and Natural History.
In several recent posts I have written about Paul Davies’ book The Eerie Silence: Renewing Our Search for Alien Intelligence (e.g., Silent Worlds, Empty Worlds). Paul Davies’ perspective represents what Davies calls “new SETI” in contradistinction to “old SETI,” which is represented by Shostak. We can recast the distinction between old and new SETI a bit by characterizing the traditional SETI undertaking of listening for alien radio broadcasts or sending our own radio signals out into space as a narrowly conceived search for peer civilizations to our own.
Under this interpretation, the traditional SETI undertaking can be seen as a process of elimination, and this process of elimination extends back into history before radio technology. Before our technology gave us the level of knowledge that we have of Mars today, it was widely speculated that there might be a technological civilization on Mars. There have been several proposals for what has generally been called extraterrestrial signaling before radio technology. Karl Friedrich Gauss, the great mathematician, suggested laying out a diagram of the Pythagorean Theorem in the wilderness of Siberia, with appropriately contrasting colors of vegetation. Joseph Johann Littrow suggested flaming trenches carved into the Sahara desert as a way to signal the inhabitants of Mars. Neither scheme was pursued.
Subsequent technological advances have made it possible for us to eliminate the possibility of a peer technological civilization within our solar system. While we cannot yet rule out the possibility of life deep within the aquifers of Mars or in the ocean postulated to exist under the ice of Europa, any life that would exist under these conditions would not have given rise to industrial-technological development.
Traditional SETI searches for alien radio signals have, by this time, similarly extended the process of elimination of peer civilizations from nearby stars. That is to say, however disappointing it is for folks like me, we can say with a high degree of confidence that there are no peer industrial-technological civilizations associated with the nearest stars pictured in the diagram above. Had there been a radio-capable peer civilization on a planet orbiting Barnard’s Star, for instance, it would only take six years for a radio signal to reach us, and another six years for that civilization to receive our answer. While that rate of communication is slow compared to our familiar modes of communication, since we’ve been broadcasting our signals for more than a hundred years there has been plenty of time to send and receive several messages. Similar considerations hold for all the stars within a radius of the reach of our radio signals, which radius is now a little larger than a hundred light years.
Of course, we could receive a signal from Barnard’s Star tomorrow, of an only-just-recently radio-capable civilization, but we have other reasons now (lack of extra-solar planets, for instance, and being a red dwarf star) for eliminating other local stars as homes for peer civilizations. This does not eliminate the possibility of non-peer civilizations, which could include either non-radio capable civilizations (like the quasi-neolithic alien societies in the film Avatar) or civilizations so different from our own that we could not recognize them as peers to our particular species of technological civilization.
As our technology improves, it extends the traditional SETI task of the process of elimination farther and farther into the cosmos. It has been this gradually increasing range of technology and the implicit process of elimination that has gotten SETI researchers to thinking and coming up with the ideas that are part of what Davies calls new SETI. Similar considerations hold for the discovery of peer life, even if not intelligent or civilized life. By “peer life” I mean life more or less biologically similar to what we know on earth. The arrival of the Viking landers on Mars largely discounted the possibility of peer life on Mars, although, as I wrote above, there remains the possibility of luxuriant caves buried deep beneath the Martian surface, heated by the residual heat of the molten core. The imagination quickly jumps to visions like those of Journey to the Center of the Earth in contemplating such a scenario. But even this scenario will eventually be either confirmed or disconfirmed by science.
Exobiology and astrobiology are sciences uniquely dependent upon technology. Technological advances brought these sciences into being, and only further technological advances will be able to settle the questions posed by nascent exobiology and astrobiology. For example, when we become able to take spectra from the atmosphere of earth-like planets orbiting other stars — a technological possibility within the next few decades — we would be able to determine the presence of certain kinds of life on other planets, even if that life has not produced a technological civilization that could communicate after the fashion of traditional SETI assumptions.
As far as technological innovation, as well as scientific ingenuity, has pushed the SETI process of elimination outward, the bubble of the extent of our knowledge is still quite small in the galaxy. The map of our spiral arm within the Milky Way galaxy, showing cepheid variable stars as “light houses” in the cosmos, includes a scale that shows a thousand light years in the lower right hand corner. By this scale you can judge by eye a sphere of a hundred light years radius which is our “radio bubble” in the cosmos. As you can see, there remains plenty of space even in our nearest cosmic “neighborhood” for peer civilizations from which we have not heard, and which would not have had an opportunity to hear from us. And this is just the Milky Way. There are galaxies in the cosmos like stars in the Milky Way: almost too many to comprehend. Most of these will remain beyond our scientific knowledge except in the most abstract and schematic form of knowledge. Radical developments and departures in science would be necessary for human technological civilization, however far extended in space, to make an adequate survey of the universe and extend even the traditional SETI process of elimination to a statistically significant percentage of the universe.
However, although our scientific sample of the universe is very small in comparison to the whole, if the Principle of Mediocrity holds good, it is a valid sample. That is something to think about. If we could produce a rigorous and comprehensive statement of the principle of mediocrity, we would have a better idea of what exactly is eliminated by the SETI process of elimination.
. . . . .
. . . . .
. . . . .