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The Wow! signal was a strong
narrowband Narrowband signals are signals that occupy a narrow range of frequencies or that have a small fractional bandwidth. In the audio spectrum, ''narrowband sounds'' are sounds that occupy a narrow range of frequencies. In telephony, narrowband is ...
radio signal detected on August 15, 1977, by
Ohio State University The Ohio State University (Ohio State or OSU) is a public university, public Land-grant university, land-grant research university in Columbus, Ohio, United States. A member of the University System of Ohio, it was founded in 1870. It is one ...
's Big Ear radio telescope in the United States, then used to support the
search for extraterrestrial intelligence The search for extraterrestrial intelligence (usually shortened as SETI) is an expression that refers to the diverse efforts and scientific projects intended to detect extraterrestrial signals, or any evidence of intelligent life beyond Earth. ...
. The signal appeared to come from the direction of the constellation Sagittarius and bore expected hallmarks of extraterrestrial origin. Astronomer Jerry R. Ehman discovered the anomaly a few days later while reviewing the recorded data. He was so impressed by the result that he circled on the computer printout the reading of the signal's intensity, "6EQUJ5", and wrote the comment "Wow!" beside it, leading to the event's widely used name. The entire signal sequence lasted for the full 72-second window during which Big Ear was able to observe it, but has not been detected since, despite many subsequent attempts by Ehman and others. Several hypotheses have been advanced on the origin of the emission, including natural and human-made sources.


Background

In a 1959 paper,
Cornell University Cornell University is a Private university, private Ivy League research university based in Ithaca, New York, United States. The university was co-founded by American philanthropist Ezra Cornell and historian and educator Andrew Dickson W ...
physicists Philip Morrison and Giuseppe Cocconi had speculated that any extraterrestrial civilization attempting to communicate via radio signals might do so using a frequency of
megahertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base ...
(-centimeter
spectral line A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission (electromagnetic radiation), emission or absorption (electromagnetic radiation), absorption of light in a narrow frequency ...
), which is naturally emitted by hydrogen, the most common element in the universe and therefore likely familiar to all technologically advanced civilizations. In 1973, after completing an extensive survey of extragalactic radio sources, Ohio State University assigned the now-defunct
Ohio State University Radio Observatory The Ohio State University Radio Observatory was a Kraus-type (after its inventor John D. Kraus) radio telescope located on the grounds of the Perkins Observatory at Ohio Wesleyan University in Delaware, Ohio from 1963 to 1998. Known as Big Ear ...
(nicknamed "Big Ear") to the scientific
search for extraterrestrial intelligence The search for extraterrestrial intelligence (usually shortened as SETI) is an expression that refers to the diverse efforts and scientific projects intended to detect extraterrestrial signals, or any evidence of intelligent life beyond Earth. ...
(SETI), in the longest-running program of this kind in history. The radio telescope was located near the
Perkins Observatory Perkins Observatory is an astronomical observatory in Delaware, Ohio, United States. It is owned and operated by Ohio Wesleyan University. In 1931 it had the third largest telescope in the World; the 69 inch aperture Perkins telescope came onli ...
on the campus of
Ohio Wesleyan University Ohio Wesleyan University (abbrevriated OWU) is a private university, private Liberal arts colleges in the United States, liberal arts college in Delaware, Ohio, United States. It was founded in 1842 by Methodist leaders and Ohio Valley, Centra ...
in
Delaware, Ohio Delaware is a city in Delaware County, Ohio, United States, and its county seat. It is located near the center of Ohio, about north of Columbus as part of the Columbus metropolitan area. The population was 41,302 at the 2020 census. Delaware ...
. By 1977, Ehman was working at the SETI project as a volunteer; his job involved analyzing by hand large amounts of data processed by an
IBM 1130 The IBM 1130 Computing System, introduced in 1965, was IBM's least expensive computer at that time. A binary 16-bit machine, it was marketed to price-sensitive, computing-intensive technical markets, like education and engineering, succeeding th ...
computer and recorded on
line printer A line printer Printer (computing), prints one entire line of text before advancing to another line. Most early line printers were printer (computing)#Impact printers, impact printers. Line printers are mostly associated with unit record eq ...
paper. While perusing data collected on August 15 at 22:16  EDT (02:16 
UTC Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communica ...
), he spotted a series of values of signal intensity and frequency that left him and his colleagues astonished. The event was later documented in technical detail by the observatory's director.John Kraus, Director, Ohio State Radio Observatory 31, January 1994
"The Tantalizing WOW! Signal"
Copy of letter to Carl Sagan containing an unpublished paper describing the event.


