Wow! signal

Background

In 1973, after completing an extensive survey of extragalactic radio sources, The Ohio State University assigned the now-defunct Big Ear telescope, then located near the Perkins Observatory in Delaware, Ohio, to the scientific search for extraterrestrial intelligence (SETI), in what would have become the longest-running program of this kind in history.

Over a decade earlier, in a 1959 paper, Cornell physicists Philip Morrison and Giuseppe Cocconi had also speculated that any extraterrestrial civilization attempting to communicate via radio signals might choose to do so using a frequency of 1420 megahertz, which is naturally emitted by hydrogen, the most common element in the universe and therefore likely familiar to all technologically advanced civilizations.

By 1977, Ehman was working at the SETI project as a volunteer; his job involved analysing by hand large amounts of data processed by an IBM 1130 mainframe computer and printed on perforated paper. While perusing data collected on August 15 at 22:16 EDT (02:16 UTC), Ehman spotted a series of values of signal intensity and frequency that left him and his colleagues astonished.

Signal measurement

The alphanumeric sequence circled by Ehman, 6EQUJ5, represents the intensity variation of the radio signal over time, measured as unitless signal-to-noise ratio and ranging from 0 to 36, with the noise averaged over the previous few minutes. Each individual character corresponds to a sample of the signal, taken every 12 seconds. A whitespace character on the printout denotes an intensity between 0 and 1; the numbers "1" to "9" denote the correspondingly numbered intensities (from 1 to 9); intensities of 10 and above are indicated by a letter: "A" corresponds to intensities between 10 and 11, "B" to 11 to 12, and so on. The highest measured value was "U" (an intensity between 30 and 31), that is thirty times stronger than normal background noise.

A common misconception is that the Wow! signal constitutes some sort of message. In fact, what was received appears to be an unmodulated, continuous wave signal with no encoded information; essentially a flash of radio energy. The string "6EQUJ5" is merely the representation of the expected variation of signal intensity over time, expressed in the particular measuring system adopted for the experiment.

Frequency

Two different values for the signal's frequency have been given: 1420.36 MHz (J. D. Kraus) and 1420.46 MHz (J. R. Ehman), both very close to the value of 1420.41 MHz of the hydrogen line, as predicted by Morrison and Cocconi. The two values are in fact the same distance apart from the hydrogen line – the first 0.04975 MHz (49.75 kHz) below and the second 0.04985 MHz (49.85 kHz) above.

Ehman analyzed the discrepancy between the two published frequencies. He concluded that an oscillator, which became the first local oscillator (LO), was ordered for the frequency of 1450.4056 MHz. However, the university's purchasing department made a typographical error in the order and wrote 1450.5056 MHz (i.e., 0.1 MHz 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 into account this error. Thus, he has also concluded that Kraus did not account for it. Hence, the correct value for the frequency of the Wow! signal is 1420.46 MHz.

Time variation

At the time of the observation, the Big Ear radio telescope was only adjustable for declination (or height above the horizon), and relied instead on the rotation of the Earth 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. All these characteristics are present in the Wow! signal.

Bandwidth

The Wow! signal was a narrowband emission: its bandwidth was less than 10 kHz. The Big Ear telescope was equipped with a receiver capable of measuring fifty 10 kHz-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.

Celestial location

The precise location in the sky where the signal apparently originated is uncertain due to the Big Ear telescope's design, which featured two feed horns, each pointing in a slightly different direction, while following Earth's rotation. The Wow! signal was detected by one of the horns but not by 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 (RA) values for the location of the signal (expressed below in terms of the two main reference systems):

In contrast, the declination was unambiguously determined to be as follows:

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.

Hypotheses on the signal's origin

Interstellar scintillation of a weaker continuous signal—similar in effect to atmospheric twinkling—could be an explanation, but that would not exclude the possibility of the signal's being artificial in origin. But even the significantly more sensitive Very Large Array 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 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 recanted his skepticism somewhat, after further research showed an Earth-borne signal to be very unlikely, given the requirements of a space-borne reflector being bound to certain unrealistic requirements to sufficiently explain the signal. Also, it is problematic to propose that the 1420 MHz signal originated from Earth since this is within the "protected spectrum": a bandwidth reserved for astronomical purposes in which terrestrial transmitters are forbidden to transmit. 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 humans. However, Ehman thinks that the most likely explanation for the signal is from an extraterrestrial civilization.

In a 2012 podcast, scientific skeptic author Brian Dunning concluded that a radio transmission from deep space in the direction of Sagittarius, as opposed to a near-Earth origin, remains the best technical explanation for the emission, although there is no evidence to conclude that an alien intelligence was the source.

In a 2016 paper, Antonio Paris and Evan Davies proposed that the diffuse head of a comet could produce H I emission like the Wow! signal, and identified a pair of comets that were in the right area of the sky by extrapolating the orbits back to the 1977 date. The paper acknowledges that the comet-as-an-emission-source hypothesis has not been empirically tested, and that a potentially long-lived emission source does not explain why only one of the feed horns detected the Wow! signal. On January 7, 2017 335P/Gibbs was expected to pass through the star system, and on January 25, 2017 266P/Christensen is expected. Paris has embarked on a mission to test his theory.

Ehman's analysis of the Paris and Davies paper indicates that it is highly unlikely that either of the two comets could have been the cause/source of the Wow! signal. Ehman's colleagues at the Ohio State University Radio Observatory agree with his analysis and conclusions.

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, but did not detect it. In a July 1995 test of signal detection software to be used in its upcoming Project Argus search, 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 in Green Bank, West Virginia, also achieving a null result.

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 recurrences of the event in 1999 using the 26 m radio telescope at the University of Tasmania's Mount Pleasant Radio Observatory. Six 14-hour observations were made at positions in the vicinity, but nothing like the Wow! signal was detected.

Response

In 2012, the 35th anniversary of the Wow! signal, Arecibo Observatory beamed a digital stream toward the area of the signal's origin. The transmission consisted of approximately 10,000 Twitter messages solicited for the purpose by the National Geographic Channel, bearing the hashtag "#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 wattage of the most powerful commercial radio transmitter.

In popular culture

In a 2017 Super Bowl commercial spoofing conspiracy theories such as the moon landing hoax, Area 51, and subliminal advertising, the base of a stone monolith carries the inscription "6EQUJ5".