The Wow! signal is a strong, narrowband radio signal picked up by Ohio State University's Big Ear radio telescope on 15 August 1977, close to the 1420 MHz hydrogen line that SETI researchers had long treated as a likely channel for interstellar communication. It lasted about 72 seconds as the sky drifted through the telescope's beam, it has never been detected again, and its source is still unknown. The name comes from the word "Wow!" that astronomer Jerry R. Ehman wrote in red ink beside the reading on the computer printout.
It matters because it is still the best-known candidate signal in the search for extraterrestrial intelligence (SETI), and a clean test of how science handles an anomaly. Since 2017 a comet explanation has been proposed and rejected, a candidate star has been named, and a hydrogen cloud hypothesis has been published. None settles the case, and seeing why shows what a real detection would require.
What was the Big Ear telescope?
Big Ear was a large, fixed radio telescope near Delaware, Ohio, run by Ohio State University, and from 1973 it carried out a long-running survey for narrowband signals of possible artificial origin. It was not a dish that tracked the sky. It used a tiltable flat reflector, a fixed parabolic reflector and a pair of feed horns, so it could be aimed north or south but not east or west. Earth's rotation carried each patch of sky through the beam.
By 1977 Ehman was working on the project as a volunteer, checking the computer output by hand for anything that stood out against the noise. The observatory no longer exists: Big Ear was torn down in 1998 when the land was developed for an expanded golf course and housing.
What happened on 15 August 1977?
The signal arrived at about 22:16 Eastern Standard Time on 15 August 1977, which was 23:16 local daylight time in Ohio. Nobody saw it live. The computer logged it, and Ehman found it a few days later, probably on 19 August, while reviewing the printouts. He circled the string "6EQUJ5" in one of the channel columns and wrote "Wow!" in the margin. The note became the signal's name.
What does 6EQUJ5 mean?
"6EQUJ5" is not a message; it is six successive intensity readings from a single receiver channel. Each character records how strong the signal was relative to the background noise during one interval, with digits for 0 to 9 and letters for higher values, so "A" stood for 10, "B" for 11, and so on.
Each reading took about 12 seconds: roughly 10 seconds of averaging and around 2 seconds for the computer to process the data. Read in order, the characters show the signal climbing to a peak and falling away. The "U" at the top means an intensity between 30 and 31 times the background noise, exceptionally strong for the survey. The signal sat in one of the receiver's 50 channels, each 10 kHz wide, without spreading into its neighbours. That made it narrowband, a property researchers associate with transmitters rather than with most natural radio sources.
Why did it last 72 seconds?
The signal lasted 72 seconds because that is how long Big Ear could see any fixed point in the sky. Six 12-second readings cover 72 seconds, the time Earth's rotation took to sweep the beam across a distant source.
That timing is one of the strongest pieces of evidence in the record. A source fixed among the stars should rise as it enters the beam, peak at the centre and fade on the way out, tracing the antenna's beam pattern. The Wow! signal did exactly that. Interference from a nearby transmitter, an aircraft or a satellite would not normally trace that profile so cleanly. It does not prove the source was interstellar, but it fits a source at astronomical distance.
The puzzle of the two feed horns
Big Ear had two feed horns pointing at slightly different parts of the sky, so any celestial source should have passed through both beams, one after the other, a few minutes apart. The Wow! signal appeared in only one, so either the source faded or switched off within those minutes, or something else was going on.
The data also do not say which horn received the signal, leaving two possible positions side by side in Sagittarius. The region lies northwest of the globular cluster M55, roughly 2.5 degrees south of the Chi Sagittarii group of stars, with Tau Sagittarii the nearest star easy to see with the naked eye. The ambiguity has followed every follow-up search since.
How close was it to the hydrogen line?
Very close. Ehman later gave its frequency as 1420.4556 ± 0.005 MHz, about 50 kHz above the 1420.4058 MHz rest frequency of neutral hydrogen. Hydrogen is the most abundant element in the universe, and in 1959 Giuseppe Cocconi and Philip Morrison argued in Nature that a civilisation wanting to be found might broadcast near this line, which any radio astronomer would know. The hydrogen line sits at the lower edge of the quiet radio band later nicknamed the "water hole," which SETI researchers have long favoured for the same reason.