Signal measurement

The string 6EQUJ5, commonly misinterpreted as a message encoded in the radio signal, represents in fact the signal's
intensity Intensity may refer to: In colloquial use * Strength (disambiguation) *Amplitude * Level (disambiguation) * Magnitude (disambiguation) In physical sciences Physics *Intensity (physics), power per unit area (W/m2) *Field strength of electric, m ...
variation over time, expressed in the particular measuring system adopted for the experiment. The signal itself appeared to be an unmodulated
continuous wave A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency, typically a sine wave, that for mathematical analysis is considered to be of infinite duration. It may refer to e.g. a laser or particl ...
, although any
modulation Signal modulation is the process of varying one or more properties of a periodic waveform in electronics and telecommunication for the purpose of transmitting information. The process encodes information in form of the modulation or message ...
with a period of less than 10 seconds or longer than 72 seconds would not have been detectable.


Intensity

The signal intensity was measured as
signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to noise power, often expressed in deci ...
, with the noise (or baseline) averaged over the previous few minutes. The signal was
sampled Sample or samples may refer to: * Sample (graphics), an intersection of a color channel and a pixel * Sample (material), a specimen or small quantity of something * Sample (signal), a digital discrete sample of a continuous analog signal * Sample ...
for 10 seconds and then processed by the computer, which took 2 seconds. The result for each frequency channel was output on the printout as a single
alphanumeric Alphanumericals or alphanumeric characters are any collection of number characters and letters in a certain language. Sometimes such characters may be mistaken one for the other. Merriam-Webster suggests that the term "alphanumeric" may often ...
character, representing the 10-second average intensity, minus the baseline, expressed as a
dimensionless Dimensionless quantities, or quantities of dimension one, are quantities implicitly defined in a manner that prevents their aggregation into units of measurement. ISBN 978-92-822-2272-0. Typically expressed as ratios that align with another sy ...
multiple of the signal's
standard deviation In statistics, the standard deviation is a measure of the amount of variation of the values of a variable about its Expected value, mean. A low standard Deviation (statistics), deviation indicates that the values tend to be close to the mean ( ...
. In this particular intensity scale, a
space character A whitespace character is a character data element that represents white space when text is rendered for display by a computer. For example, a ''space'' character (, ASCII 32) represents blank space such as a word divider in a Western scri ...
denoted an intensity between 0 and 1, that is between baseline and one standard deviation above it. The numbers 1 to 9 denoted the correspondingly numbered intensities (from 1 to 9); intensities of 10 and above were indicated by a letter: "A" corresponded to intensities between 10 and 11, "B" to 11 to 12, and so on. The Wow! signal's highest measured value was "U" (an intensity between 30 and 31), which is thirty standard deviations above background noise.


