On the Probability, Semiotics, and Politics of Detecting Extraterrestrial Intelligence
I. The Universal Language That Isn’t
There is a story we tell ourselves about first contact, and it goes like this: mathematics is the universal language. Any sufficiently advanced civilisation, regardless of its biology, its sensory apparatus, its evolutionary history, will have discovered the same primes, the same geometric constants, the same physical relationships. If we want to speak to the stars, we encode mathematics, because mathematics is what we share. It is the lingua franca of the cosmos, the one thing that is true everywhere and independent of the speaker.
This is a cultural claim dressed as a scientific one. It is, more precisely, a semiotic claim, a claim about the relationship between signs, meaning, and minds, and it has rarely been examined as one.
The discipline that would examine it is semiotics: the study of how meaning is made, transmitted, and received. Semiotics, in the tradition of Charles Sanders Peirce, holds that a sign requires three things, a sign vehicle (the physical form of the sign, the signal itself), an object (the thing the sign refers to), and an interpretant (the meaning constructed by the receiver). A sign is not a sign in itself; it is a sign only in the act of interpretation. Meaning does not reside in the signal. It is constructed, by the receiver, from the signal, using the only tool available: context.
This matters more than it might seem. When we say that mathematics is universal, we are making a claim about the sign vehicle, that the same mathematical structures will be independently discovered by any intelligence. This is plausible, though not as certain as we like to think; even mathematics, as a human practice, is shaped by culture, by language, by the contingent history of the species that produced it. The ancient Greeks did geometry differently from the medieval Islamic mathematicians, who did it differently from the calculus tradition that emerged in Europe. These are not different mathematics, the relationships are the same, but they are different semiotic systems, different ways of encoding and interpreting the same underlying structures. If mathematics were truly transparent, if the sign vehicle mapped unambiguously to the object, then mathematical notation would not have a history. It does.
But the deeper problem is not at the level of the sign vehicle. It is at the level of the interpretant. Even if an extraterrestrial intelligence has discovered the same mathematical relationships, the meaning they construct from a signal encoding those relationships depends on their context, their biology, their sensory world, their temporal experience, their cognitive architecture. A signal that encodes prime numbers, for us, means “intelligence, because primes are a non-natural pattern that requires a mind to generate.” This interpretation depends on a chain of assumptions: that the receiver recognises primality as a non-natural pattern; that they associate non-natural patterns with intelligence; that they associate intelligence with communication; that they associate communication with the intent to be understood. Each of these is a cultural inference, grounded in the specific semiotic world of a species that evolved on a particular planet with a particular evolutionary history.
The Estonian biologist Jakob von Uexküll coined the term Umwelt to describe the perceptual world inhabited by an organism, the specific slice of reality available to it through its particular sensory and cognitive apparatus. A tick’s Umwelt consists of temperature, light, and the smell of butyric acid. A bat’s Umwelt is built from echolocation. A human’s Umwelt is built from vision, hearing, and a peculiar cognitive architecture that is, as far as we know, unique: the capacity for recursive symbolic thought, which has produced language, mathematics, and the stories we tell about the stars.
The assumption underlying SETI, the Search for Extraterrestrial Intelligence, is that the Umwelt of a technological alien species would overlap sufficiently with ours that a signal meaningful in one would be meaningful in the other. This is an assumption, not a fact. We have no evidence for it, because we have no data. We have one data point: ourselves. And the one data point we have is not a sample; it is a case study, and a case study of one tells you nothing about the distribution of the population from which it is drawn.
II. The Imperative of Life
There is a second assumption, deeper and less examined than the first, that underlies the entire enterprise. It is the assumption that life, once it arises, tends toward intelligence, and that intelligence, once it arises, tends toward communication, and that communication, once it is possible, tends toward the stars. This is a narrative of progress, a kind of cosmic teleology, and it is, in its structure, remarkably similar to the Great Chain of Being, the medieval concept that all of creation is arranged in a hierarchy from the lowest to the highest, with humanity (naturally) near the top. We have secularised the Great Chain. We no longer place God at the top. We place technological civilisation there instead, and we assume that the universe, if it contains life at all, will produce civilisations like ours, because ours is the natural endpoint of the chain.
