Beyond the SETI Paradigm
In my Centauri Dreams post Stagnant Supercivilizations and Interstellar Travel I described what I call the “SETI paradigm,” as follows:
“…this cluster of assumptions that I call the SETI paradigm — that interstellar travel is difficult or impossible, that communication is easy, and therefore SETI and METI are, or ought to be, the focus of the efforts of advanced civilizations to interact with peers — hang together by mutual implication. If we reject any one aspect of the paradigm, it falls apart.”
According to the SETI paradigm (which is not the view of everyone involved in SETI research, but which is nevertheless a common attitude within SETI), the most common technosignature we ought to find when we look out into the universe would be EM spectrum transmissions (primarily radio or light) of artificial origin. Carl Sagan and others imagined an Encyclopedia Galactica shared among civilizations communicating over interstellar distances by radio signals.
It must be acknowledged that this is an expansive vision of civilization in the universe, and it is a vision accomplished without interstellar travel, in accord with the SETI paradigm. It is rewarding to imagine that such an expansive vision of civilizational interaction is possible even in a universe in which interstellar travel is difficult or impossible.
I have often argued that, if we lived in a densely populated universe (i.e., a universe densely populated by technological civilizations, which is the kind of universe in which an Encyclopedia Galactica would be possible), the first time a radio telescope was switched on, it would have been barraged by a superfluity of SETI signals. It would have taken time to understand the cacophony of signals from other worlds, but we would have eventually sorted them out and begun to understand what it was upon which we unexpectedly found ourselves eavesdropping.
If civilizations are common in the universe, we might expect there to be one or several civilizations nearby us, within a few dozen lightyears, say, so that we might receive signals only a decade or two old. But even if only one in a hundred civilizations developed electrical technology, or even one in a thousand, and of these only a small fraction decided to broadcast signals, there would still be a welter of signals. Some of these would be relatively easy to isolate and translate, and some of these would probably verge on being untranslatable, but natural variation would produce a wide range of different kinds of civilizations (as there are and have been many different civilizations on Earth), and these different kinds of civilizations would produce a wide range of SETI signals, so we would have our choice of which ones to focus on — sort of like our choice of what artifacts in the archaeological record we wish to focus on.
But while radio astronomy has found a great many interesting things that had eluded prior optical astronomy, and has contributed much to the growth of scientific knowledge, it has not detected any signals of an unambiguously artificial origin. There have been many false alarms, which later were determined to be natural phenomena, but no technological civilizations. Instead of a dizzying variety of signals from which to choose, we have only the eerie silence between the stars.
Even if there are signals yet to be found, we can nevertheless affirm that we do not live in a universe dense with EM spectrum technosignatures, which, by the SETI paradigm, ought to be the most common technosignatures in the universe, if there are any technosignatures to be found at all. But if we abandon the presuppositions of the SETI paradigm, we may have to rethink the presumption of EM spectrum signals as the most common technosignature. What is the most common kind of technological artifact that we ought to expect? What ought we to consider as the most likely, or most obvious, technosignatures that could be detected?
One possibility is suggested by the recent kerfuffle over the paper, “Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration?”
by Shmuel Bialy and Abraham Loeb. One sentence plucked from this paper was the topic of dozens of clickbait articles about aliens. Bialy and Loeb briefly considered the possibility that ‘Oumuamua could have been artificial, and more specifically they focused on the possibility of it being a lightsail. Here is the passage from the paper in which they introduce this idea:
“If radiation pressure is the accelerating force, then ‘Oumuamua represents a new class of thin interstellar material, either produced naturally, through a yet unknown process in the ISM or in proto-planetary disks, or of an artificial origin. Considering an artificial origin, one possibility is that ‘Oumuamua is a lightsail, floating in interstellar space as a debris from an advanced technological equipment. Lightsails with similar dimensions have been designed and constructed by our own civilization, including the IKAROS project and the Starshot Initiative. The lightsail technology might be abundantly used for transportation of cargo between planets or between stars.”
And then they continued to what they explicitly identify as a more “exotic” scenario:
“…a more exotic scenario is that ‘Oumuamua may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization. Based on the PAN-STARRS1 survey characteristics, and assuming natural origins following random trajectories… the interstellar number density of ‘Oumuamua-like objects should be extremely high… larger than predicted by theoretical models… This discrepancy is readily solved if ‘Oumuamua does not follow a random trajectory but is rather a targeted probe.”
This is the passage that stirred up a great deal of commentary. The authors conclude with this reflection:
“A survey for lightsails as technosignatures in the Solar System is warranted, irrespective of whether ‘Oumuamua is one of them.”
Bialy and Loeb were taken to task by many who felt this was an overly speculative passage to insert into a scientific paper. Critics have rightly noted that the artificial origin scenario should be an explanation of last resort, but It is to be noted that the authors couched their language in careful terms, always phrasing an artificial origin for ‘Oumuamua in hypothetical terms, and explicitly identifying the “alien probe” scenario as exotic. We know, however, that the media always cherry picks the juiciest items they can find, and all subtlety is lost. In this case, it really ought to be the mass media that is held to a higher standard, and not the authors of the paper.
