The Wow! Signal

On August 15, 1977, a radio telescope in Ohio detected a signal that appeared to be exactly what SETI researchers had been searching for—a powerful, narrowband transmission from the direction of the constellation Sagittarius, bearing all the characteristics expected of a deliberate extraterrestrial broadcast. Astronomer Jerry Ehman circled the anomalous data and wrote “Wow!” in the margin. Despite decades of searching, the signal has never repeated. It remains the strongest candidate for extraterrestrial communication ever recorded.

The Detection

Big Ear Radio Telescope

The Big Ear radio telescope at Ohio State University had been conducting SETI (Search for Extraterrestrial Intelligence) observations since 1973. The telescope scanned the sky automatically, recording data for later analysis.

August 15, 1977

The Signal: At 11:16 PM Eastern Time, Big Ear recorded a signal unlike anything in its four years of operation:

• Duration: 72 seconds (the maximum time any source could be observed by Big Ear)
• Frequency: 1420.4556 MHz—the emission frequency of hydrogen
• Bandwidth: Less than 10 kHz (extremely narrow)
• Intensity: 30 times louder than background noise
• Location: Chi Sagittarii region

The Discovery: Astronomer Jerry Ehman reviewed the computer printout several days later. When he saw the data sequence—6EQUJ5—he immediately recognized its significance. He circled the sequence and wrote “Wow!” in the margin.

The name stuck.

Why It Matters

The Perfect Signal

The Wow! signal matched SETI predictions exactly:

1420 MHz: This is the “hydrogen line”—the frequency at which neutral hydrogen atoms emit radiation. SETI researchers had long theorized that intelligent civilizations might broadcast at this frequency because:

• Hydrogen is the most abundant element in the universe
• Any technological civilization would know this frequency
• It’s relatively quiet (less interference than other frequencies)
• It’s a universal “meeting point” for interstellar communication

Narrowband: Natural radio sources produce broadband emissions across many frequencies. The Wow! signal was extremely narrow—characteristic of artificial transmission.

Non-repeating: A beacon would likely pulse or scan the sky. The Wow! signal appeared once—either a single broadcast, a rotating beacon that hasn’t pointed our way again, or a signal we only caught part of.

Location: The direction (Sagittarius constellation) is toward the galactic center—a reasonable location for a civilization wanting to broadcast to many star systems.

What Could It Be?

Extraterrestrial Hypothesis

The possibility that SETI exists to find:

Arguments For:

• The signal has all expected characteristics of ET communication
• The frequency choice is logical for interstellar contact
• No natural phenomenon reproduces all characteristics
• The statistical probability of a random signal matching all criteria is extremely low

Arguments Against:

• The signal never repeated despite extensive searches
• We haven’t found other signs of civilization in that direction
• A beacon would logically repeat
• 72 seconds is a short window to establish communication

Natural Explanations

Interstellar Scintillation: Like stars twinkling, radio signals can vary due to interstellar medium effects. Could natural signals be briefly amplified?

Problem: Doesn’t explain the narrow bandwidth or specific frequency.

Hydrogen Cloud: Perhaps an unusual hydrogen cloud produced the signal naturally.

Problem: Hydrogen clouds don’t produce such narrow, intense signals.

Human Origin Explanations

Secret Satellite: Perhaps a classified satellite transmitted at 1420 MHz.

Problem: Transmitting at the protected hydrogen frequency would be illegal and impractical.

Terrestrial Interference: Local human radio sources can contaminate astronomical observations.

Problem: Big Ear’s location and methods were designed to filter such interference; the signal came from the sky.

The Comet Hypothesis (2017)

Astronomer Antonio Paris proposed that comets 266P/Christensen and P/2008 Y2 (Gibbs) might have produced the signal—their hydrogen comas emitting at 1420 MHz.

The Theory: These comets were in the right part of the sky in August 1977.

The Critique: Many astronomers dispute this explanation:

• The comets weren’t identified until decades later
• Comet emissions wouldn’t be as narrow or intense
• The hypothesis doesn’t fully account for the signal characteristics
• Other astronomers observing comets haven’t reproduced such signals

The comet theory remains controversial and is not widely accepted.

The Search Continues

Follow-Up Observations

Since 1977, astronomers have repeatedly searched the Wow! signal’s origin point:

Searches:

• Big Ear observed the region hundreds of times
• Other radio telescopes have monitored the area
• SETI@home processed signals from the region
• Modern facilities have conducted targeted surveys

Results: The signal has never repeated. Whatever produced it hasn’t broadcast again—at least not when we were listening, at that frequency, from that direction.

The Implications

If Extraterrestrial: The Wow! signal represents proof that we’re not alone—that somewhere in the cosmos, intelligence exists that can build radio transmitters.

If Natural: An unknown astronomical phenomenon exists that can mimic artificial signals—scientifically interesting in its own right.

If Neither: Some mundane explanation we haven’t identified accounts for the data.

Legacy

SETI’s Holy Grail

The Wow! signal remains:

• The strongest SETI candidate ever detected
• Unexplained after nearly 50 years
• The standard against which other signals are measured
• Motivation for continued searching

Cultural Impact

The signal has influenced:

• Popular culture’s image of first contact
• Scientific discussion of interstellar communication
• Public interest in SETI research
• Philosophy about our place in the universe

The Question

On a summer night in 1977, something from the direction of Sagittarius sent a signal that looked exactly like what we’d expect from an extraterrestrial civilization.

It lasted 72 seconds. It never came again. We still don’t know what it was.

Somewhere in the universe, something might have called out. We might have heard it. And we’ve been listening ever since, hoping—and fearing—that it might call again.

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“Wow!” Jerry Ehman wrote on the printout. One word to capture humanity’s closest brush with proof that we’re not alone. A 72-second signal from the stars that bore every hallmark of intelligent origin. It came once, from the direction of the galactic center, at the frequency of hydrogen—the universal element. Then silence. For nearly fifty years, we’ve listened. The cosmos hasn’t spoken again.