The Wow! signal was a strong radio signal received on August 15, 1977, by Ohio State University's Big Ear radio telescope in the United States, which was being used at the time in the search for extraterrestrial intelligence. The signal appeared to come from the direction of the constellation Sagittarius and met all the expectations of a signal of extraterrestrial origin.

Astronomer Jerry R. Ehman found the signal a few days after it was received while reviewing printout of the data collected by the telescope. He was so impressed by the strength of the signal that he circled the text that represented it, "6EQUJ5", and wrote the comment "Wow!" on its side, leading to the event's widely used name.

The entire signal was detected for the full 72-second window during which Big Ear was able to observe it, but has not been detected since, even though many, such as Ehman, have tried. Many explanations have been given on where the signal comes from, including natural and human-made sources, but none of them have been accepted by scientists as good enough to fully explain the signal.

Although the Wow! signal did not have any known encoded message, it remains the strongest candidate for an alien radio transmission ever detected.

Background 
In a 1959 paper, Cornell University physicists Philip Morrison and Giuseppe Cocconi hypothesized that any extraterrestrial civilization trying to communicate with radio signals might do so using a frequency of  megahertz ( centimeters), because is naturally emitted by hydrogen, which is the most common element in the universe, and is likely familiar to all technologically advanced civilizations.

In 1973, after completing a survey of radio sources coming from other galaxies, Ohio State University assigned the Ohio State University Radio Observatory (nicknamed "Big Ear") to the scientific search for extraterrestrial intelligence (SETI), in the longest-running program of this kind in history. The radio telescope was located near the Perkins Observatory on the campus of Ohio Wesleyan University in Delaware, Ohio.

In 1977, Ehman was working at the SETI project as a volunteer; his job involved going through large amounts of data processed by a computer and recorded on paper. While looking at the data collected on August 15 at 22:16 EDT (02:16 UTC), he noticed a series of values showing signal strength and frequency that left him and others amazed.
The event was later documented in detail by the observatory's director.

The string 6EQUJ5 is commonly misunderstood as a message encoded in the radio signal. This string actually represents the signal's strength over time, expressed in the particular measuring system adopted for the experiment. The signal itself did not appear to change in a way that would have allowed for a message to be encoded, although any change that would have happened in less than 10 seconds or longer than 72 seconds would not have been detectable. The signal did, however, have a continuous wave.

Intensity 
The signal strength was measured by how much more powerful it was than the average background noise over the previous few minutes. The signal was sampled for 10 seconds and then processed by the computer, which took 2 seconds. Meaning that every 12 seconds the strength for each different frequency channel was printed as a single number or letter that represented the 10-second average strength, minus the strength of the background noise.

On this scale, an empty space was left for strengths between 0 and 1 standard deviations higher than the background noise. The numbers 1 to 9 showed how many standard deviations above the background noise the signal strength was. If the strength of the signal was 10 or above it would be indicated by a letter: "A" corresponded to a strength of 10, "B" to 11, "C" to 12, and so on. The Wow! signal's highest measured value was "U" (a strength of 30), making it thirty standard deviations above background noise.John Kraus, the director of the observatory, gave a value of  in a 1994 summary written for Carl Sagan. However, Ehman in 1998 gave a value of . This is () above the hydrogen line value (with no red- or blue-shift) of . If due to blue-shift, it would mean the source was moving about  towards Earth. An explanation of the difference between Ehman's value and Kraus's can be found in Ehman's paper.

The Wow! signal was a narrowband emission: meaning its bandwidth was less than  kHz. The Big Ear telescope was able to measure fifty  kHz-wide channels. The output from each channel was represented in the computer printout as a a different column with the number or letters printed on it representing the strength of the signal at that range. The Wow! signal is only in one column.

Time variation 
At the time the signal was heard, the Big Ear radio telescope was only adjustable for altitude (or height above the horizon), and used the spinning of the Earth to listen across the sky. Given how fast the Earth spins, and how big the telescope's observation window is, the Big Ear could listen to any given point in the sky for just 72 seconds. This means that a continuous extraterrestrial signal would only be able to be heard for exactly 72 seconds, and the recorded strength of such signal would display an increase for the first 36 seconds—peaking at the center of the observation window—and then a decrease as the telescope moved away from it. All these characteristics are present in the Wow! signal.

Celestial location 

The exact spot in the sky where the signal came from is unclear because of the design of the Big Ear telescope. The telescope had two different structures (called feed horns) that listened to the sky, and both listened to the sky in slightly different directions, and would be moved along the sky by the Earth's spin. The Wow! signal was heard in one of these feed horns, but not in the other, and the data was processed in a way that makes it is impossible to determine which of the two horns heard the signal. Making two different spots in the sky where the signal could have come from.

The galactic coordinates for the positive horn are =11.7°, =−18.9°, and for the negative horn =11.9°, =−19.5°, both being therefore about 19° toward the southeast of the galactic plane, and about 24° or 25° east of the galactic centre. The region of the sky in question lies northwest of the globular cluster M55, in the constellation Sagittarius, roughly 2.5 degrees south of the fifth-magnitude star group Chi Sagittarii, and about 3.5 degrees south of the plane of the ecliptic. The closest easily visible star is Tau Sagittarii.

