The Signal

On August 15, 1977, at 11:16 p.m. Eastern time, the Big Ear radio telescope at Ohio State University detected a radio signal from space so unusual that volunteer astronomer Jerry Ehman circled the alphanumeric sequence on the computer printout and wrote “Wow!” in the margin.[1]

The signal has never been satisfactorily explained. Nearly 50 years later, it remains the strongest candidate for non-natural extraterrestrial radio contact in recorded history.

The Technical Details

The Big Ear telescope was part of the SETI (Search for Extraterrestrial Intelligence) program at Ohio State, scanning the sky for narrowband radio signals that might indicate artificial transmission.

The signal’s characteristics:

ParameterValueSignificance
Frequency~1420.456 MHzThe hydrogen line (1420 MHz) — the emission frequency of neutral hydrogen, considered the most logical frequency for interstellar communication
Bandwidth<10 kHzExtremely narrowband — natural radio sources are broadband; narrowband signals suggest artificial origin
Duration72 secondsConsistent with the telescope’s beam width for a fixed celestial source as Earth rotated
Intensity30σ above backgroundThe sequence “6EQUJ5” encoded the signal strength, peaking at approximately 30 times the normal background level
DirectionSagittarius constellationNear the star group Chi Sagittarii, roughly in the direction of the galactic centre
ModulationNone detectedA continuous, unmodulated carrier — no embedded message identified

Why These Parameters Matter

In 1959, physicists Giuseppe Cocconi and Philip Morrison published a foundational paper arguing that if extraterrestrial civilisations wanted to signal their presence, they would logically transmit at 1420 MHz — the hydrogen line — because:[2]

  1. Hydrogen is the most abundant element in the universe
  2. Any technologically advanced civilisation would know this frequency
  3. It is a “quiet” part of the radio spectrum with low natural background
  4. It represents a universal constant that any civilisation studying physics would independently discover

The Wow! signal was detected at exactly this frequency. It was narrowband. It was strong. It came from the sky. It matched, point by point, the predicted profile of an extraterrestrial beacon.

What Has Been Ruled Out

Terrestrial Interference

The Big Ear telescope had two feed horns, separated by about three minutes of sky time. The signal appeared in only one horn, which is consistent with a celestial source (it would pass through each horn sequentially as Earth rotated). However, it appeared in only the positive horn, not the negative — which has never been fully explained.

Terrestrial radio transmitters at 1420 MHz are prohibited — the hydrogen line is internationally protected for radio astronomy. While illegal transmissions are possible, a terrestrial signal strong enough to register at this intensity would likely have been detected by other instruments.[1]

Satellite Reflection

No known satellite was in the beam at the time. Satellite transmissions at 1420 MHz would violate international frequency allocations.

Aircraft

Aircraft do not produce narrowband 1420 MHz signals at this intensity.

The Comet Hypothesis (2017)

In 2017, astronomer Antonio Paris proposed that the signal could have been produced by hydrogen gas in the coma of a comet — specifically 266P/Christensen or 335P/Gibbs — that happened to be in the beam at the time.[3]

This hypothesis was widely criticised:

  • The comets in question were not confirmed to have been in the beam at the precise time
  • Cometary hydrogen emission is broadband, not the narrowband signal observed
  • The signal intensity (30σ) is orders of magnitude stronger than what a cometary hydrogen cloud would produce
  • Other radio astronomers noted that if comets produced signals this strong at 1420 MHz, they would be detected routinely — and they are not
  • Paris’s own follow-up observation of 266P/Christensen reportedly detected a signal, but the methodology was criticised as insufficiently controlled[4]

The comet hypothesis is considered unlikely by most radio astronomers who have reviewed it.

What Has Not Been Ruled Out

A One-Time Extraterrestrial Transmission

The signal matches the predicted profile of an ET beacon. The principal objection is that it was never detected again despite extensive follow-up observations.

Over 100 subsequent observations of the same region of sky — by the Big Ear telescope, the Very Large Array (VLA), and other instruments — have failed to detect a repeat.[5]

However, the non-repetition is not necessarily fatal to the ET hypothesis:

  • A rotating beacon (like a lighthouse) would only be detectable when aimed at Earth — the timing of subsequent observations may have missed the beam
  • A one-time or infrequent transmission would not repeat on human timescales
  • The Big Ear telescope had limited sensitivity and sky coverage — the signal was detected serendipitously, and the follow-up observations may not have been sufficient to detect a weaker or intermittent source

An Unknown Natural Phenomenon

It is possible that the signal was produced by a natural astrophysical process not yet identified. This is the default sceptical position. However, no known natural mechanism produces narrowband signals at this intensity at the hydrogen line frequency. Pulsars, quasars, and other energetic objects produce broadband emission, not the narrowband signature observed.

Instrumental Artefact

A malfunction or anomaly in the Big Ear telescope’s electronics could theoretically produce a false signal. However, the signal’s characteristics (celestial source behaviour, correct beam response, expected duration for a point source) argue against this.

Why It Matters

The Wow! signal occupies a unique position in the SETI archive:

  1. It is the only signal ever detected that matches the theoretical prediction for an extraterrestrial beacon
  2. It was detected by a legitimate scientific instrument during a systematic search
  3. It has survived nearly 50 years of attempted explanation without a satisfactory natural cause being identified
  4. Every proposed conventional explanation has significant problems

At the same time:

  1. It was detected only once — and a single detection cannot be verified
  2. It could represent an unknown natural phenomenon — the universe is full of things we haven’t catalogued
  3. No information content was extracted from the signal — if it was a message, we cannot read it
  4. SETI’s fundamental challenge — distinguishing a genuine ET signal from noise — has not been solved by this detection

The State of Play

The Big Ear telescope was demolished in 1998 to make way for a golf course — a fact that says something about institutional priorities.

The Wow! signal remains:

  • The strongest candidate for a non-natural extraterrestrial radio signal
  • Unexplained by any proposed natural mechanism
  • Unrepeated despite extensive follow-up
  • Insufficient on its own to constitute proof of extraterrestrial intelligence

It is, essentially, the most interesting 72 seconds in the history of radio astronomy — and it may remain so indefinitely.

Research Verdict

AssessmentUnexplained — strongest SETI candidate, but a single unrepeated detection
ConfidenceModerate
SummaryThe Wow! signal matched the predicted profile of an extraterrestrial transmission in every measurable parameter. No conventional explanation has survived scrutiny. However, a single unrepeated detection cannot constitute proof. It remains the most significant anomaly in SETI history.
The Wow! signal is exactly what scientists predicted an alien signal would look like. It is also exactly what a single anomalous data point looks like. Both statements are true, and the tension between them is the reason this signal still matters nearly 50 years later.

Sources

  1. Wikipedia — Wow! signal.
  2. Cocconi, G. & Morrison, P. (1959). “Searching for Interstellar Communications.” Nature, 184, 844–846.
  3. Paris, A. (2017). “Hydrogen clouds from comets 266/P Christensen and P/2008 Y2 (Gibbs) and the Wow! signal.” Journal of Atmospheric and Solar-Terrestrial Physics.
  4. SETI League — Critique of the comet hypothesis.
  5. Gray, R.H. & Marvel, K.B. (2001). “A VLA search for the Ohio State ‘Wow’ signal.” Icarus, 161(2), 252–257.