The Wow! Signal

How a 72-second radio burst from deep space shocked SETI scientists and remains unexplained after 47 years

On August 15, 1977, at 11:16 PM Eastern time, the Big Ear radio telescope at Ohio State University detected a radio signal from space so powerful, so precisely tuned, and so apparently artificial that astronomer Jerry Ehman, reviewing the computer printout data the next day, circled the signal's alphanumeric designation and wrote "Wow!" in red pen in the margin, giving the transmission its now-famous name and creating what remains the most compelling potential evidence of extraterrestrial intelligence ever detected despite nearly five decades of attempts to find the signal again or explain it through natural phenomena. The signal lasted exactly seventy-two seconds, the maximum time any object could be observed by the Big Ear telescope as Earth's rotation carried that section of sky through the telescope's field of view, and it was detected at a frequency of 1420 megahertz, the exact frequency that hydrogen atoms emit radiation, and this frequency is significant because hydrogen is the most abundant element in the universe and because international agreements prohibit terrestrial radio transmissions at this frequency precisely because scientists believe any intelligent civilization would use this frequency for interstellar communication, making it the logical channel to monitor when searching for alien signals.

The signal's intensity was dramatic, registering as "6EQUJ5" on the computer printout, a code that represented signal strength variations over the seventy-two-second observation window, with the strength starting at background levels, rising to thirty times stronger than background noise at its peak, then fading back to background levels, creating exactly the pattern you would expect from a narrowband radio transmission from a fixed source as the telescope's beam swept across it during Earth's rotation. The signal appeared to originate from the constellation Sagittarius, specifically from a region of sky toward the center of our galaxy where the density of stars is high and where theoretically the probability of finding intelligent civilizations would be greater than in the sparse outer regions of the Milky Way, and the precise coordinates were carefully documented so that follow-up observations could be conducted to see if the signal repeated or if additional transmissions could be detected from the same source.

THE SEARCH FOR REPETITION AND EXPLANATION

In the days and weeks immediately following the Wow! Signal detection, the Big Ear telescope was repeatedly pointed at the same coordinates where the signal originated, and dozens of additional observation sessions were conducted, but the signal never appeared again, and in the forty-seven years since that August night in 1977, despite numerous attempts by Big Ear and other radio telescopes around the world including much more sensitive instruments than were available in the 1970s, no one has ever detected another transmission from that location or anything remotely similar to the original Wow! Signal, creating a profound mystery about what could produce such a powerful, apparently artificial radio transmission exactly once and then never again. The possibility that the signal was caused by terrestrial interference, such as aircraft radio, satellite transmissions, or ground-based radar, was extensively investigated and is considered extremely unlikely because the signal's frequency was in a protected band where such transmissions are prohibited internationally, and because the signal's characteristics matched what would be expected from an astronomical source rather than a terrestrial one, showing the proper rise and fall pattern as the telescope's beam swept across a fixed point in space rather than the irregular characteristics of moving interference sources.

The hypothesis that the signal might have been caused by a passing satellite was examined and largely dismissed because satellites in 1977 did not typically transmit at the hydrogen line frequency, and because the signal appeared to come from a fixed celestial location rather than moving across the sky as a satellite would, and because extensive searches of satellite tracking data from that time period found no satellites that could account for a transmission from that direction at that time. The possibility of equipment malfunction or data processing error was considered, but the Big Ear telescope was operating normally before and after the Wow! Signal detection, showing no other anomalies in the data, and the signal's characteristics were consistent with a real astronomical event rather than an artifact of malfunctioning equipment, and multiple computer systems and data recording methods all showed the same signal, making a technical glitch an inadequate explanation.

Natural astronomical phenomena that could potentially produce narrowband radio emissions at specific frequencies were investigated as possible explanations, and while pulsars, quasars, and various other exotic stellar objects can produce radio waves, none of them produce the type of continuous narrowband signal that was detected, and extensive surveys of the region where the Wow! Signal originated have never found any natural radio source that could account for the transmission. The most recent natural explanation proposed in 2017 suggested that the signal might have been caused by a comet passing through the telescope's field of view, with the comet's hydrogen cloud emitting radiation at the characteristic hydrogen frequency, and this theory gained some attention because it provided a natural explanation that didn't require invoking equipment malfunction or extraterrestrial intelligence, but the hypothesis was quickly challenged by other astronomers who pointed out that comets do not produce the type of narrowband signals that were detected and that no comet was known to be in that region of sky at that time, and subsequent analysis has largely discredited the comet explanation.

THE IMPLICATIONS IF ARTIFICIAL

If the Wow! Signal was indeed an artificial transmission from an extraterrestrial civilization, it raises profound questions about the nature and intentions of whoever sent it, and why it was transmitted only once or at least was only detected once by our instruments despite decades of listening. One possibility is that it was a directed beacon intentionally aimed at our solar system by a civilization that had detected signs of our presence or our technology and was attempting to initiate contact, and that the transmission was brief because the senders were using enormous amounts of energy to broadcast across interstellar distances and could only sustain the signal for a limited time, or because they were rotating their beam to target multiple star systems and we only caught their transmission during the brief window when the beam was pointed our direction. Another possibility is that it was not intended for us at all but was incidental radiation from some alien technology, perhaps equivalent to the radar or radio transmissions that leak from Earth into space as unintended byproducts of our civilization's activities, and we happened to be in the right place at the right time to detect it as it passed through our region of space.