Frequency

John Kraus, the director of the observatory, gave a value of in a 1994 summary written for
Carl Sagan Carl Edward Sagan (; ; November 9, 1934December 20, 1996) was an American astronomer, planetary scientist and science communicator. His best known scientific contribution is his research on the possibility of extraterrestrial life, including e ...
. However, Ehman in 1998 gave a value of . This is () above the
hydrogen line The hydrogen line, 21 centimeter line, or H I line is a spectral line that is created by a change in the energy state of solitary, electrically neutral hydrogen atoms. It is produced by a spin-flip transition, which means the dire ...
value (with no red- or blue-shift) of . If due to blue-shift, it would correspond to the source moving about towards Earth. An explanation of the difference between Ehman's value and Kraus's can be found in Ehman's paper. The first
local oscillator In electronics, the term local oscillator (LO) refers to an electronic oscillator when used in conjunction with a Frequency mixer, mixer to change the frequency of a signal. This frequency conversion process, also called Heterodyne, heterodyning ...
in the telescope's radio receiver was specified to a frequency value of . However, the university's purchasing department made a
typographical error A typographical error (often shortened to typo), also called a misprint, is a mistake (such as a spelling or transposition error) made in the typing of printed or electronic material. Historically, this referred to mistakes in manual typesettin ...
in the order form, instead obtaining an oscillator with frequency (i.e., higher than desired). The software used in the experiment was then written to adjust for this error. When Ehman computed the frequency of the Wow! signal, he took this error into account.


Bandwidth

The Wow! signal had a
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
of less than . It is considered
narrowband Narrowband signals are signals that occupy a narrow range of frequencies or that have a small fractional bandwidth. In the audio spectrum, ''narrowband sounds'' are sounds that occupy a narrow range of frequencies. In telephony, narrowband is ...
emission in the sense that its fractional bandwidth was relatively small (~1%). However, the bandwidth is not small compared to the bandwidth of some astrophysical masers (~) or to the frequency resolution of modern narrowband SETI searches (~). The Big Ear telescope was equipped with a receiver capable of measuring fifty -wide channels. The output from each channel was represented in the computer printout as a column of alphanumeric intensity values. The Wow! signal is essentially confined to one column.


Time variation

At the time of the observation, the Big Ear radio telescope was only adjustable for
altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...
(or height above the horizon), and relied on the
rotation of the Earth Earth's rotation or Earth's spin is the rotation of planet Earth around its own axis, as well as changes in the orientation of the rotation axis in space. Earth rotates eastward, in prograde motion. As viewed from the northern polar star ...
to scan across the sky. Given the speed of Earth's rotation and the spatial width of the telescope's observation window, the Big Ear could observe any given point for just 72 seconds. A continuous extraterrestrial signal, therefore, would be expected to register for exactly 72 seconds, and the recorded intensity of such signal would display a gradual increase for the first 36 seconds—peaking at the center of the observation window—and then a gradual decrease as the telescope moved away from it. All these characteristics are present in the Wow! signal.


Celestial location

The precise location in the sky where the signal apparently originated is uncertain due to the design of the Big Ear telescope, which featured two feed horns, each receiving a beam from slightly different directions, while following Earth's rotation. The Wow! signal was detected in one beam but not in the other, and the data was processed in such a way that it is impossible to determine which of the two horns received the signal. There are, therefore, two possible
right ascension Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the equinox (celestial coordinates), March equinox to the (hour circle of the) point in questio ...
(RA) values for the location of the signal (expressed below in terms of the two main reference systems): In contrast, the
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. The declination angle is measured north (positive) or ...
was unambiguously determined to be as follows: 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 Center The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a ...
. 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 ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...
. The closest easily visible star is Tau Sagittarii. Initially, no nearby Sun-like stars were known to lie within the antenna coordinates, although in any direction the antenna pattern would encompass about six distant Sun-like stars as estimated in 2016. In 2022, a paper published in the '' International Journal of Astrobiology'' identified three likely Sun-like stars within the antenna-pointed coordinates. The better characterized star, 2MASS 19281982-2640123, is located 1,800 light years away, only 132 light years away from Maccone's estimation where an intelligent civilization is more likely to exist. The other two candidates, 2MASS 19252173-2713537 and 2MASS 19282229-2702492, were insufficiently characterized but still likely to be Sun-like stars. Also, 14 other catalogued stars at the antenna coordinates may still turn out to be similar to the Sun after more data becomes available. As a response to the discovery, Breakthrough Listen conducted the first targeted search for the Wow! Signal in its first collaboration between the Green Bank Telescope and the Allen Telescope Array of the SETI Institute. The observations were performed on May 21, 2022, lasting 1 hour from Greenbank, 35 minutes from ATA, and 9 minutes and 40 seconds simultaneously. No technosignature candidates were found.