The companion piece on why SETI scientists tune into cosmic radio waves explains the logic of listening here.
Why has it never repeated?
No search has picked the signal up again, which is the main reason scientists do not count it as a detection. Ehman and the Big Ear team looked at the same region many times afterwards. Robert H. Gray searched again with Kevin Marvel using the far more sensitive Very Large Array in New Mexico in 1995 and 1996. In 1999 Gray and Simon Ellingsen used the Mount Pleasant Radio Observatory in Tasmania for six 14-hour observations of the area. None of them found anything comparable.
Ehman himself has been careful. In his 30th-anniversary report on the signal, first written in 2007, he says an extraterrestrial intelligence might have sent it, but that he chooses not to "draw vast conclusions from 'half-vast' data," and that its origin is still an open question for him.
What would SETI need to call it a detection?
Repeatability. A signal that cannot be observed again cannot be checked by other teams or instruments, and independent confirmation is the standard SETI sets for itself. A single event, however clean, leaves room for a rare natural process, an instrument fault or unidentified interference.
That is why the hydrogen cloud hypothesis described below matters even to people who hope the signal was artificial. It makes predictions that can be checked against the sky: if the predicted cloud is found, the case moves toward a natural explanation; if not, the field narrows the other way.
The theories: comets, a Sun-like star and hydrogen clouds
Three proposals have been tested against the record since 2017; one is still under active study.
The comet hypothesis
In 2017 Antonio Paris, then at St. Petersburg College in Florida, proposed that the signal came from the hydrogen cloud around one of two comets, 266P/Christensen and P/2008 Y2 (Gibbs), now numbered 335P/Gibbs, which were in that part of the sky in 1977. Astronomers, including members of the original Big Ear team, rejected the idea: the comets were not in the beam at the right time, comets emit only weakly at that frequency, and a comet cannot explain a signal seen in one horn but not the other.
A Sun-like candidate star
In a 2022 paper in the International Journal of Astrobiology, Alberto Caballero searched catalogues for Sun-like stars within the signal's possible positions and singled out one candidate, catalogued as 2MASS 19281982-2640123, about 1,800 light-years away. It offered a target for listening, not evidence of a source. Searches by Breakthrough Listen in 2022, using the Green Bank Telescope and the Allen Telescope Array, found nothing.
The hydrogen cloud hypothesis
The most developed recent explanation comes from the Arecibo Wow! project, led by Abel Méndez of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo. In "Arecibo Wow! I," a preprint first posted on 16 August 2024, Méndez, Kevin Ortiz Ceballos and Jorge I. Zuluaga reanalysed archival Arecibo Observatory data recorded between February and May 2020. They found narrowband signals near 1420 MHz that resembled the Wow! signal in form but were roughly a hundred times fainter, and traced them to small, cold clouds of neutral hydrogen.
They propose that the 1977 event was a sudden, maser-like brightening of one such cloud in the hydrogen line, triggered by a strong transient source such as a magnetar flare or a soft gamma repeater. The authors frame this as a testable hypothesis: they describe the kind of cloud that should sit in the signal's field and suggest instruments such as the VLA or China's FAST telescope to look for it. If confirmed, it would mean a new kind of natural false positive for technosignature searches.
A follow-up, "Arecibo Wow! II," posted on 14 August 2025 by a larger team, digitised and reanalysed archival Ohio SETI printouts from the days around the detection. They reported a peak flux density above 250 janskys, higher than earlier estimates, a revised frequency of 1420.726 ± 0.005 MHz, about 0.27 MHz above Ehman's figure, and a tighter pair of candidate positions. Citing the beam profile and the absence of plausible satellite, broadcast or solar sources, they judged radio interference unlikely, while still describing the origin as unexplained.
Was there ever a reply?