This is not what the observable facts suggest.
The observable facts are these. Life on Earth arose approximately 3.8 billion years ago, possibly earlier. For the first 2.5 billion years of that history, more than half the planet’s lifetime, life consisted of single-celled organisms. Multicellularity arose perhaps 600 million years ago. Complex multicellular life, with differentiated tissues and organs, arose perhaps 550 million years ago. The first animals appeared in the Cambrian, roughly 540 million years ago. From the Cambrian to the present, life on Earth has experienced at least five major mass extinction events, each of which reset the trajectory of evolution and eliminated a significant fraction of existing species.
Intelligence, in the sense relevant to SETI, which is to say, intelligence capable of building technology capable of transmitting or receiving interstellar signals, has arisen exactly once in 3.8 billion years. Not twice. Not in multiple lineages. Once. In a single species, Homo sapiens, which has existed for approximately 300,000 years and has been technologically capable of radio transmission for approximately 120 years.
Let us sit with those numbers. 3.8 billion years of life. One instance of the kind of intelligence we are searching for. 120 years of the kind of technology we are searching with. The fraction of life’s history on this planet during which it has been detectable by the methods SETI uses is approximately:
120 / 3,800,000,000 ≈ 3.2 × 10⁻⁸
Three ten-millionths of the time life has existed. And that is the fraction for our own planet, where we know life exists. For any other planet, we do not even know whether life exists, let alone intelligence, let alone technology.
This is not an argument that life is rare. It is an argument that the specific thing SETI is searching for, a technological civilisation, producing detectable electromagnetic signals, during the narrow window in which we are listening, is, on the evidence available to us, vanishingly improbable. Not impossible. Not absent. But the probability is not a number we can calculate, because we have one data point, and one data point cannot populate a distribution.
What we can say is this: the assumption that intelligence is convergent, that it arises independently in many lineages the way that eyes have evolved independently dozens of times, is not supported by the evidence. On Earth, we have many examples of social complexity (eusocial insects, cetaceans, elephants, corvids, cephalopods), many examples of tool use (chimpanzees, crows, otters, dolphins), many examples of cognitive sophistication (parrots, octopuses, pigs, dogs). We have exactly one example of technology-building intelligence. Eyes have evolved independently because light is a physical constant and the selective advantage of detecting it is overwhelming. Technology-building intelligence is not eyes. It is a specific, contingent, historically bounded phenomenon that arose in a specific lineage under specific conditions, and the evidence that it is a convergent adaptation, that it arises reliably, like vision, wherever the conditions permit, is, at present, zero.
This does not mean it is rare. It means we do not know. And the not-knowing is the honest position, and the not-knowing is the position that the SETI industry, as a matter of institutional logic, cannot afford to occupy.
III. The Signal Problem, What We Are Actually Looking For
Setting aside the semiotic and philosophical questions, let us consider the physics. What would it actually take to detect an extraterrestrial signal?
The problem has three components: attenuation, noise, and coincidence.
Attenuation
Electromagnetic radiation propagates according to the inverse square law. A signal that leaves a transmitter with power P will arrive at a distance d with power density P / (4πd²). At interstellar distances, this is a brutal fact. A signal transmitted from a star 6,000 light-years away, roughly the distance to the Cygnus arm, a reasonable estimate for the scale of any plausible target, arrives at Earth with a power density that is a factor of roughly 10⁻¹⁵ of its transmitted value. To detect a signal at that distance, the transmitter must either emit with enormous power or beam the signal tightly in our direction.