Hypotheses on the signal's origin

Interstellar of a weaker
continuous signal In mathematical dynamics, discrete time and continuous time are two alternative frameworks within which variables that evolve over time are modeled. Discrete time Discrete time views values of variables as occurring at distinct, separate "poi ...
—similar in effect to atmospheric
twinkling Twinkling, also called scintillation, is a generic term for variations in apparent brightness, colour, or position of a distant luminous object viewed through a medium.Wang, Ting-I; Williams, Donn"Scintillation technology bests NIST" , ''InTec ...
—could be an explanation, but that would not exclude the possibility of the signal being artificial in origin. The significantly more sensitive
Very Large Array The Karl G. Jansky Very Large Array (VLA) is a centimeter-wavelength radio astronomy observatory in the southwestern United States built in the 1970s. It lies in central New Mexico on the Plains of San Agustin, between the towns of Magdalena, Ne ...
did not detect the signal, and the probability that a signal below the detection threshold of the Very Large Array could be detected by the Big Ear due to interstellar scintillation is low. Other hypotheses include a rotating lighthouse-like source, a signal sweeping in frequency, or a one-time burst. Ehman said in 1994: "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 Space debris (also known as space junk, space pollution, space waste, space trash, space garbage, or cosmic debris) are defunct human-made objects in spaceprincipally in Earth orbitwhich no longer serve a useful function. These include dere ...
." He later somewhat recanted his skepticism, after further research showed the unrealistic requirements that a space-borne reflector would need to have to produce the observed signal. The signal's frequency of is also part of a protected spectrum: a frequency range reserved for astronomical research in which terrestrial transmissions are forbidden, although a 2010 study documented several instances of terrestrial sources either interfering from adjacent frequency bands or illegally transmitting within the spectrum. In a 1997 paper, Ehman resists "drawing vast conclusions from half-vast data"—acknowledging the possibility that the source may have been military or otherwise a product of Earth-bound sources. In a 2019 interview with John Michael Godier, Ehman stated: "I'm convinced that the Wow! signal certainly has the potential of being the first signal from extraterrestrial intelligence."
METI Meti is a town in south-western Ethiopia, located in Godere woreda of the Gambela Region The Gambela Region, also spelled Gambella, and officially the Gambela Peoples' Region (), is a regional state in western Ethiopia. Previously known as ...
president Douglas Vakoch told that any putative SETI signal detections must be replicated for confirmation, and the lack of such replication for the Wow! signal means it has little credibility. In August 2024, the Planetary Habitability Laboratory published a preprint reporting observations made in 2020 at the
Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
in Puerto Rico—where they conclude that the Wow! signal was likely caused by a rare astrophysical event, in which stellar emissions energizing a cold hydrogen cloud caused it to suddenly surge in brightness.


Discredited hypotheses

In 2017, Antonio Paris, Assistant Professor of Astronomy and Astrophysics at St. Petersburg College, Florida, proposed that the hydrogen cloud surrounding two
comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...
s, 266P/Christensen and 335P/Gibbs, now known to have been in the same region of the sky, could have been the source of the Wow! signal. This hypothesis was dismissed by astronomers, including members of the original Big Ear research team, as the cited comets were not in the beam at the correct time. Furthermore, comets do not emit strongly at the frequencies involved, and there is no explanation for why a comet would be observed in one beam but not in the other.


