The public image of science is all too often colored by an extreme empirical bias arising from misleading and often ignorant representations in the media: a chemist is usually shown as a person in a laboratory coat, astronomers are pictured in an observatory dome, etc. Likewise, SETI (Search for ExtraTerrestrial Intelligence) projects have traditionally been associated with very large – usually white – radio telescope dishes. A simple Google search is sufficient to confirm the pervasiveness of this imagery. Such stereotyping is devoid of any real cognitive, historical, or moral sense and usually only misleading and confusing. A thinker is, contra Auguste Rodin, not colorful or photogenic enough for the contemporary media, or their consumers with short attention spans. And yet, while thinking is the basis of all human culture, it is in the development of scientific theory that it reaches the peak of its clarity and power. There is, however, consistent and persistent confusion surrounding the need for a theoretical basis for SETI projects.

The Drake equation – sound theory or surrogate science?

Appararently, there cannot be a single scientific book or review article dealing with SETI issues that does not include the customary, or even ritual, mention of the Drake equation. This equation provides a theoretical estimate of the number of technologically advanced civilizations likely to exist within the universe (or in the Milky Way, to start with), which are the targets of our SETI efforts. Somewhat paradoxically, referring to this equation often does not have to do with the actual substance of the discussion and in at least some cases seems to be motivated exclusively by the fear that a discourse that does not include any equations will not be taken seriously in the "real" scientific world. Such reverence for mathematics and numbers is entirely misplaced; there have been many similarly grotesque instances of authors trying to force mathematical language and formalism on fields as diverse as literary criticism, art history, and class struggle. Even in those fields in which mathematical expression has gradually taken root such as, for example, evolutionary biology, this has occurred for deep methodological or historical reasons and not because practitioners felt insecure or uncomfortable if they didn't repeat ad nauseam some simple piece of mathematical regularity such as, for example, the Hardy–Weinberg equilibrium.

While it is rarely stated, invocation of the Drake equation is these days largely an admission of failure. It is important, however, to establish the precise character of this failure. It is not a failure to not yet have detected any extraterrestrial signals since it would have been foolish from the outset to expect success on such small timescales of a few years to a few decades, especially with such limited detection capacities and funding. It is rather a failure to not yet have developed any genuine theoretical grounding for the search, which could be described as a failure of imagination, or a philosophical/methodological failure. Consider the structure of the Drake equation:

N = R* fp ne fl fi fc L

where

N is the “predicted” number of technologically advanced extraterrestrial civilizations in the Milky Way;

R* is the star-formation rate in the Milky Way, appropriately averaged;

fp is the proportion of stars possessing planets of any kind;

ne is the average number of habitable planets per planetary system;

fl is the proportion of habitable planets actually supporting life (generated either through local abiogenesis or by panspermia);

fi is the proportion of inhabited planets supporting intelligent life;

fc is the proportion of intelligent communities developing the technology required for detection and communication over interstellar distances;

and the (in)famous factor

L denoting the lifespan of a detectable civilization (which is often misinterpreted in a variety of ways).

What do all these fractions in the Drake equation really represent? They are in fact shorthand for a multitude of processes that are too complex and contingent to be explicitly modeled. The problem with the Drake equation is similar to that posed by Fermi’s other famous question: "How many piano tuners are there in Chicago?" We can never hope to model all the psychological, economic, and other factors that result in an individual in Chicago becoming a piano tuner, so we use probabilistic reasoning instead. By analogy, each fraction in the Drake equation is an expression of the relevant probability distribution function, which should be integrated over the relevant region in the parameter space – a fact that should be clearly stated! There should generally be an integro-differential equation for each of the probability terms in the equation. For instance, the average number of habitable planets is an integral over the rate at which planets become habitable, minus the rate at which they cease to be habitable (as a result of a variety of processes, such as runaway greenhouse effects, stellar evolution, and so on); this should be integrated over the entire volume of the Milky Way and the entire course of its history. In this manner we can finally make the transition from a guess (even an educated guess) to computation within the context of quantitative astrobiological models. In fact each of the terms in the Drake equation, with the possible exception of the average lifetime of technological societies, quite clearly falls into the field of astrobiological research, which is becoming more and more sophisticated and precise. It is therefore increasingly legitimate to ask those who continue to cite or rewrite the Drake equation without offering any deeper theoretical insight to either “put up" or "shut up”!