A tight beam is energy-efficient but covers a tiny fraction of the sky. The beamwidth of a transmitting antenna is inversely proportional to the ratio of the antenna’s diameter to the wavelength. To beam a signal at 1.42 GHz (the hydrogen line, a popular SETI frequency) to a target the size of the Earth’s orbit from a transmitter 6,000 light-years away requires an antenna with an effective diameter of approximately 500,000 kilometres, larger than any structure any civilisation we can imagine could build, unless it is distributed across a solar system. And even then, the beam would only reach the Earth if the transmitter knew exactly where the Earth was and was pointed at it with precision.
The alternative is an omnidirectional beacon, a signal broadcast in all directions, detectable from any point in the sky. But an omnidirectional signal at interstellar distances requires power that beggars the imagination. A signal detectable by the Arecibo telescope (when it existed) at 1,000 light-years, broadcast omnidirectionally, would require a transmitter power of approximately 10¹⁵ watts, roughly 100 times the total current electrical generating capacity of human civilisation. At 6,000 light-years, the requirement scales by a factor of 36, to roughly 3.6 × 10¹⁶ watts, a significant fraction of the total solar output intercepted by the Earth.
This is not impossible. A civilisation that has access to the energy of a star, a Kardashev Type II civilisation, which is the kind that science fiction routinely assumes, could, in principle, broadcast omnidirectionally at interstellar distances. But we are now no longer doing science. We are doing speculation, and the speculation is unconstrained by any observation, which means it is, in Karl Popper’s sense, unfalsifiable, which means it is not, strictly speaking, science at all.
The attenuation problem means that the vast majority of plausible transmission scenarios are undetectable. A civilisation that is broadcasting toward us, with a beam pointed at our solar system, using a power level we could detect, at a frequency we are monitoring, at a time we are listening, is a civilisation that is, by construction, very similar to us in its assumptions about how communication works. We are, in effect, searching for ourselves.
Noise
The universe is loud. The cosmic microwave background provides a floor of approximately 2.7 K of thermal noise at all frequencies. Galactic synchrotron radiation adds frequency-dependent noise that is particularly strong at lower frequencies. Earth’s atmosphere emits thermal noise. The receiver itself contributes thermal noise, quantified by its system temperature. And, increasingly, our own civilisation contributes radio frequency interference, satellites, radar, broadcast transmitters, industrial equipment, that contaminates the very frequencies we are searching for.
The signal-to-noise ratio determines detectability. A signal is detectable only if it is significantly stronger than the noise floor integrated over the observation time. For a narrowband signal, the kind SETI traditionally searches for, because a narrowband signal is an indicator of engineering, of a transmitter that has been deliberately designed, the noise in a single frequency bin is proportional to the system temperature divided by the integration time. Longer observations reduce noise, but longer observations cost more telescope time, and telescope time is the scarcest resource in the SETI enterprise.
The “water hole”, the band of frequencies between the hydrogen line (1.42 GHz) and the hydroxyl line (1.66 GHz), is relatively quiet, because galactic background noise decreases with frequency and the atmosphere is transparent in this range. The choice of the water hole is, however, a semiotic choice: it assumes that an alien civilisation would choose to broadcast in this band for the same reasons we would choose to listen in it. The reasoning is that hydrogen and hydroxyl are the components of water, and water is the solvent of life (as we know it), and therefore the band between them is a “cosmic meeting place”, a frequency band that any water-based life would find significant.
This is a lovely idea. It is also, in its structure, an argument from analogy: we find the hydrogen line significant because hydrogen is the most abundant element in the universe and its spectral line is a natural reference frequency. We assume that an alien intelligence would find it significant for the same reasons. But the significance is not in the frequency; it is in the interpretation. And the interpretation depends on the interpretant, which depends on the Umwelt, which depends on the species.
Temporal Coincidence
A civilisation must be transmitting during the window in which another civilisation is listening. We have been listening, in one form or another, for approximately 60 years. If technological civilisations have lifetimes of, say, 10,000 years (a number chosen for illustrative purposes, because we have no basis for choosing any number), and if they arise at some rate per galaxy per unit time (a rate we cannot estimate because we have one data point), then the probability of temporal overlap depends on the ratio of the combined listening-and-transmitting window to the total time available. If both civilisations are rare and short-lived, the probability of overlap is small. If both are common and long-lived, it is large. We do not know which, and the range of plausible values spans many orders of magnitude.