The fact that the signal has never repeated could indicate that it was a one-time event such as a test transmission, a malfunction of alien equipment, or even a death cry from a civilization that existed briefly and then was destroyed by war, catastrophe, or some other extinction event, and this darker interpretation adds an element of cosmic loneliness to the mystery, the possibility that we detected the final gasp of intelligence from somewhere in the galaxy and that by the time we heard their signal they were already gone. Alternatively, the signal might repeat on a timescale longer than we have been monitoring, perhaps transmitting once every century or once every thousand years, and we were lucky enough to catch one repetition but will not live to see another, or the signal might be continuously transmitted but is so tightly beamed that it only intersects Earth during rare moments when our planet's position in its orbit aligns with the transmission direction, and we caught one such moment in 1977 but have not been properly positioned to receive the beam since then.

THE FRUSTRATION OF LOST OPPORTUNITY

One of the most frustrating aspects of the Wow! Signal is that the Big Ear telescope, which detected it, was designed to survey large areas of sky rather than to study specific sources in detail, and the telescope's configuration meant it could only observe any given point for seventy-two seconds before Earth's rotation carried that point out of view, so even if there had been operators monitoring in real-time rather than discovering the signal in archived printouts the next day, there would have been nothing they could do to extend the observation or to gather additional data about the signal's characteristics. If the signal had been detected by a modern telescope with digital recording equipment and steerable antennas, scientists could have captured far more information including precise frequency structure, polarization, modulation patterns, and other characteristics that might have provided clues about whether the signal was natural or artificial and potentially even what kind of technology produced it, but the limitations of 1977 technology mean we have only the basic information about signal strength and frequency, leaving enormous gaps in our knowledge about this tantalizing detection.

The Big Ear telescope was demolished in 1997 to make way for a golf course expansion, a decision that scientists interested in SETI (Search for Extraterrestrial Intelligence) considered tragic because it meant the loss of the only instrument that had ever detected what might be an alien signal, though by that point Big Ear was obsolete compared to newer telescopes and had not made any additional significant detections in the twenty years since the Wow! Signal. Modern SETI efforts use far more sophisticated equipment including the Allen Telescope Array, dedicated to scanning millions of star systems for signals, and these instruments are many times more sensitive than Big Ear and can simultaneously monitor multiple frequencies and directions, yet none of them have detected anything comparable to the Wow! Signal despite having much greater capability, and this absence of similar detections in the modern era adds to the mystery of what the 1977 signal might have been.

THE BROADER SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE

The Wow! Signal exists within the larger context of humanity's search for evidence of life beyond Earth, a search that has been systematically conducted since the 1960s through radio astronomy projects scanning for artificial signals, and the fact that this search has been underway for over sixty years with no definitive detections except the ambiguous Wow! Signal is itself significant, raising questions about whether intelligent life is extremely rare in the universe, whether such life exists but does not broadcast detectable signals, whether we are looking in the wrong places or at the wrong frequencies, or whether the vast distances and timescales involved make detection incredibly unlikely even if the galaxy contains many civilizations. The silence that has characterized most SETI observations is sometimes called the Great Silence or the Fermi Paradox, the contradiction between the high probability that extraterrestrial civilizations should exist based on the billions of stars and planets in our galaxy, and the apparent absence of any evidence for such civilizations in our observations of space.

Some scientists argue that the Wow! Signal should not be taken as strong evidence of extraterrestrial intelligence because a single non-repeating detection is insufficient to rule out all possible natural explanations and because the standards of proof for such an extraordinary claim should be correspondingly high, requiring either repetition of the signal or detection of clear artificial modulation patterns that could not be produced by natural phenomena. Others counter that dismissing the Wow! Signal because it doesn't repeat sets an impossible standard, as we cannot expect alien civilizations to continuously broadcast for our convenience, and that the signal's characteristics are precisely what SETI scientists have been looking for, and its rarity might simply reflect the genuine rarity of detectable transmissions in our galaxy rather than indicating the signal was not real or not artificial.

The mystery of the Wow! Signal persists because it represents a tantalizing glimpse of something extraordinary that appeared for seventy-two seconds and then vanished, leaving us with a piece of data that we cannot definitively explain through either natural phenomena or equipment malfunction but also cannot confirm as artificial intelligence without additional evidence, and this ambiguity places the signal in a liminal space between the known universe of natural radio sources and the hypothetical universe where we are not alone in the cosmos. Whether the Wow! Signal was a message from aliens, an unknown natural phenomenon, or some combination of terrestrial interference and coincidence may never be resolved unless we detect it again or unless someone develops a definitive natural explanation that accounts for all the signal's characteristics, and until then it remains one of the most intriguing mysteries in the search for extraterrestrial intelligence, a reminder that the universe occasionally presents us with phenomena we cannot explain and that the possibility of contact with alien civilizations, however remote, is not entirely confined to science fiction but exists as a genuine scientific question with potential real-world evidence, even if that evidence consists of a single seventy-two-second radio transmission detected one August night in Ohio in 1977.