On August 15, 1977, a strong narrowband radio signal was received by the Big Ear radio telescope of the Ohio State University, United States, then assigned to a SETI project. The signal appeared to come from the constellation Sagittarius and bore the 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 the reading on the computer printout and wrote the comment Wow! on its side, which is how the event has since been referred to.
The entire signal sequence lasted for the full 72-second window that Big Ear was able to observe it, but has not been detected since, despite several subsequent attempts by Ehman and others.
Various hypotheses on the source of the emission have been put forward. Although the possibility of a natural origin has not been completely discounted, to date the Wow! signal is considered the best candidate for an alien radio transmission ever received.
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 familiar to all its inhabitants.
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 EST, Ehman spotted a series of values of signal intensity and frequency that left him and his colleagues astonished.
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 value "U" (an intensity between 30 and 31) was the highest detected by the radio telescope; on a linear scale it was over 30 times stronger than normal deep space.
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 typo in the order and wrote 1450.5056 MHz (i.e., 100 kHz 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.
Contrary to a common misconception, the Wow! signal is not actually a message. What was received appears to be an unmodulated, continuous wave signal, with no information encoded in it. The string "6EQUJ5" is simply the representation of the expected Gaussian distribution of signal intensity versus time.
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 the sky across. Given the speed of Earth's rotation and the 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 show 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.
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.
Location of the signal
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 to 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):
|B1950 equinox||J2000 equinox|
|RA (positive horn)||19h22m24.64s ± 5s||19h25m31s ± 10s|
|RA (negative horn)||19h25m17.01s ± 5s||19h28m22s ± 10s|
The declination, instead, was unambiguously determined to be as follows:
|B1950 equinox||J2000 equinox|
|dec||−27°03′ ± 20′||−26°57′ ± 20′|
The region of the sky in question lies northwest of the globular cluster of 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 Very Large Array level 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 2016 paper, Paris and Davies proposed that the diffuse head of a comet could produce HI 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 the hypothesis must be tested, including the detail that only one of the feed horns detected the signal which is an issue for any long-lived emission source.
Ehman has analyzed that paper by Paris and Davies. His analysis reveals that it is highly unlikely that either of the two comets could have been the cause 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 as well as by other astronomers to detect and identify the signal again. The signal was expected to appear three minutes apart in each of the telescope's feed horns, but that did not happen. Ehman unsuccessfully looked 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.
In 2012, on the 35th anniversary of the Wow! signal, Arecibo Observatory beamed a response from humanity, containing 10,000 Twitter messages, in the direction from which the signal originated. Arecibo scientists have attempted to increase the chances of intelligent life receiving and decoding the celebrity videos and crowd-sourced tweets by attaching a repeating sequence header to each message that will let the recipient know that the messages are intentional and from another intelligent life form.
- Arecibo message, a 3-minute-long message sent into space
- LGM-1, the first pulsar discovered, mistaken for an alien radio signal
- Radio signal from HD 164595
- KIC 8462852, "Tabby's star"
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