The one observable fact we have is that we have been listening for 60 years and have heard nothing. This is a null result, and null results in science are informative but notoriously difficult to interpret. A null result can mean the phenomenon does not exist. It can mean the phenomenon exists but is below the detection threshold. It can mean the phenomenon exists and is detectable but we are looking in the wrong place, at the wrong frequency, at the wrong time, or with the wrong assumptions. The history of science is rich with null results that were, in retrospect, failures of imagination rather than failures of the phenomenon, the Michelson-Morley experiment, the early null results in the search for exoplanets, the decades of null results in gravitational-wave detection before LIGO.
But the history of science is also rich with null results that were, in retrospect, correct: the search for the ether, the search for Vulcan, the search for N rays. The difference between a null result that precedes a detection and a null result that confirms an absence is visible only in retrospect, and we are not in retrospect. We are in the present, and the present is ambiguous.
IV. The Semiotic Trap
Here is the trap, and it is a semiotic one: we cannot search for a signal without a model of what a signal looks like. And our model of what a signal looks like is, inescapably, a model of what our signal would look like. We search for narrowband carriers because we build narrowband carriers. We search in the water hole because we find the water hole significant. We search for prime numbers because we see primes as a signature of intelligence. We search for structured, repetitive, obviously non-natural patterns because structured, repetitive, obviously non-natural patterns are what we produce when we want to be noticed.
The logic is circular, and the circularity is not a flaw that can be engineered away. It is structural. To search is to assume. To assume is to project. To project is to search for yourself.
This is not an argument against searching. It is an argument for understanding what searching means, and for being honest about the fact that a null result in a search conditioned on a model that is derived from a sample of one tells you only that the model’s specific predictions are not confirmed. It does not tell you that the phenomenon is absent. It does not tell you that the model is wrong. It tells you nothing, in the strict statistical sense, because the prior is unconstrained and the likelihood is dominated by assumptions that are not testable.
The semiotic literature has a term for the specific kind of error this produces: abduction, as defined by Peirce, is the process of inferring the best explanation for an observation. SETI is not, in practice, abductive; it is hypothetical-deductive. We hypothesise a model of alien communication, deduce what the signal would look like, and search for it. When we do not find it, we do not revise the hypothesis, we refine the search. We build more sensitive receivers. We search more frequencies. We observe more stars. The hypothesis, that a technological civilisation would produce a signal recognisable to us, is never tested, because it cannot be tested. It is the precondition of the search, not a result of it.
The Italian semiotician Umberto Eco, in his work on the limits of interpretation, argued that a text can be interpreted in an unbounded number of ways, but not all interpretations are equally valid. The constraint on interpretation is the text itself, the sign vehicle, but also the community of interpreters, the shared context that makes some readings plausible and others not. For SETI, the “text” is the signal (if one exists), and the “community of interpreters” is the scientific community of Earth. The constraint on our interpretation is our own semiotic world, our physics, our mathematics, our engineering, our cultural assumptions about what communication is and why beings communicate. We cannot step outside this constraint. We can only be aware of it.
V. The One Data Point
Let us return to the observable facts, because they are all we have, and they deserve more respect than they typically receive.
We know that life exists on at least one planet. We know that, on that planet, life has produced one technological civilisation. We know that that civilisation has been capable of radio transmission for approximately 120 years. We know that, during those 120 years, the civilisation has not deliberately broadcast a continuous, high-power, omnidirectional signal intended for reception by other stars. (We have sent short, targeted messages, the Arecibo message in 1974, the more recent Breakthrough Message competition, but these are not continuous beacons and would not be detectable by a civilisation like ours at interstellar distances.)
This last fact is, in the context of SETI, almost never discussed. If we are the model for what a technological civilisation does, then the model predicts that a technological civilisation does not broadcast. It listens. It searches. It does not transmit, or it transmits only sporadically and with low power, in short bursts, toward specific targets.