Many researchers, however, do not want to put up. It is particularly discouraging to come across publications with titles such as "The Drake Equation: The Theory of SETI", or labelling of the Drake equation as a "SETI creed", when it in fact appears to be used instead of any real SETI theory, i.e. theoretical framework for SETI projects, and when it is the reputation of the entire research field that too often suffers from such quasi-religious associations. That SETI has historically been considered too complex or too unworthy of attention for the development of a real theoretical framework only serves to fuel the scepticism of those who are opposed to SETI on other grounds. Either the lack of any proper theoretical underpinning is taken to demonstrate that the search for extraterrestrial intelligent life is not a real science, or it is claimed that although some theoretical underpinnings do exist for such searches, they are all expressed within the Drake equation, despite its meagre informational content and, at best, vague predictions. The latter argument is particularly insidious since it implies a real insight into SETI research, despite this "insight" being both outdated and prejudiced. Taken together, both arguments give the impression that the search for extraterrestrial intelligent life should not be taken seriously.

To the extent that the Drake equation represents any elements of a SETI theory, in contrast to using it as a fig leaf, it does so in a manner that is constraining and impractical, as has even been noted previously by researchers who are otherwise quite sympathetic to SETI, e.g. . A typical example is that the equation as it stands does not take into account the possibility of interstellar colonization and the resulting emergence of independent SETI targets. Attempts have been made to rectify this omission, for example by Walters and co-workers in 1980 , but the predominant state of mind in the majority of the SETI community has been amply demonstrated by the sad fact that this study was not widely cited until just a few years ago, and has not been used to justify practical searches.

This is in itself quite strange, and even bizarre. If the cornerstone of a scientific theory is modified in such an important way it is only rational, and indeed mandatory, to use the modified version from that moment on. It must not be regarded as just "an option", or something to mention in footnotes (if at all). When Fischer, Haldane, and Wright modified the old Darwinian theory, thus creating what subsequently became known as the Modern Synthesis, anyone interested in evolution from that time onwards was obliged, if intellectually honest, to use the modified theory and acknowledge the resulting conclusions. There is no disrespect to Darwin and Wallace in this: if anything, the modified insight provided by the Modern Synthesis leads to a better appreciation of their original achievement.

The same principle applies in other fields. For example, since 1998 almost all cosmological formulae have been modified to take into account the large contribution of dark energy to the total energy budget of the universe. Such high methodological standards have not been applied to SETI investigations. The work by Walters et al. should have permanently altered the understanding and citing of the Drake equation; the fact that it was largely ignored reflects the immaturity of this particular research field.

Even without the specifically astrobiological arguments discussed above, the tendency by both SETI proponents and opponents alike to portray the Drake equation as a cornerstone of such research should give pause for thought. After all, in other grand controversies in the history of science a particular aspect of a theory has been invoked by either critics (as a weakness) or defenders (as a strength), but not by both alike. Consider, for instance, epicycles in the old geocentric cosmology of Ptolemy: they were introduced to "save the phenomena" (a phrase that was also used by Copernicus in his apologetic dedicatory letter to Pope Paul III when publishing in 1543 De Revolutionibus Orbium Coelestium, the deeply heretical nature of which Copernicus was well aware of), and no rhetorical manoeuvring could make them an asset to the theory. The opponents, Copernicans such as Galileo or Kepler, claimed that the reliance on excessive epicycles was one of the major weaknesses of the old theory, while supporters such as Clavius or Riccioli at best muttered something along the line that "they enable us to make accurate predictions". It would be a strange situation indeed if geocentrists were to emphasize epicycles as one of their achievements!

It is therefore high time for the ritual invocation of the Drake equation to cease. Exceptions are, of course, those innovative works that use the equation effectively as a provocation in order to reach deeper theoretical insights. While the SETI community may not be able to prevent abuses of history and its Whiggish reinterpretation, it can at least start to establish a more serious and strict theoretical framework for its research. Since the Drake equation serves as a general rule of thumb that should be derivable from any such theoretical framework through a chain of approximations, integrations, and averaging, it should not be promoted as anything more than that. We should instead strive to achieve a deeper understanding of the theoretical underpinnings of SETI programs through greater precision, more numerical models, more simulations, more quantifications of specific scenarios, and so on. Fortunately, the winds finally seem to be changing in this area.

Faking the golden age

Nostalgia for simpler and happier times is the leitmotif of many great artistic and scientific works. The SETI community has indulged in it to a very large – and perhaps unhealthy – extent, especially since the onset of the SETI winter following the cancellation of NASA’s High Resolution Microwave Survey in 1993. The perceived "golden age" of the founding fathers of the entire SETI discipline spanned roughly the years from 1959 to 1971, from the pioneering paper by Cocconi and Morrison to the Byurakan Soviet-American SETI conference. The central idea that we should use radio waves to search for our peer civilizations (especially at those frequencies associated with water molecules), and even attempt two-way communication with them, seemed entirely natural and reasonable at the time.

