SETI as a Process of Elimination
While it has become common to identify SETI — the search for extraterrestrial intelligence — with the search for civilizations capable of communicating by technological means, and most especially the use of the electromagnetic spectrum as the means of communication, some of the earliest proposals that might be considered the origins of SETI predate radio telescopes and the early paradigm of SETI based upon them.
Proposals were made in the early nineteenth century to inscribe an enormous representation of the Pythagorean theorem into the Siberian tundra, to signal the moon or Mars with sunlight reflected by large mirrors, or by filling canals configured as meaningful symbols with oil and setting them on fire at night. These projects were proposed when human beings knew very little about the other planets of our solar system and the conditions for life generally, when it was still plausible to believe that these celestial bodies might be inhabited not only with other beings, but also with other civilizations. Indeed, our understanding of the conditions on the other planets orbiting our star is quite recent.
It was only a little over a hundred years ago when Percival Lowell looked at Mars through his telescope, thought he saw canals, and constructed a melancholy story of an alien species at risk of extinction, building canals to channel water from the poles to the arid equatorial regions of Mars. Lowell’s books describing this scenario were quite popular, and traces of the idea persist in classic science fiction stories from the first half of the twentieth century.
While no large scale efforts at SETI were undertaken by these relatively primitive means — no doubt then, as today, it was difficult to find the funding — it is worthwhile to point out the counterfactuals involved in these ideas. The projects so described could have been built, and if they had have been built, astronomers on Earth peering with telescopes at the surface of the moon or Mars, waiting for a signal to be returned by similar means, would have been disappointed, and this would have been the first negative result from a SETI research project.
But let me consider a slightly more fantastical counterfactual. Suppose that our planetary system had had a very different structure than that which it does in fact have, suppose that there were many inhabitable worlds orbiting our sun, and suppose further that, on many of these inhabitable worlds, intelligent beings had emerged and civilizations had arisen. In this case, only the most rudimentary technological achievements on the part of these counterfactual civilizations would have been sufficient to detect large scale visual signaling from Earth. Presumably they would have responded in kind, and a dialogue would have emerged. The eventual development of spacefaring technology, then, would have meant not the exclusive human exploration of our planetary system, but rather a reaching out to fellow beings with whom we shared a single star and a single planetary system.
Subsequent science has dashed any such hopes. There may yet be microbes under the surface of Mars, or some kind of life in the subsurface oceans of Europa or Enceladus, but we can confidently rule out any other technological civilization in our planetary system. We cannot yet completely rule out the possibility of complex life in subsurface oceans in our planetary system, so we cannot yet completely rule out the slim possibility that there could be other intelligent beings with whom we share our planetary system, but, if they are there, they have made no large scale effort to signal through the ice under which they would be trapped. It makes for an interesting thought experiment, however, to formulate methods by which such signaling might be accomplished by a marine intelligence under kilometers of ice.
Only slowly did we come to a realization that, beyond Earth, the rest of solar system seems to be sterile. Planetary science has opened up new worlds for us, and increased our knowledge of the solar system exponentially, but everything that it has shown us of our own solar system has confirmed the absence of other intelligent beings and other civilizations. The accumulated knowledge we possess today has been a process of the elimination of these and similar scenarios which once seemed possible.
When SETI research projects were eventually taken up in earnest, by means of radio telescopes rather than by gigantic mirrors or geoglyphs, essentially the same endeavor was pushed outward, beyond the diminutive confines of our solar system, looking at other stars rather than merely at other planets, but the same considerations that held for SETI within our solar system holds for SETI in the universe entire, albeit on a far larger scale. Thus we can interpret the results of SETI in the same spirit, as a process of elimination, the incremental, step-by-step elimination of the possibility of intelligent beings and their civilizations, in the same way that these possibilities have been eliminated within our own solar system.
Another way to formulate this is to say that the process of elimination that is SETI first eliminated the possibility of a peer technological civilization within our solar system. By a “peer” civilization I mean a civilization more or less like our own — that is to say, a civilization belonging to the same classification as our civilization, assuming that we can work out a plausible taxonomy for civilizations — and a civilization falling within a reasonable developmental approximation of our own civilization.
While we cannot rule out the possibility of some particular individual civilization lost among the countless stars of our universe, which individual civilizations may be outliers, incorporating surprising exceptions to the lessons we believe that we can draw from our own experience of civilization, if we conceptualize SETI as a process of elimination we can exclude entire classes of civilizations; even if we cannot say that there are no other civilizations at all within our light cone, we can say that there are no other civilizations of a certain class. The class of peer civilizations within our own solar system has been excluded in this way.