The implication is uncomfortable. If the one civilisation we know does not behave the way SETI assumes civilisations behave, i.e., continuously broadcasting detectable signals, then the search is predicated on a model that is contradicted by the only available data. We are searching for a behaviour that we ourselves do not exhibit.
There are responses to this. The most common is that we are young, technologically, and that older civilisations would have moved beyond the listening phase into the broadcasting phase. This is plausible. It is also unfalsifiable, because we have no older civilisation to observe. Another response is that we are, in fact, broadcasting, our radio, television, and radar emissions leak into space continuously, and have been doing so for a century. This is true, but the power of these leaked signals at interstellar distances is far below the detection threshold of any plausible receiver. A civilisation like ours, at 1,000 light-years, would not be detectable by our own instruments. The leakage is real but too faint to be the signal we are searching for.
The third response is the one that the semiotic analysis suggests: that we are not broadcasting because broadcasting, as a strategy for interstellar communication, does not make sense. The energy costs are prohibitive. The probability of being heard is unknown. The temporal coincidence required is, on any reasonable estimate, small. And the assumption that another civilisation would be listening, in the same frequency band, with the same model of what a signal looks like, during the same century, is a tower of assumptions, each individually uncertain, that collectively approaches the kind of improbability that, in any other scientific context, would be treated as fantasy.
VI. The SETI Industry
SETI has existed, as a formal enterprise, since 1960, when Frank Drake conducted Project Ozma, the first systematic radio search for extraterrestrial signals, using the 26-metre radio telescope at Green Bank, West Virginia. Drake listened to two stars, Tau Ceti and Epsilon Eridani, for a total of approximately 150 hours, at a single frequency (1.42 GHz), and heard nothing. The search has grown since then, in telescope size, in frequency coverage, in target number, in computational sophistication. It has not, in sixty years, found anything.
This is not a failure, or not only a failure. It is a fact, and the fact has a context: the search space is vast, and the fraction of it we have explored is small. Jill Tarter, the longtime director of the Center for SETI Research, compared the explored volume of SETI’s search space to a glass of water taken from the ocean. If you take one glass of water from the ocean and find no fish, you have not proven that the ocean contains no fish. You have proven that this glass of water contains no fish. The ocean is large.
The metaphor is apt, and it is also, in its way, a defence of the enterprise against its own null results. The ocean is large. We have sampled a glass. The absence of fish in the glass is not evidence of the absence of fish in the ocean. This is true. It is also, in a practical sense, the kind of argument that can be used to justify indefinite searching, because the ocean is always larger than the glass, and the glass can always be refilled, and the fish can always be somewhere else.
The question is not whether the search should continue. The question is what the search costs, and what it produces, and whether the cost and the production are in a relationship that is sustainable or honest.
Funding
SETI has historically been funded by a mix of NASA, the National Science Foundation, and private sources. NASA’s funding for SETI was terminated by Congress in 1993, largely through the efforts of Senator Richard Bryan of Nevada, who declared that the search had found nothing and was a waste of taxpayer money. The field survived on private funding, the SETI Institute, founded in 1984, built the Allen Telescope Array with support from Paul Allen; the Breakthrough Listen initiative, launched in 2015 with $100 million from Yuri Milner, is the largest and best-funded SETI project in history, and it has access to some of the world’s most powerful radio telescopes, including the Green Bank Telescope in West Virginia and the Parkes Telescope in Australia.
One hundred million dollars is, by the standards of scientific funding, not a great deal of money. The James Webb Space Telescope cost approximately $10 billion. The Large Hadron Collider cost approximately $9 billion. Breakthrough Listen, spread over ten years, costs $10 million per year. In the context of global scientific expenditure, this is a rounding error.
