How the 'Wow!' Signal Works

By: Dave Roos  | 
wow signal
Jerry Ehman's original writing next to the sequence that seemed to suggest an extraterrestrial communication. Today it's known as the Wow! signal. SETI

Thanks to NASA's Kepler mission, astronomers have identified more than 5,000 exoplanets in our tiny corner of the Milky Way galaxy alone [source: NASA]. Multiply those 5,000 potentially inhabitable planets by the vastness of space and time, and it seems inconceivable that we are the only intelligent life-forms in creation.

Are we alone in the universe? This tantalizing question is what launched the first serious scientific search for extraterrestrial intelligence (SETI) in the 1970s. In 1973, the Ohio State University Radio Observatory — known as "Big Ear" — began scanning the skies for the slightest traces of extraterrestrial transmissions, blips in the deafening silence of space [source: Kawa]. And on an August night in 1977, the first astonishing signal they received wasn't a whimper, but a roar.


Jerry Ehman was an Ohio State professor volunteering with the Big Ear SETI experiment that summer in 1977 [source: Krulwich]. Every couple of days, a bike messenger would arrive at Ehman's office with a pile of printouts generated by the telescope's mainframe computer. Ehman's thankless job was to scan the mind-numbing numbers for anomalies, anything that stood out from the constant low hum of background radiation.

On Aug. 18, 1977, Ehman was scanning readouts from three days earlier when he came across something radically different. Instead of the usual 1s and 2s and occasional 4s, there was a stream of both letters and numbers signaling a radio transmission 30 times louder than the background buzz of deep space [source: Krulwich]. Grabbing a red pen — he was a teacher, after all — Ehman circled the mysterious sequence "6EQUJ5" and excitedly scribbled next to it the single word "Wow!"

More than 35 years later, the so-called "Wow!" signal remains the "closest encounter" mankind has ever had with what may or may not be an alien species. The powerful blast of radio waves lasted just 72 seconds, but many astronomers and amateur Ufologists believe the unique characteristics of the signal point to a celestial origin. In the decades since that original Wow! moment, no one has been able to replicate the signal or identify its definitive source, cosmic or Earthly.


How SETI Works

This diagram compares the planets of our inner solar system to Kepler-186, a five-planet star system about 500 light-years from Earth in the constellation Cygnus. The five planets of Kepler-186 orbit an M dwarf, a star that is half the size and mass of the sun. NASA/JPL-Caltech/T. Pyle

The nearest planet that is similar in size to Earth and located within the narrow habitable zone of its star is unromantically named Kepler-186f. If there is life on this planet, none of us will ever know. That's because Kepler-186f is 493 light-years away [source: NASA].

When the SETI began in the 1960s, astronomers quickly dismissed the idea of physically visiting an alien planet. The technological advances necessary to shoot humans across the galaxy is, like the nearest habitable planet, still light-years away.


Instead, the SETI sciences decided to stay on Earth, but keep an ear on the heavens. If intelligent life is out there, SETI decided, then it must have an understanding of radio waves and the electromagnetic spectrum. Like us, the alien species probably doesn't have unlimited energy resources to travel around the universe looking for friends. The most efficient way to say, "Hello, universe. We're here!" is to send a radio transmission.

The next question for SETI scientists was where to listen? The best guess was promoted by two Cornell physicists in the early 1960s, Philip Morrison and Guiseppi Cocconi. The two men assumed that an extraterrestrial life-form intelligent enough to master the electromagnetic spectrum would try to craft its message in a "common language" that anyone could understand [source: Selle].

The most common electromagnetic frequency, Morrison and Cocconi reasoned, is emitted by the most common element in the universe, hydrogen. If an alien was trying to communicate with us over an open channel, it would choose 1420 megahertz, also known as the "hydrogen line."

And so began the search for alien life. Using large radio telescopes, astronomers focus on one tiny patch of sky and listen for the faintest sign of an unusual transmission coming over the 1420 MHz frequency. After listening for a few minutes, the telescope moves on to the next tiny patch of sky, and so on and so on [source: Andersen].

