The Wow! Signal: 72 Seconds That Changed Everything

Classification: THE WATCHERS | Confidence: ASTRONOMY — UNEXPLAINED 1977 SIGNAL

On the night of August 15, 1977, the Big Ear radio telescope at Ohio State University was sweeping a narrow band of the radio spectrum looking for extraterrestrial intelligence. The telescope was a fixed structure — half a football field of metal mesh anchored to a concrete pad, designed to be a passive collector as the Earth’s rotation moved the sky across it. The telescope was not pointed at anything in particular. The telescope was pointed at everything in a particular direction. The data came in as a stream of numbers, printed on reams of continuous-feed printer paper, three columns to the inch, in a continuous strip across the floor of the operator’s office. The operator was a young astronomer named Jerry Ehman.

Ehman was reading the printer output. The signal was, by every standard the SETI community uses, anomalous. It lasted 72 seconds — the maximum duration a source could remain in the telescope’s beam as the Earth rotated. It was at 1420 MHz — exactly the hydrogen line frequency, the most logical frequency for any civilization trying to be noticed. It was in a part of the sky away from the galactic plane, where interference from natural sources would be low. It was approximately 30 times stronger than the background noise. It never repeated. Ehman circled the printout in red pen and wrote “Wow!” on it. The circle and the word are still visible on the printout. The printout is the most famous single page of data in the history of radio astronomy.

The Wow! Signal has been re-examined for 48 years. It has been re-analyzed, re-observed, re-modeled, re-debunked, and re-mystified. It is the most famous unexplained radio transient in scientific history. It has been explained by comet emissions (Antonio Paris, 2017), by reflected terrestrial signals, by satellite glints, by meteor trails, by instrumental artifact. Each explanation has been refuted. The signal has not been refuted. The signal has also not been confirmed. The signal has not been seen again. The signal is, after 48 years, exactly what it was on the night Ehman wrote on the printout: a single, narrow-band, hydrogen-line, off-galactic-plane, 72-second, strong radio transient from a part of the sky with no known source.

The Cocconi-Morrison Paper

The Wow! Signal is the kind of signal SETI was founded to find. The SETI project itself began with a 1959 paper in Nature by the physicists Giuseppe Cocconi and Philip Morrison, titled “Searching for Interstellar Communications.” The paper made a simple argument: any civilization attempting to signal its existence to other civilizations would pick a frequency that other civilizations would also be listening to. The most logical frequency is the 21-cm line of neutral hydrogen, at 1420 MHz. Hydrogen is the most abundant element in the universe. Hydrogen’s emission line is the most prominent natural radio frequency. Any civilization with radio astronomy would have noticed hydrogen’s emission line. Any civilization wanting to be noticed would transmit at that frequency. The paper proposed that SETI searches should focus on 1420 MHz.

Eighteen years later, the Big Ear telescope — a SETI project funded by Ohio State — recorded a signal at 1420 MHz. The signal was exactly at the frequency Cocconi and Morrison had predicted. The signal was strong. The signal was narrow-band. The signal was not natural. The signal did not repeat. The signal is the closest any SETI project has come to detecting an extraterrestrial civilization. The signal also, by SETI’s own standards, cannot be considered a detection. The signal has not repeated. The signal cannot be verified. A single non-repeating signal is not a detection. A single non-repeating signal is a candidate. The candidate is still on the table. The candidate has been on the table for 48 years.

The 2017 Paris Comet Hypothesis

In 2017, the astronomer Antonio Paris of St. Petersburg College proposed that the Wow! Signal was actually the emission from a hydrogen cloud surrounding a comet. The comets 266P/Christensen and P/2008 Y2 (Gibbs) were both in the same part of the sky as the Wow! Signal in August 1977. Both comets are surrounded by clouds of neutral hydrogen. The hydrogen emits radio at 1420 MHz. The emission would be narrow-band. The emission would be transient. The hypothesis was testable: point a radio telescope at the comets in their next close approach and see if the 1420 MHz emission recurs.

Paris and a collaborator did test it. In 2016-2017, they pointed the Star Scan-1 telescope at the two comets. The comets were at different parts of their orbits than in 1977. The comets did emit 1420 MHz radiation. The radiation was weaker than the Wow! Signal. The hypothesis was not confirmed. The hypothesis was not refuted. The hypothesis was not falsifiable in any straightforward way — the comets in 1977 could have emitted radiation of the right intensity; the comets in 2017 could not be used to verify this. The hypothesis remains one possibility. The hypothesis has been criticized on the grounds that the comets’ predicted positions in 1977 do not match the direction of the signal as accurately as the hypothesis requires. The criticism is not conclusive. The hypothesis is not conclusive.