But the comparison cuts both ways. If $100 million is a rounding error, then it is also $100 million that is not being spent on other things. The opportunity cost is not theoretical, it is the science that could have been done with the telescopes, the computing power, and the human capital that Breakthrough Listen has absorbed. The Green Bank Telescope, which spends a portion of its observing time on SETI, is a general-purpose radio telescope that could be used for pulsar timing, molecular spectroscopy, galaxy surveys, and the study of the interstellar medium. The time it spends listening for aliens is time it is not spending on those things. The computing infrastructure that processes SETI data, and SETI generates enormous volumes of data, requiring significant processing, could be processing data from other astronomical observations.
The question is whether the expected return on SETI justifies the opportunity cost. The expected return is the probability of a detection multiplied by the value of a detection. The value of a detection is, by any measure, immense, a confirmed signal from an extraterrestrial intelligence would be the most significant scientific discovery in human history, and its implications would extend far beyond science into philosophy, religion, politics, and culture. But the probability is, as we have established, not a number we can estimate. It is, at best, a range so wide that the expectation value is not well-defined. If the probability is 10⁻²⁰, the expected return is negligible. If the probability is 10⁻², the expected return is enormous. We do not know where, in that range, the truth lies, and the range is so wide that multiplying it by any value produces a result that can be used to justify either continuing or stopping, depending on which end of the range you prefer.
This is not a scientific question. It is a question about the allocation of resources in the face of radical uncertainty, and it is a question that the SETI industry, as an institution, has a structural incentive to answer in one direction.
The Industry
SETI is, at this point, an industry. It has institutions (the SETI Institute, the Berkeley SETI Research Center, the International Centre for Radio Astronomy Research). It has a workforce, scientists, engineers, software developers, administrators. It has a public profile, maintained through documentaries, popular books, conference talks, and the occasional viral news story about a “candidate signal” that turns out to be terrestrial interference or a known astrophysical phenomenon. It has a narrative, and the narrative is compelling: we are searching, the search is hard, the universe is vast, and the answer, if it comes, will change everything.
Industries do not, as a rule, argue for their own dissolution. The SETI industry is no exception. The null results of the last sixty years have been interpreted not as evidence to revise the fundamental assumptions of the search but as evidence to expand it: more telescopes, more frequencies, more stars, more sensitivity, more computing power. The logic is that the search space is large and we have explored a small fraction of it, and therefore the appropriate response to null results is to search more, not to question whether the thing we are searching for exists in the form we are searching for.
This is not unreasonable. It is also not science, in the strict sense, because the core hypothesis, that a technological civilisation would produce a signal recognisable to us, is not falsifiable by the search. If we search the entire sky, at all frequencies, for a century, and find nothing, the response can always be: the signal is there, but we are not looking at the right time, or the right modulation, or the right encoding, or with the right model. The hypothesis adapts to survive the evidence, and a hypothesis that adapts to survive all evidence is not a scientific hypothesis. It is an article of faith.
There is a comparison to be made with particle physics. The search for the Higgs boson was, for decades, a search for a predicted but unobserved phenomenon. The difference is that the Higgs was predicted by a theory, the Standard Model, that made other, testable predictions, and the search for the Higgs was constrained by those predictions. If the Higgs had not been found in the predicted mass range, the Standard Model would have been falsified, and the search would have had a defined endpoint. SETI has no equivalent. There is no theory that predicts, with specific parameters, what a signal from an alien civilisation would look like, at what frequency, with what modulation, from what direction. There are conjectures, the water hole, the hydrogen line, the beacon hypothesis, but they are not predictions in the scientific sense, because they are not derived from a testable theory. They are arguments from analogy and from aesthetics, and arguments from analogy and aesthetics, however appealing, are not falsifiable.
The Effect on Science
The more subtle cost of SETI is not the money or the telescope time. It is the effect on the scientific culture, specifically, on the culture of astrobiology, the broader field that studies the origin, evolution, and distribution of life in the universe.