The conceptualization of SETI as a process of elimination also places a particular emphasis on negative results, and there has been a problem with reporting negative results, which are interpreted as a form of failure. Confirming the elimination of the possibility of an entire class of civilizations allows us to narrow and hence to refine our conception of civilization. Once a class of civilizations has been eliminated as a possibility, SETI can focus on other classes of civilizations that have not yet been eliminated, thus moving the process forward in a via negativa to civilization, that is to say, determining what civilization is not in the universe.
An advantage, from a scientific standpoint, of conceptualizing SETI in terms of a process of elimination of entire classes of civilizations, is that, if the charge is made against SETI that it involves no definitively falsifiable hypotheses, SETI can point to specific classes of excluded civilizations — like other technological civilizations within our solar system — as evidence that SETI does indeed have definitively falsifiable hypotheses, even if these hypotheses have not typically been expressed in this way, that is to say, expressed in terms of disconfirmation or falsification.
Traditional SETI searches for alien radio signals have, in our time, extended the process of elimination to peer civilizations from nearby stars. That is to say, however disappointing it is for those of us who once dreamed of nearby alien civilizations, we can say with a high degree of confidence that there are no peer industrial-technological civilizations associated with the nearest stars to our sun. The first SETI project, Frank Drake’s OZMA in 1960 looked for signals from Tau Ceti and Epsilon Eridani, two sunlike stars relatively nearby. No alien transmissions were detected. Most of the nearby stars, with which we might communicate on a reasonable scale of time and at power levels attainable with contemporary technology, have been surveyed for radio broadcasts. Nothing has been found.
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, so that about twelve years would elapse between a message broadcast and a response. 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 might receive a signal from Barnard’s Star tomorrow, from an only-just-recently radio-capable civilization, or a civilization that has only recently passed the threshold of generating EM spectrum signals detectable over interstellar distances, but we have other reasons now for eliminating other local stars as homes for peer civilizations, such as the question of extra-solar planets, for instance, and of any exoplanet being tidally locked to a red dwarf star, in the case of Barnard’s Star. This does not eliminate the possibility of non-peer civilizations, which could include either non-radio capable civilizations, i.e., a pre-industrial civilization, or civilizations so different from our own that we could not recognize them as peers to our particular species of technological civilization. These are classes of civilizations, or perhaps non-civilizations, not yet excluded by the process of elimination.
As our technology improves, it extends the traditional SETI task of the process of elimination farther and farther into the cosmos. New instruments and new techniques of observation further expand the scope of SETI. We cannot yet put a numerical value on the zone of exclusion from which the nearby process of elimination can be said to safely rule out the possibility of peer civilizations by the means of conventional radio astronomy, but as the technology of the search improves, and SETI matures as a science, it can pass beyond the classificatory concept of excluded classes of civilizations, and the comparative concept of peer civilizations, to fully quantitative concepts that will make it possible to assign numerical values, or a range of numerical values, to the zone of exclusion.
The science itself of interstellar EM spectrum communication is still in its infancy. We know from the existence of extremely distant astronomical radio sources that detectable radio signals can be effectively propagated over vast astronomical distances, as long as they are sufficiently powerful; this is the existence proof of long distance radio transmission. For example, active galactic nuclei, pulsars, supernova remnants, and radio galaxies all produce radio signals detectable over astronomical distances. The radio galaxy 3C 295 is about five billion light years’ distance from us, so we know, from this, that radio sources from half way across the known universe can be detected by us.
Given this existence proof of the transmission of very powerful signals, it would be an interesting research project to determine the weakest astronomical radio sources that are propagated over cosmological distances, what kind of naturally-occurring astrophysical processes produce relatively weak signals, and how powerful these signals were at the source. A sufficiently thorough research program of this kind would start to give us a range of numerical values for the propagation of radio signals over cosmological distances. In other words, we need to construct a cosmic distance ladder specific to EM spectrum signals, i.e., an EM spectrum distance ladder, and to do this we need a “standard candle” for radio astronomy based not on visible spectrum luminosity, but on radio frequency strength.
There is, then, much work to be done in the basic science underlying SETI. We already have undertaken an experiment in long distance radio communication, though that was not the intention of the mission that is indirectly providing this experiment for us. The Voyager 1 spacecraft has passed through the heliopause and into interstellar space (this is now called the “interstellar mission” and Voyager 1 will be joined by Voyager 2 in interstellar space sometime in 2016). Voyager 1 has taken its 23 watt radio transmitter with it into interstellar space and continues to remain in communication with Earth. The farther the Voyager spacecraft travel into interstellar space, through the interstellar medium, and continue to remain in radio communication with Earth, the more we will learn about the transmission of radio signals through interstellar space. Unlike astronomical radio sources, the radio signals we receive from Voyager are known to emanate from an artificial source with a known transmitter power and a known antenna array. These known parameters will contribute to establishing a metric for the detectability of faint artificial radio sources.
