SETI

← Encyclopedia

In 1959 two physicists worked out that the conversation was already physically possible. Giuseppe Cocconi and Philip Morrison, writing in Nature, argued that a radio telescope of the day could, in principle, exchange signals across interstellar distances — that the engineering for hearing another civilization existed, if only one knew where in the spectrum to point. They proposed a frequency. Neutral hydrogen, the commonest stuff in the universe, radiates at 1420 megahertz — the 21-centimeter line — and any species advanced enough to build a radio telescope would know it. Make that the meeting place, they suggested: a frequency no one chose, that everyone with the equipment would arrive at independently. Their paper closed on a sentence the field has carried ever since. “The probability of success is difficult to estimate; but if we never search, the chance of success is zero.” That is the whole wager. It has been pressed for more than sixty years, and the central fact about the result must be stated without softening: no confirmed detection of an extraterrestrial technosignature has ever been made.

A radio astronomer named Frank Drake had reached the same idea almost at the same moment, and he had a telescope. In the spring of 1960, from Green Bank in West Virginia, he pointed the 85-foot Tatel dish at Tau Ceti and Epsilon Eridani — two nearby Sun-like stars — and listened near the hydrogen line. Project Ozma was the first organized scientific search, and it set the pattern in miniature: an early thrill, a signal that turned out to be a high-altitude aircraft, and then nothing. What Ozma proved was not that anyone was there but that the looking was workable. The following year Drake convened a small meeting at Green Bank and wrote on the board the parameter chain now known as the Drake equation — an attempt to estimate how many civilizations might be broadcasting at any one time. It was an agenda, not a result, and the site treats it in its own entry; what matters here is that it gave the search a shape and a vocabulary.

The instrument under all of this is patience pointed at noise. Radio SETI hunts for a narrowband signal — a tone too pure to be natural — standing out against the broad hiss of the galaxy. The classic listening band lies between the 21-centimeter hydrogen line and the 18-centimeter lines of the hydroxyl radical. Hydrogen plus hydroxyl makes water, so the band was nicknamed the “water hole,” with the conceit that water-based life might meet there; the poetry is incidental, but the band is genuinely quiet and a sensible place to listen on plain physical grounds. Later searches added optical SETI — watching for nanosecond laser pulses bright enough to briefly outshine a star. The premise throughout is deliberate, repeatable, predictive observation, and this separates the discipline from the louder cultural noise around it. UFO and UAP reports are a different category of claim entirely — unpredictable sightings, radar and photographic anomalies, no controlled program — and SETI scientists are at pains to keep the two apart. Seth Shostak of the SETI Institute has been blunt on the point. A telescope aimed at a known star for a measured interval is not the same activity as a blurry video, and the field’s credibility depends on the distinction holding.

Once, the search reached the other way. On 16 November 1974, at a ceremony for the upgraded Arecibo dish in Puerto Rico, a single message went out toward the globular cluster M13 in Hercules. It ran 1,679 bits — the product of two primes, 73 and 23, so that a recipient could lay it out as a grid and read the picture — broadcast at 2380 megahertz by a team led by Drake, with Carl Sagan among them. No one mistook it for a conversation. M13 is some 25,000 light-years off; a reply, were anyone there to send one, could not return for fifty thousand years. The Arecibo message was a demonstration and a gesture, and it opened a quarrel that has not closed. To transmit deliberately — active SETI, or METI, messaging extraterrestrial intelligence — is a different act from listening, and a genuinely contested one. Stephen Hawking, among others, warned against announcing ourselves to a cosmos that might hold something hostile; a 2015 statement called for a moratorium until the matter could be discussed worldwide. Defenders such as Shostak and Jill Tarter answer that the danger is speculative, that a civilization able to cross the stars would likely have outgrown predation, and that Earth’s own leakage radiation has been announcing us for decades anyway. The argument is unresolved — an ethical dispute with named positions on both sides, not a settled question.