Astrobiology is a legitimate and thriving science. It studies the chemistry of life’s origins, the conditions under which life can arise, the limits of life as we know it (extremophiles, subsurface biospheres, alternative biochemistries), and the detectability of life on other planets through biosignatures, atmospheric, geological, spectral. It is a field constrained by data: we can study extremophiles in the lab, we can analyse the atmospheres of exoplanets with telescopes like JWST, we can model the conditions on Mars, Europa, Enceladus, Titan. The data are limited but real, and the hypotheses are testable.
SETI, as a subfield of astrobiology, has a tendency to dominate the public perception of the whole. When people think about the search for extraterrestrial life, they think about radio signals and intelligent aliens, not about methane plumes on Mars or phosphine on Venus or the spectral signature of vegetation on an exoplanet. This is a problem, because the more immediately testable and productive lines of astrobiological research, the search for biosignatures, the study of extremophiles, the characterisation of exoplanet atmospheres, are, in the public mind, subordinated to the more speculative and less productive search for engineered signals.
The effect is compounded by the media, which prefers the SETI narrative because it is dramatic: signals from the stars, first contact, the question of whether we are alone. The discovery of a biosignature on a distant exoplanet would be a profound scientific result, but it would not, in the public imagination, compete with a signal from an alien intelligence. The SETI industry, by maintaining its public profile, shapes the public’s understanding of what astrobiology is and what it should fund, and the shape it imposes is one that privileges the speculative over the empirical.
There is a further effect, less visible but more insidious: the SETI framing can distort the kinds of questions that scientists ask. The assumption that intelligence is the thing to search for, that intelligence is the endpoint of life, the thing that matters, can bias the kinds of biosignatures we look for and the kinds of planets we prioritise. We search for Earth-like planets because we assume that Earth-like planets are the most likely to produce Earth-like intelligence. This is reasonable, but it is also circular: we search for ourselves, and we justify the search by the assumption that the universe produces beings like us, and the assumption is grounded in the only data point we have, which is ourselves.
VII. What If the Signal Is Not a Signal?
Let us return to semiotics, because semiotics is the discipline that can most productively interrogate the assumptions of the search.
If an alien intelligence exists, and if it communicates, the communication need not take the form of a narrowband radio signal modulated with a pattern recognisable to human cryptanalysis. It could take a form that we do not recognise as communication, because the form is grounded in a semiotic world that does not overlap with ours.
Consider: a civilisation that communicates through gravitational wave modulation. This is theoretically possible, a sufficiently advanced civilisation could, in principle, manipulate massive objects to produce gravitational waves with a structured pattern. We have only recently developed the technology to detect gravitational waves at all (LIGO, 2015), and our sensitivity is limited to the most violent astrophysical events, merging black holes and neutron stars. A modulated gravitational-wave signal from an alien civilisation would be, with current technology, undetectable. But it is not impossible, and if it is happening, we would not know.
Consider: a civilisation that communicates through neutrino beams. Neutrinos interact so weakly with matter that they pass through planets undisturbed, which makes them, in principle, an excellent medium for interstellar communication, no absorption, no scattering, no interference. But the difficulty of generating and detecting neutrinos is, by current technology, prohibitive. We can detect neutrinos from the sun and from supernovae, but only with enormous detectors buried deep underground, and only at very low event rates. An engineered neutrino signal would be, with current technology, indistinguishable from background.
Consider: a civilisation that communicates through modifications to its environment that are detectable at interstellar distances, a Dyson structure that blocks or modulates the light of its star in a non-natural pattern. This is the technosignature approach: rather than searching for a signal, search for the artefact. A megastructure around a star would produce a distinctive light curve, a non-periodic, non-natural dimming pattern that could not be explained by planetary transits or stellar variability. The Kepler mission, which surveyed a portion of the sky for transiting planets, produced light curves for hundreds of thousands of stars, and a few, most notably Tabby’s Star (KIC 8462852), showed dimming patterns that were, briefly, consistent with a megastructure hypothesis. The dimming was later explained by dust, but the episode illustrates the principle: technosignatures are, in principle, detectable with existing technology, and they do not require the alien civilisation to be deliberately communicating. They require only that it is doing something big enough to see.