Abraham Loeb over his career has published a great many exotic ideas (I regularly post links to his papers), and his papers are a treasure trove of original thinking. The idea of ‘Oumuamua as an alien spacecraft is not especially strange or unusual in the context of Loeb’s work. Moreover, the scientific process is better if scientists can feel free to float marginal and even exotic explanations.
Science is a creative process. Exoticism should be carefully weighed against more mundane explanations, but that doesn’t mean that we exclusively consider the mundane explanation at the expense of the exotic explanations — we simple hold the exotic explanations to a higher standard. As Sagan said, “Extraordinary claims require extraordinary evidence.” (Eric Berger wrote a good piece more-or-less to this effect for Ars Technica: Predictably, online media go nuts over ‘Oumuamua and Harvard scientists “Scientists are perfectly happy to publish an outlandish idea.”)
Suppose we take Bialy and Loeb’s suggestion seriously that, “…lightsail technology might be abundantly used for transportation of cargo between planets or between stars.” Lightsail technology is even now under development for human beings to send an artificial probe to another star, specifically, the Breakthrough Starshot project, and what they call a, “nanocraft concept, combining light beamer, lightsail and StarChip.” One lightsail has been deployed in Earth orbit, and another is in development, though these are only technology demonstrators and are not intended to go beyond Earth orbit.
Nevertheless, lightsails can be considered contemporaneous technology, and other human spacecraft have already left our solar system:
“…now the Voyagers are leaving the protective bubble of our sun and crossing over into the uncharted territory between the stars… we have all entered the Interstellar Age.” (Jim Bell, The Interstellar Age: Inside the Forty-Year Voyager Mission, New York: Dutton, 2015, p. 3)
It was just announced a few days ago that Voyager 2 may have also passed into the interstellar medium and become an interstellar spacecraft.
There is a sense, then, in which human beings already have employed lightsail technology, and have already launched interstellar probes, so that an interstellar lightsail probe is not something speculative, but a technology that can be built today. Thus we can plausibly regard lightsails as an “entry level” interstellar technology that presents the possibility of interstellar travel for peer civilizations to our own. This observation has potentially significant implications for the interaction of civilizations in the universe.
If it is more probable that there are fewer civilizations at a higher level of technological maturity than there are civilizations at a lower level of technological accomplishment, we would expect to see a greater number of civilizations pursuing rudimentary interstellar travel, i.e., a greater number of civilizations employing entry-level interstellar technologies. Thus, if we were to encounter an interstellar probe from another civilization, the likelihood would be that it would be a rudimentary technology like a light sail, as a greater number of civilizations would be likely to achieve this level of technological development, as compared to some more advanced level of technological development.
A more sophisticated artificial interstellar probe would require technological resources not yet available to us or to a peer civilization. Thus if the Great Filter lies in the near future, the galaxy could be dense with peer civilizations, but nothing more than that. Of course, we don’t yet know if the Great Filter is in the past or the future, or if there is a single Great Filter or not. Nevertheless, a near-future Great Filter is a plausible hypothesis that we can use to formulate a scientific research program addressing the Fermi paradox.
A universe in which many peer civilizations sent out rudimentary lightsail craft for exploration would be a universe in which we would be more likely to detect a lightsail technosignature than any other kind of technosignature. This is at least as plausible as SETI, which requires present or near-term technology to project a detectable SETI beacon over interstellar distances. SETI technology and lightsail technology may be considered to be nearly on a par, so that one civilization might develop the one, and another might develop the other, and still be peer civilizations.
My remarks on the SETI paradigm may give the reader the impression that I am critical of SETI or that I consider it a waste of time. This is not the case. On the contrary, I would like to see SETI funded and vigorously pursued because a single unambiguous technosignature detection would instantly falsify the thesis of humanity’s cosmic isolation. This is low-likelihood/high payoff form of science — risky science, if you will. In the same way, exotic explanations for astronomical phenomena constitute risky science, and we are epistemically better off for always being on the verge of Kuhnian revolutionary science rather than exclusively confining ourselves to the normal science defined by mundane explanations.
Though I strongly support SETI research, and I regard SETI as among the most interesting sciences (and certainly among the sciences that have influenced me the most), I do not expect SETI to be successful. The idea that we live in a densely populated universe in which civilizations share an Encyclopedia Galactica among themselves by radio or lasers is, fortunately, a testable and falsifiable hypothesis. I regard SETI research as engaging in the incremental falsification of this hypothesis: it cannot be entirely dismissed as of yet, but we can cut it down by process of elimination.
On the other hand, the technosignatures of lightsails, even if they are quite common in the universe, are likely to be very subtle and difficult to detect. It would be a beautiful and compelling response to the Fermi paradox if diaphanous lightsails filled our galaxy like dragonflies and butterflies flitting around a meadow at dusk, and we are only now on the cusp of developing the means to catch them in our net. I do not regard this as being in the least bit likely, much less demonstrated by argument or evidence, but it is an idea worth considering, and it also poses further questions about what other rudimentary technosignatures might betray civilizations that do not broadcast their existence to the wider universe.