The Arguments Against Earth-Based Sources

Most explanations of the Wow! Signal invoke terrestrial sources: satellite transmissions, terrestrial interference, aircraft reflections, instrumentation error. Each has been examined and excluded.

The signal was not at any known satellite transmission frequency at the time. The signal was at exactly 1420.405 MHz. No terrestrial transmitter in 1977 was broadcasting at that frequency. The signal was in a part of the spectrum specifically reserved by international agreement for astronomical observation — the protected band. Terrestrial transmitters were not permitted in the protected band. The Big Ear telescope operated in the protected band specifically to avoid terrestrial interference.

The signal was not consistent with aircraft reflections. Aircraft reflections of terrestrial signals produce Doppler-shifted copies of the source signal. The Wow! Signal was not Doppler-shifted. The signal’s frequency was stable to within 10 Hz over the 72 seconds of observation. Doppler shift from an aircraft would have produced frequency variation of at least several hundred Hz.

The signal was not consistent with satellite glints. A satellite glint would have been brief — typically less than a second as the satellite passed through the antenna beam. The Wow! Signal was 72 seconds long. The signal was also modulated by the antenna’s beam pattern in a way that is consistent with a fixed celestial source and inconsistent with a moving satellite.

The signal was not consistent with instrumentation error. The Big Ear telescope had been operating for years before the Wow! Signal. The telescope had never produced a similar anomaly. The telescope’s electronics were redesigned in the late 1970s. The Wow! Signal was the only signal of its kind ever recorded by the original telescope.

The Pattern of the Single Signal

The Wow! Signal is the only signal of its kind. SETI has been running for 65 years. SETI has used dozens of telescopes. SETI has surveyed millions of stars. SETI has never recorded another signal like the Wow! Signal. SETI has recorded thousands of candidate signals. None of the candidates have the Wow! Signal’s combination of strength, narrow bandwidth, frequency accuracy, and absence of repetition. The Wow! Signal is, by the SETI community’s own metrics, the closest the field has come to a detection. The Wow! Signal is also, by the SETI community’s own standards, not a detection. A single signal is a candidate. A candidate is not a detection. The SETI community has not declared a detection. The SETI community has not retracted the candidate. The candidate is still on the table.

The pattern is the pattern of the single signal. The pattern is also the pattern of single signals in other fields. The pattern of fast radio bursts (FRBs) — extremely bright, millisecond-duration radio transients from distant galaxies — is similar. The first FRB was recorded in 2007 by the Parkes telescope. The Lorimer Burst, as it was called, was initially treated as instrumental artifact. Subsequent FRBs were recorded. The pattern of single signals became a pattern of multiple signals. The FRBs are now known to be real astrophysical events. The causes of the FRBs are still debated. The Wow! Signal has not yet entered this pattern. The Wow! Signal remains single. The pattern of single signals does not yet include the Wow! Signal. The pattern may include the Wow! Signal if the Wow! Signal repeats. The pattern does not include the Wow! Signal as long as the Wow! Signal does not repeat. The Wow! Signal has not repeated. The pattern is the same pattern.

The Pattern of the Printout

The printout is still in the archives at Ohio State. The printout has been photographed, scanned, and re-photographed. The red circle and the word “Wow!” are visible in every photograph. The printout is the document. The printout is also a record of how rare the event was. Three columns to the inch. Continuous feed. Multiple hours of observation compressed into a single strip of paper. The strip would have to be scanned for hundreds of meters to find any single anomaly. The anomaly was found by a single astronomer in real time. The astronomer circled it. The astronomer wrote on it. The circle and the word are still visible. The signal is still on the table. The pattern of single signals is still the pattern. The pattern has been running for 48 years. The pattern has not changed.

SOURCES

• Giuseppe Cocconi and Philip Morrison (1959). “Searching for Interstellar Communications.” Nature, 184(4690).
• Jerry R. Ehman (1977). The “Wow! Signal” — original computer printout, August 15, 1977, Big Ear Radio Observatory, Ohio State University. Document preserved at the Ohio State University Radio Observatory archives.
• John D. Kraus (1979). “The Wow! Signal.” Cosmic Search Magazine, 1(1).
• Robert H. Gray and Kevin B. Marvel (2001). “A Search of the Sky for the Wow! Signal.” Astrophysical Journal, 546(2).
• Antonio Paris (2017). “Hydrogen Clouds from Comets 266P/Christensen and P/2008 Y2 (Gibbs) Are Candidates for the Source of the 1977 ‘Wow!’ Signal.” Journal of the Washington Academy of Sciences, 103(2).
• James M. Cordes et al. (2017). “The Magnetosphere of the Galaxy and the Origin of Fast Radio Bursts.” Bulletin of the American Astronomical Society, 229.

Sources & Further Reading

• Google Scholar — Academic research database