The technosignature approach is, in semiotic terms, a shift from searching for a sign that is intended as communication to searching for an index, a sign that bears a physical connection to its object, like smoke to fire. An index does not require intent. It requires only a causal relationship between the sign vehicle and the thing it signifies. A Dyson structure is an index of a technological civilisation, whether or not the civilisation wants to be seen.
This is, arguably, a more honest search, because it does not require us to assume that an alien intelligence shares our semiotic assumptions about communication. It requires only that an alien intelligence, if it exists and is sufficiently advanced, will do things that are detectable, will modify its environment in ways that are, to a sufficiently careful observer, non-natural. The search for technosignatures is, in this sense, a search for the side effects of intelligence rather than for intelligence itself. It is less ambitious, less romantic, and more grounded in observable fact.
VIII. The Honest Position
The honest position, given everything we know, is this:
We do not know whether there is other intelligent life in the universe. We have one data point, ourselves, and one data point cannot populate a distribution. We do not know whether intelligence is rare or common, whether it tends toward communication or toward silence, whether it produces signals we would recognise or signals we would not. We do not know whether the assumptions underlying SETI, that mathematics is a universal language, that the hydrogen line is a cosmic meeting place, that an alien intelligence would broadcast in a form we can detect, are correct or are projections of our own semiotic world onto the void.
What we know is that the universe is large, that the conditions for life exist in many places, and that life, once it arises, is persistent and adaptable. We know that the search for life, not necessarily intelligent life, but life in any form, is a scientific enterprise with testable hypotheses and available data. We can search for biosignatures in the atmospheres of exoplanets. We can study the chemistry of the interstellar medium. We can explore the moons of our own solar system for subsurface oceans and the chemical signatures of metabolism. We can do these things now, with existing technology, and the results, whether positive or negative, will be informative.
The search for extraterrestrial intelligence, in its current form, is a search conditioned on assumptions that are not testable, funded by resources that have opportunity costs, and sustained by an industry that has a structural incentive to continue regardless of results. It is not, in its current form, a science, because its core hypothesis is not falsifiable. It is a practice, a disciplined, methodical, technologically sophisticated practice, and the practice may, one day, produce a result. But the practice is not the same as the science, and the distinction matters, because the conflation of the two, the tendency to treat SETI as if it were as rigorously grounded as, say, the search for exoplanets or the study of cosmic microwave background anisotropies, distorts the allocation of scientific resources and the public understanding of what we know and what we do not.
The semiotic perspective offers a way out of this, not a solution, but a clarity. If we understand that the search is, inescapably, a search for ourselves, for a mind like ours, producing a signal like ours, in a form we can recognise, then we can be honest about what a null result means. It means that the specific kind of mind we are searching for has not been found in the specific places we have looked. It does not mean that minds do not exist. It does not mean that the universe is empty. It means that our model of what a mind looks like, when it communicates across the void, has not been confirmed, and that the model is derived from a sample of one and may be, in ways we cannot detect, wrong.
The most productive thing we can do, in the face of this uncertainty, is to broaden the search, to look not only for the signals we expect but for the indices we do not. To search for biosignatures as well as technosignatures. To search for the side effects of life as well as the deliberate productions of intelligence. To search, in short, not for a mirror but for a window, a way of seeing the universe that does not assume that what we see will look like us.
This is not a argument for stopping SETI. It is an argument for situating it, for understanding its place in the larger enterprise of astrobiology, for being honest about its assumptions, for acknowledging its opportunity costs, and for refusing the temptation, which all industries feel, to justify their own continuation by the infinity of the search space and the impossibility of proving a negative.
The universe may be full of minds. It may be empty of them. We do not know, and the not-knowing is the honest position, and the honest position is the one from which the best science is done, because the best science is done not by those who are certain of what they will find but by those who are honest about what they do not.
The signal, if it comes, will not be what we expect. It will be what it is. The question is whether we are listening in a way that allows us to hear it.