The 26 m Howard E. Tatel Radio Telescope at Green Bank, West Virginia, used by Frank Drake and collaborators in the Ozma Project, the first ever systematic SETI project ( Image credit: Z22, Wikimedia Commons ).

As always with such misconceptions, there is an archetypal kernel of truth buried under all those nostalgic sentiments: the first steps are always the most difficult. The original Ozma Project was certainly much more heroic than anything that has followed, especially when compared with using the robotic equipment currently employed - in particular automated sky surveys such as those used in the Sloan Digital Sky Survey. There was also an atmosphere of truly sincere international cooperation, especially between the USA and the USSR, in stark contrast to the Cold War suspicion and paranoia that was prevalent at the time. The famous Shklovsky-Sagan joint 1966 book Intelligent Life in the Universe, with its comprehensive covering of the subject and air of cheerful cosmic optimism, was rightly regarded as symbolic of the era. It is hard not to feel a touch of nostalgia for such a simple vision.

It is of course easy in retrospect to list all the problems and deficiencies of the early SETI approaches. There was no understanding of the distribution of planets and planetary systems within our galaxy, and hence the now unjustifiable insistence on exclusively "sun-like" stars. We now know, for instance, that most planets exist around red dwarf stars, which are themselves the most numerous type of star in the Milky Way. We also know that gas giants can possess at least marginally habitable satellite worlds, far removed from the conventional circumstellar habitable zone and analogous to Jupiter's Europa, or Saturn's Titan and Enceladus. No less importantly, we now know far more about one habitable planet – our Earth – than we did in the 1960s and that knowledge actually provides the most important clues to all of our SETI dilemmas. The emergence of what is known as Earth System Studies, which reveal a complex ecological interconnectedness of many seemingly unrelated components of our planetary environment, testifies to an entirely new level of understanding of not only baseline habitability, but also the necessary conditions for the evolution and diversification of life. Awareness of climate change and other complex anthropogenic risks has also increased over the last quarter of a century, which cannot but moderate the immense early optimism of the founding fathers with respect to the general prospect of discovering advanced technological civilisations in the universe.

Finally, a point that would be obvious in almost any other context has, in the SETI context, become hidden by subsequent developments: It is not down to the founding fathers that the original SETI precepts have all too often been adopted dogmatically, but to the subsequent generation(s) of SETI researchers. To borrow an expression from Steven Jay Gould in his description of the history of the Modern Synthesis in evolutionary biology, the orthodox SETI philosophy "hardened" during the period from about 1971 to 2010. Things that were deemed "too speculative" or "outlandish" (such as searches for alien astroengineering artefacts, or von Neumann probes, or exhaust from interstellar engines) have been ignored, or even banned completely from orthodox discussions.

New horizons of astrobiology

Why should one bother, though? Do we really need some complex mathematical theoretical framework in order to search for extraterrestrials? The answer is, of course, in the affirmative. There is no real science without a theoretical grounding. As Darwin famously wrote in 1861: "How odd it is that anyone should not see that all observation must be for or against some view if it is to be of any service!" What Darwin called "a view" corresponds to what we would today call a hypothesis, put forward on the basis of one or another scientific theory.

It is rather intuitive and obvious that we need a theoretical framework in fields such as astrobiology and in the search for extraterrestrial intelligent life. Such a theoretical framework will come with specific predictions of what we can expect to find, and consequently, the most efficient way to search for it. This is a simple point that serious philosophers have known for a long time and which goes by all kinds of high-brow titles, such as the Duhem-Quine thesis (that you cannot seek something without having a prior theoretical reason for doing so). The time for "spontaneous" empirical work, such as simply going into a forest to study trees, or placing a telescope on a hilltop in the hope that you will discover something new (effectively by pure chance), is long gone, and we should be happy about that! Although people still occasionally make discoveries by chance, these need to be kept in perspective: such serendipitous discoveries are perhaps one in a thousand, or less. Most things in science are achieved through careful theoretical predictions and planning; SETI is not, and cannot be, an exception in this regard.

Fortunately though, we appear to be living in a time of sea-change, in which the conservative facade is crumbling ever more rapidly. The astrobiological revolution started with the discovery of a large number of extrasolar planets since 1995, and we now have less and less reason to believe that our own solar system and planet Earth are exceptional within a cosmic context. Prospects for the discovery of simple, microbial forms of life now appear to have dramatically increased, not only on Mars (which was, we now know for sure, much more habitable in the past), but also on such astrobiologically interesting bodies as Europa, Titan, and Enceladus. The increasing interest in extremophiles living in hardy corners of Earth’s biosphere, from deep within the crust to many miles above the stratosphere, on the very border of outer space, has also been an important factor in our realization that life itself is probably a much less rare, exceptional, or fragile occurrence than has hitherto been assumed. Despite having become so conservative and hardened, the SETI sector could not entirely escape all of this buzz and fury of astrobiological activity.