In 2012, for the 35th anniversary, the Arecibo Observatory was reported to have beamed a digital stream of roughly 10,000 Twitter messages toward three nearby stars catalogued by the Hipparcos mission. It was a publicity event, with no bearing on what caused the 1977 reading. For an earlier overview of the case and the public interest it drew, see the Wow! signal mystery.
Billy Carson's reading
Billy Carson has not, so far as the 4BK archive shows, built a dedicated program around the Wow! signal, and this article does not attribute a Wow!-specific claim to him. His broader reading of possible artificial signals and objects is on the record elsewhere, and it is consistent: keep the question open, separate evidence from conclusion, and do not let one debunked item close a whole line of inquiry.
On the Black Knight satellite, Carson accepts NASA's identification of the STS-88 photographs as a lost thermal blanket, but argues that the history of reported signals is a separate question. On 'Oumuamua, he has praised the willingness of Harvard astrophysicist Avi Loeb to put an artificial hypothesis into the peer-reviewed literature so it could be tested, as covered in the 'Oumuamua explainer. On 3I/ATLAS he goes further, offering his personal opinion that the object is artificial, while NASA has said it behaves like a comet and that no technosignatures were seen. See NASA's 3I/ATLAS briefing and Billy Carson's reaction and his take on Loeb's anomalies and the delayed Mars photos.
Read alongside those, the Wow! signal is the kind of case his approach is built for: a well-documented anomaly with a natural hypothesis on the table and no confirmation either way.
Watch, read and go deeper
For more unexplained cases in space history, UFOlogy and disclosure, start with the UNSOLVED series on 4BK TV. The documentary Black Knight Satellite (Beyond The Signal) takes up Billy Carson's interest in artificial signals at length. For the bigger questions about information, reality and observation, browse the Holographic Universe topic hub.
Frequently asked questions
What does the Wow signal say?
Nothing, as far as the record shows. "6EQUJ5" is six intensity readings taken about 12 seconds apart, showing how strong the signal was against background noise. The survey recorded only averaged intensity, so any encoded content would not have been captured.
Where in the sky did the Wow signal come from?
The constellation Sagittarius, northwest of the globular cluster M55 and about 2.5 degrees south of the Chi Sagittarii group. Because the data do not show which feed horn received it, there are two adjacent candidate positions.
Was the Wow signal caused by a comet?
Very unlikely. Antonio Paris proposed two comets in 2017, but astronomers including members of the Big Ear team noted that the comets were not in the beam at the right time, that comets emit weakly at that frequency, and that a comet would not explain the one-horn detection.
Has anyone found the source of the Wow signal?
No. The Arecibo Wow! project's hydrogen cloud hypothesis is the leading natural explanation under study, but no cloud or star has been confirmed as the source, and 2022 searches of Alberto Caballero's candidate star found nothing.
Could the Wow signal have been aliens?
It cannot be ruled out, and it cannot be confirmed, because it never repeated. Jerry Ehman's own position is that the origin remains open and the data are too limited for firm conclusions.
Sources and further reading
- Jerry R. Ehman, "The Big Ear Wow! Signal (30th Anniversary Report)" (2007, revised 2010), Big Ear Radio Observatory archive
- Giuseppe Cocconi and Philip Morrison, "Searching for Interstellar Communications," Nature (1959)
- Robert H. Gray, The Elusive Wow: Searching for Extraterrestrial Intelligence (2012)
- Robert H. Gray and Kevin Marvel, "A VLA Search for the Ohio State 'Wow'," The Astrophysical Journal (2001)
- Robert H. Gray and Simon Ellingsen, "A Search for Periodic Emissions at the Wow Locale," The Astrophysical Journal (2002)
- Alberto Caballero, "An approximation to determine the source of the WOW! Signal," International Journal of Astrobiology (2022)
- Abel Méndez, Kevin Ortiz Ceballos and Jorge I. Zuluaga, "Arecibo Wow! I: An Astrophysical Explanation for the Wow! Signal," arXiv:2408.08513 (2024)
- Abel Méndez et al., "Arecibo Wow! II: Revised Properties of the Wow! Signal from Archival Ohio SETI Data," arXiv:2508.10657 (2025)