While much remains to be done, known distant radio sources suggest that, in the pursuit of SETI as a process of elimination, we are not limited to the exclusion only of certain kinds of peer civilizations within a certain parochial galactic range of relatively nearby stars. Kardashev, in the 1964 paper, “Transmission of Information by Extraterrestrial Civilizations,” in which he introduced his famous classifications of civilizations as Type I, Type II, and Type III, suggested that we might already, at the time his paper was written, have discovered artificial radio sources:
“…even at the present time we have knowledge of about twenty or thirty or so radio sources, with the upper limit of the angular dimensions in the range of 1 to 10 sec of arc. Some of these sources have been identified with peculiar optical objects the nature of which is still obscure. Most of them have not yet been successfully identified with optical objects. For example, two sources of radio-frequency emission from outer space, CTA-21 and CTA-102, were recently discovered at the California Technological Institute, and display angular dimensions not less than 20”, and have not been identified with a single one of the optical objects in the Palomar sky charts and, even more intriguing, these sources exhibit a spectrum highly similar to the anticipated artificial spectrum.”
A year after Kardashev’s paper appeared, Gennady Sholomitskii published a paper, “Variability of the Radio Source CTA-102,” announcing that the radio signal from CTA-102 varied over time, which some took to be evidence of information encoded in the signal. CTA-102 was subsequently identified as a quasar, however, and is no longer believed to be an artificial radio source, but the important point here is that a Kardashevian supercivilization of a sufficient magnitude would radiate as powerfully as a natural radio source, and so would be detectable across very great astronomical distances.
CTA-102 was not the only SETI false alarm resulting from a natural radio source being mistaken for an artificial source. When the pulsar PSR B1919+21 was discovered two years later, in 1967, its periodicity of 1.3373 seconds was eerily reminiscent of a beacon, and so it was christened “LGM-1,” “LGM” being an acronym for “little green men.” (I would like to note, as an aside, that the idea of extraterrestrials as little green men has fallen out of favor in popular culture, to have been replaced by aliens gray in color, though “little gray men” would still be abbreviated as LGM.)
Our universe, then, is alive with radio emissions, and the challenge of SETI is the find the signal possibly buried in the pervasive radio noise, though, as I pointed out above, a supercivilization could be radiating as powerfully as a quasar, and not hidden in the background radio noise. If a supercivilization of this magnitude wanted to send a clearly identifiable artificial radio beacon across astronomical distances, it could unquestionably do so.
In the same Kardashev paper quoted above, Kardashev wrote,
“…should there exist even one type II civilization within the confines of the local system of galaxies, there will be a realistic possibility of securing an enormous quantity of information. The same holds for the existence of even one single type III civilization in the portion of the universe accessible to observation.”
Another way to formulate this is to say that we can now eliminate the possibility of there being any type II civilizations in the local system of galaxies and the possibility of there being any type III civilizations in the observable universe. Seen in this light, these are powerful results.
At some point in time — and again, as with size of the zone of exclusion, we cannot yet assign an exact numerical value to this point in time — the distant radio objects in the observable universe date to such an early stage in the evolution of the universe (due to lookback time involved in our observations) that we can be confident that an insufficient period of time has elapsed for the emergence and expansion of a distant supercivilization.
The universe must be old enough to produce the chemical elements necessary to life and civilization before these can arise, and once civilization arises it requires a cosmologically significant period of time to develop into a supercivilization (unlike our diminutive civilization, which has not yet persisted for a cosmologically significant period of time).
It is likely that our present ability to survey the observable universe extends sufficiently far into the past that we can already see beyond the threshold for the earliest emergence of a billion-year-old supercivilization, which means that any supercivilization within our cosmological horizon should be observable. If such a civilization were transmitting at the energy levels available to it, we would know about it.
As part of the SETI process of elimination, we can with a high level of confidence assert that the class of type III civilizations can be eliminated from the known universe. There are no true supercivilizations out there. Of course, we must make certain qualifications to this sweeping conclusion: for example, it only applies to radio loud supercivilizations making no attempt to suppress their emissions. These qualifications are part of the process of refinement in our conception of civilizations that are the potential objects of a SETI search.
Conceptualizing SETI as a process of elimination, then, gives us a powerful tool for the refinement of the concept of a supercivilization discoverable by SETI means, as well as a method of disconfirmation that can definitively falsify hypotheses concerning these supercivilizations, so long as we take sufficient care in the formulation of our hypotheses and the execution of observations that could confirm or disconfirm such hypotheses.
The above post is a revised and expanded text of my Wow! Signal Burst spoken word production of “SETI as a Process of Elimination,” which was, in turn, a revised and expanded version of my earlier blog post SETI as a Process of Elimination.