There has therefore been a noticeable resurgence of interest in SETI-related topics since about 2010. This has encompassed many different aspects of SETI and included the re-discovery of ideas proposed long ago. Perhaps the most interesting is the revival of the ideas of Freeman Dyson concerning the infrared detectability of astroengineering constructions such as the eponymous spheres, or large-scale habitats, or stellar engineering signposts. These ideas, sometimes dubbed the Dysonian SETI, have re-emerged from many different directions and points of view, as a reaction against the conservative orthodoxy. Perhaps the most influential consequence has been the inauguration of the G-HAT project (an acronym for "Glimpsing Heat from Alien Technology", sometimes denoted by the symbol Ĝ) in about 2014, under the leadership of Jason Wright of Penn State University and his collaborators. The project has analysed about 100,000 galaxies detected in an infrared survey of the sky by the WISE satellite, searching for any that produce a strong mid-infrared signature. While about 50 candidates have been identified for follow-up investigations, no clear and obvious signpost of alien technology has been found. This should not worry us unduly, since we are just at the very beginning of these new horizons investigations. What we should note is that searches such as that conducted by the G-HAT project are necessarily concentrated on the most advanced extraterrestrial civilizations, i.e. those that are high on Kardashev’s scale, and there cannot (almost by definition) be very many of them. In other words, they completely by-pass the issue of "large N" in the Drake equation. This does not mean that the conventional radio SETI is no longer continued; it is, and in a number of different forms such as, for example, searches conducted by the SETI Institute using the Allen Telescope Array .

( Left ) Freeman Dyson, a pioneer of a new and original approach to SETI through astroengineering, the first and best example of which has been the eponymous Dyson sphere, as conceived in 1960. ( Right ) Artistic illustration of a Dyson sphere. A system of radiation collectors assembled around a star would alter its light emission; specifically, it would most likely cause an increase in the amount of infrared radiation in the star's emitted spectrum, prompting the search for atypical infrared signatures in stellar emission spectra ( Image credit: Monroem, Wikimedia Commons [left] and Droneandy, Shutterstock [right] ).

It is not easy to say how much of the universe has already been covered by SETI projects , since this obviously depends on an unknown number of parameters describing the detectability of civilizations in general. In any case, on an overall cosmic (or even just galactic) scale the proportion covered is extremely small. In our local stellar neighbourhood we can do better and exclude the existence of strong radio beacons within about 30 parsecs from the sun, and any obvious astroengineering features (such as Dyson spheres) out to perhaps ten times that distance. Of course as we ascend along Kardashev’s scale the constraints on existing or former civilizations become greater; perhaps the most interesting qualitative conclusion of this kind is the obvious absence of a Kardashev Type 3 civilization within the Milky Way, in spite of there having been ample time for one to develop, at least in theory.

So, where does all of this leave us? We are now starting to consider questions that have not been taken into consideration at all in the Drake equation and its exegeses, not thereby rejecting it but rather transcending it. Some of these questions are exactly those posed by alternative approaches such as the Dysonian SETI. Other questions deal with the influence of postbiological evolution, artificial intelligence, or virtual reality on SETI studies. Still bolder researchers proclaim the essential interconnectedness between Futures studies and SETI, these being local and global aspects of the same philosophical concerns about the existence of advanced forms of intelligence in the universe. All of these issues, as well as others, are largely orthogonal to concerns of the orthodox SETI community and the adherents of the fake "golden age"; the issues that orthodoxy, by definition, cannot address (matters such as macro/astroengineering, postbiological evolution, technological singularity, etc.). Addressing these issues is not an option, this is not a luxury: this is now mandatory in order to maintain the intellectual honesty and scientific status of SETI projects.

Artistic representation of a Bernal’s sphere, a possibly detectable astroengineering feat ( Image credit: Rick Guidice, NASA Ames Research Center ).

The context of the SETI dilemma has changed dramatically in recent years. There is no going back to the simple times of the founding fathers when contact seemed to be just around the corner, and there is no longer any excuse for "guesstimates" of the Drake equation parameters. Instead, a whole wonderful new vista of astrobiology has opened up and we have the grounds and opportunities for serious quantitative analysis of all issues relevant to extraterrestrial life and intelligence. We need to arm ourselves with serious mathematics and numerics, as well as with a great deal of patience. We now have a true renaissance of interest in searches for extraterrestrial intelligent life; whether it will last long enough to provide us with the results that we all expect depends crucially on the outstanding problem that has been outlined just roughly herein: that of establishing a true SETI theory.

Welcome to the real adventure!





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