No nearby sun-like stars were within the are that was being listened to, although in any direction the antenna pattern would encompass about six stars that are far away.

Hypotheses on the signal's origin 
There are a number of hypotheses as to what the Wow! signal is, and where it came from. Scientists have not gotten behind any one of these ideas. Interstellar twinkling of a weaker signal—similar in effect to twinkling of stars in the sky—could be an explanation, but that would not exclude the possibility of the signal being from extraterrestrials. The significantly more sensitive Very Large Array did not detect the signal, and the probability that a signal not detected by the Very Large Array could be detected by the Big Ear due to interstellar twinkling is low. Other hypotheses include a rotating lighthouse-like source, a signal sweeping in frequency, or just a one-time event.

Ehman has said: "We should have seen it again when we looked for it 50 times. Something suggests it was an Earth-sourced signal that simply got reflected off a piece of space debris." He later took back his some of his doubt, after further research showed that explanation to be very unlikely, given that any space debris the signal would have bounced off of would have had to meet some unrealistic requirements. Also, other problems arise because the 1420 MHz signal is within a a bandwidth reserved for astronomical purposes, in which radios on Earth are banned from using. In a 1997 paper, Ehman suggest that not draw "vast conclusions from half-vast data"— and acknowledged the possibility that the signal may have been military or from people on Earth.

METI president Douglas Vakoch told Die Welt that any SETI signal detections must be heard again for confirmation, and the fact that it has not been heard again making the Wow! signal have little credibility.

Discredited hypotheses 
In 2017, Antonio Paris, a teacher from Florida, proposed that the hydrogen cloud surrounding two comets, 266P/Christensen and 335P/Gibbs, now known to have been in the same region of the sky, could have caused the Wow! signal. This idea was dismissed by astronomers, including members of the original Big Ear research team, as the comets he suggested were not in the exact right spot at the correct time. Also, comets do not emit strongly at the frequencies that the signal was heard, and there is no explanation for why a comet would be observed in one feed horn but not in the other.

Searches for recurrence of the signal 
Ehman and other astronomers have tried to hear the signal again and identify it. The signal was expected to occur three minutes apart in each of the telescope's feed horns, but that did not happen. Ehman searched for a second occurence of the signal using Big Ear in the months after the detection, but it was never heard again.
In 1987 and 1989, Robert H. Gray listened for the signal using the META array at Oak Ridge Observatory, but did not hear it.  In a July 1995 test of signal detection software to be used in its upcoming Project Argus, SETI League executive director H. Paul Shuch made several observations of the Wow! signal's place in the sky with a 12-meter radio telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia, also never hearing anything.

In 1995 and 1996, Gray again searched for the signal using the Very Large Array, which is significantly more sensitive than Big Ear.  Gray and Simon Ellingsen later searched for and second occurrence of the signal in 1999 using the 26-meter radio telescope at the University of Tasmania's Mount Pleasant Radio Observatory. Six 14-hour observations were made at positions in the general point in the sky, but nothing like the Wow! signal was ever heard.

Response 
In 2012, on the 35th anniversary of the Wow! signal, Arecibo Observatory beamed a message at Hipparcos 34511, 33277, and 43587. The transmission consisted of approximately 10,000 Twitter messages that National Geographic has asked fans to send in with the hashtag "#ChasingUFOs" (a promotion for one of the channel's TV series). The sponsor also included a series of videos with messages from several celebrities.

To increase the chance that any extraterrestrials that would hear it would recognize the signal as an attempt to communicate from another intelligent life form, Arecibo scientists attached a header to each individual message, and sent the transmission at roughly 20 times the power of the most powerful commercial radio transmitter.

In popular culture 

 In 1995, H. Paul Shuch composed the filk song "Ballad of the 'Wow!' Signal", which is sung to the tune of "Ballad of Springhill" by Peggy Seeger.
 In 2012, The Dandy Warhols released the song "Seti vs. the Wow! Signal".
 In 2016, Jean-Michel Jarre released the "Oxygène 17 music video" which is dedicated to the Wow! signal.
 In a 2017 Super Bowl commercial from Avocados From Mexico spoofing conspiracy theories such as the moon landing hoax, Area 51, and subliminal advertising, the base of a stone monolith carries the inscription "6EQUJ5".
 The Wow! signal was mentioned in The X-Files episode "Little Green Men".
 The Wow! signal is used as the source of the alien power suit in the movie Lazer Team.
 The Wow! signal is the subject of a 2019 documentary written by Michael Shaw and directed by Bob Dawson.
 The 2019 film Ad Astra uses the "6EQUJ5" signal as the filename of a Top Secret message reviewed by the film's main character.

 The "6EQUJ5" signal features as an Easter Egg in the 2013 video game Grand Theft Auto V, along with several other alien Easter Eggs.

Related pages 
 Arecibo message – 1974 message into space from the Arecibo Observatory, a three-minute-long message sent into space

References 

SETI