Institutionally the search has had to survive its own respectability gap. NASA built SETI programs through the 1970s and 1980s, and on 12 October 1992 — chosen as the Columbus quincentennial — its High-Resolution Microwave Survey began observing, pairing a targeted look at nearby stars with an all-sky sweep. A year later Congress killed it — a budget amendment, led by a senator who called it a hunt for “little green men,” ended the federal program in 1993. The search did not stop; it privatized. The SETI Institute, incorporated in 1984 as a California nonprofit by Thomas Pierson and Jill Tarter with Drake’s backing, took up the targeted survey under the name Project Phoenix — risen, as intended, from the ashes — and ran it on the world’s great dishes from 1995 to 2004, on private money alone. Other lines ran alongside: Berkeley’s SERENDIP riding piggyback on other telescopes’ observations, the Planetary Society’s Harvard searches, Ohio State’s long survey with the Big Ear. In 1999 came an unlikely democratization. SETI@home let anyone with a home computer chew through Arecibo data while their screen sat idle; at its height more than five million people ran it. Berkeley wound the distribution down in 2020. Across two decades and millions of machines, it returned no confirmed signal.

Dedicated hardware followed the money. The Allen Telescope Array — funded chiefly by the Microsoft co-founder Paul Allen, who gave over thirty million dollars — opened at Hat Creek in California in 2007: 42 dishes of an intended 350, the first instrument built for SETI from scratch. Then in 2015 Yuri Milner announced Breakthrough Listen at the Royal Society in London, with Hawking and Drake on the platform — a hundred million dollars over ten years, the most thorough search ever attempted. It buys time on the Green Bank and Parkes telescopes, on the Automated Planet Finder for optical work, on MeerKAT and FAST; it targets the nearest million stars, the galactic plane, the hundred closest galaxies. The sensitivity climbs with each generation, and still the result holds.

It is not for want of moments that looked like the moment. On 15 August 1977 the Big Ear recorded a 72-second narrowband burst near the hydrogen line, out of Sagittarius, so striking that the astronomer reviewing the printout circled the figures and wrote “Wow!” in the margin. The Wow! signal never repeated and has never been confirmed; the site gives it its own entry. In April and May of 2019 Breakthrough Listen’s Parkes data turned up a narrowband tone from the direction of Proxima Centauri, drifting in a way a planet might explain, with no modulation — candidate BLC-1, reported in 2020 and at first likened to the Wow! signal. By 2021 it had been traced to human radio-frequency interference. That is the shape of the whole record. Ozma’s aircraft, the Wow! signal, BLC-1: every candidate that has come up for scrutiny has, on scrutiny, dissolved — explained away, attributed to interference, or left forever unrepeated and so unconfirmable. The exception that would change everything has not arrived.

The hunt has also widened past beacons to the things a civilization might build. A sufficiently advanced one, the reasoning goes, might wrap its star in energy-collecting machinery — a Dyson sphere or swarm — which would betray itself as an odd excess of infrared; the site treats the megastructure idea, and the Kardashev scale that ranks such civilizations by their energy use, in their own entries. The clearest test case came with KIC 8462852, Tabby’s Star, whose erratic dimming in Kepler data — up to a fifth of its light, on no regular schedule — drew the dramatic suggestion of an alien construction. The Allen Array listened; optical searches watched. They found nothing, and the dust-cloud explanation now holds the field. The episode is instructive precisely because it is not evidence of aliens. It is a hypothesis raised, taken seriously enough to test, and not supported — which is what working SETI looks like when it works.

All of this presses against a silence the search keeps documenting and cannot fill. The persistent non-detection is the empirical edge of the Fermi paradox — the mismatch between a galaxy that should, on generous estimates, be loud with civilizations and a sky that says nothing — which the site handles separately. What deserves saying plainly is that the silence is not yet damning. With the best instruments, Earth’s own broadcasts would fade below detection a fraction of the way to the nearest star; success demands luck about frequency, timing, and power at once; and humanity has been listening, in cosmic terms, for an eyeblink. Absence on these terms is thin evidence of anything.

SETI is the instrument through which an old question — are we alone, or in company? — finally acquired antennas and a budget, and the apparatus carries the longing intact. The wager Cocconi and Morrison made was never really about hydrogen frequencies; it was about refusing to let the question go unasked. The contemplative traditions this archive gathers turned that question inward, through symbol and rite; the radio search turns it outward, through arithmetic and patience, and the two have more in common than either would likely claim. The dishes turn through their assigned arcs, and the next sweep is already on the schedule — sixty years of accumulated noise behind it, and the one rule the field has never had cause to revise: the only search certain to fail is the one never run.

→ Related: Drake Equation · Fermi Paradox · Wow Signal · Dyson Sphere · Kardashev Scale