Searches for recurrence of the signal

Several attempts were made by Ehman and other astronomers to recover and identify the signal. The signal was expected to occur three minutes apart in each of the telescope's feed horns, but that did not happen. Ehman unsuccessfully searched for recurrences using Big Ear in the months after the detection. In 1987 and 1989, Robert H. Gray searched for the event using the META array at
Oak Ridge Observatory The Oak Ridge Observatory (ORO, code: 801), also known as the George R. Agassiz Station, is located at 42 Pinnacle Road, Harvard, Massachusetts. It was operated by the Center for Astrophysics Harvard & Smithsonian as a facility of the Smit ...
, but did not detect 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 drift-scan observations of the Wow! signal's coordinates with a 12-meter radio telescope at the
National Radio Astronomy Observatory The National Radio Astronomy Observatory (NRAO) is a federally funded research and development center of the United States National Science Foundation operated under cooperative agreement by Associated Universities, Inc. for the purpose of radi ...
in Green Bank, West Virginia, also achieving a
null result In science, a null result is a result without the expected content: that is, the proposed result is absent. It is an experimental outcome which does not show an otherwise expected effect. This does not imply a result of zero or nothing, simply a res ...
. In 1995 and 1996, Gray again searched for the signal using the
Very Large Array The Karl G. Jansky Very Large Array (VLA) is a centimeter-wavelength radio astronomy observatory in the southwestern United States built in the 1970s. It lies in central New Mexico on the Plains of San Agustin, between the towns of Magdalena, Ne ...
, which is significantly more sensitive than Big Ear. Gray and Simon Ellingsen later searched for recurrences of the event in 1999 using the 26-meter radio telescope at the
University of Tasmania The University of Tasmania (UTAS) is a public research university, primarily located in Tasmania, Australia. Founded in 1890, it is Australia's fourth oldest university. Christ College (University of Tasmania), Christ College, one of the unive ...
's
Mount Pleasant Radio Observatory The Mount Pleasant Radio Observatory is a radio astronomy, radio-astronomy-based observatory owned and operated by the University of Tasmania, located 20 km east of Hobart in Cambridge, Tasmania. It is home to three radio astronomy antennas ...
. Six 14-hour observations were made at positions in the vicinity, but nothing like the Wow! signal was detected.


Response

In 2012, on the 35th anniversary of the Wow! signal,
Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
beamed a digital stream towards Hipparcos 34511, 33277, and 43587. The transmission consisted of approximately 10,000 Twitter messages solicited for the purpose by the
National Geographic Channel National Geographic (formerly National Geographic Channel; abbreviated and trademarked as Nat Geo or Nat Geo TV) is an American pay television network and flagship channel owned by the National Geographic Global Networks unit of Disney Enter ...
, bearing the
hashtag A hashtag is a metadata tag operator that is prefaced by the hash symbol, ''#''. On social media, hashtags are used on microblogging and photo-sharing services–especially Twitter and Tumblr–as a form of user-generated tagging that enable ...
"#ChasingUFOs" (a promotion for one of the channel's TV series). The sponsor also included a series of video vignettes featuring verbal messages from various celebrities. To increase the probability that any extraterrestrial recipients would recognize the signal as an intentional communication from another intelligent life form, Arecibo scientists attached a repeating-sequence header to each individual message, and beamed the transmission at roughly 20 times the power of the most powerful commercial radio transmitter.


In popular culture

The signal was featured in the 2024 television series ''3 Body Problem'', in which the signal is also detected in
Inner Mongolia Inner Mongolia, officially the Inner Mongolia Autonomous Region, is an Autonomous regions of China, autonomous region of China. Its border includes two-thirds of the length of China's China–Mongolia border, border with the country of Mongolia. ...
.


See also

*
List of interstellar radio messages This is a list of notable Interstellar communication, interstellar radio messages (IRMs) transmitted from Earth. Many of these were sent in the aim of Search for extraterrestrial intelligence, searching for extra-terrestrial intelligence (SETI), a ...
* * BLC1 (Breakthrough Listen Candidate 1), signal detected in 2019 * * * *
Radio source SHGb02+14a Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected to ...
* Oh-My-God particle


References


External links


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{{Portal bar, Astronomy, Stars, Outer space 1977 in Ohio 1977 in science August 1977 in the United States IBM 1130 Ohio State University Radio spectrum Sagittarius (constellation) Search for extraterrestrial intelligence Unsolved problems in astronomy