And that's exactly what Jerry Ehman and other SETI volunteers were doing with the Big Ear telescope at Ohio State back in the summer of 1977. They were listening to a sliver of the sky near the constellation Sagittarius and measuring the strength of the signal picked up on the 1420 MHz channel.

Ehman and others had been at it for years, always receiving the same 1s and 2s of normal background radiation, until Aug. 15, when the Big Ear picked up a startling signal that would echo through the decades.

Next we'll find out why the "Wow!" signal makes such a great case for being a message from ET.


The Case for the 'Wow!' Signal

For 72 seconds on Aug. 15, 1977, the Big Ear radio telescope picked up a signal that was 30 times as loud as the normal background noise. But what makes this signal worthy of Jerry Ehman's famous "Wow!"? Why does it look to many astronomers like a message from an alien planet?

First, it has to do with the hydrogen line. The frequency of the "Wow!" signal was recorded as 1420.4556 MHz, almost exactly the electromagnetic wavelength of hydrogen [source: Krulwich]. If an alien species were to choose a single frequency to broadcast a long-range message, SETI scientists concluded, that's the one.


The second striking characteristic of the "Wow!" signal is its "shape." The shape of a radio signal describes how it would look if graphed over time.

When the "Wow!" signal was first detected by Big Ear, it registered as a 6 on the telescope's "loudness" scale. A few seconds later, it jumped to an "E" (the computer could only report single digits, so when a number exceeded 9, it switched to letters). The signal peaked at "U" (the equivalent of the No. 30), then it slowly decreased back to 5. Plotting the signal on a graph, you get a nearly symmetrical pyramid shape.

Why does the signal's shape matter? Because it matches the shape you would expect from a deep space source. Here's why [source: Andersen]:

  • A radio telescope is located on Earth's surface
  • As Earth rotates, the telescope's focal range slowly drifts across the sky
  • If the origin of a radio signal is a fixed point in deep space, the signal will appear weak as it first enters the telescope's range
  • When the telescope points straight at the source, the signal will be the loudest
  • As the telescope drifts out of range of the signal, it decreases again, hence the pyramid shape

Another tantalizing characteristic of the "Wow!" signal was the sharpness of the transmission. When a radio telescope receives electromagnetic waves from a natural cosmic source, like a quasar, the radio waves are diffused over a band of frequencies.

Not the "Wow!" signal. The Big Ear telescope was listening on 50 different channels, not just 1420 MHz, and none of those other radio channels registered a blip [source: Andersen]. To many SETI scientists, this is a clear sign of an intentional radio transmission from a distant world, not an accidental cosmic event.

Next we'll hear what the skeptics say, and what we've discovered in the 35 years since we first heard the "Wow!"


The Case Against the 'Wow!' Signal

wow signal
This shows the location of the Wow! signal on a star map. There are two possible regions indicated here; the ambiguity is due to the way in which the original 1977 experiment was designed. Benjamin Crowell/Wikimedia Commons (CC BY-SA)

If you believe that we are not alone in the universe — or want to believe — then the "Wow!" signal offers thrilling proof that someone, somewhere, is trying to say "hello."

Then there's the bad news. In the more than three decades since Jerry Ehman circled the astonishing "6EQUJ5" on his printout, no SETI radio telescope has recorded anything like it since. The Big Ear even scanned the same patch of sky 100 more times, but found nothing [source: Gray and Marvel].


Robert Gray, an amateur astronomer and data analyst with a passion for the "Wow!" signal, conducted the most serious attempt to replicate the signal using one of the biggest and baddest radio telescopes on Earth, the Very Large Array (VLA) in New Mexico.

In 1995 and 1996, Gray aimed the VLA at Sagittarius, the first time the telescope was used expressly to search for signs of extraterrestrial life. The VLA — which combines the power of 27 separate radio antennas — is 100 times more sensitive than the Big Ear, which was retired in 1997 [sources: NRAO, Gray and Marvel].

Sadly, Gray found no trace of the "Wow!" signal with the VLA. But that wasn't enough to convince him that the original recording was some kind of glitch.

In a 2012 interview published in The Atlantic, Gray argued that our assumptions about extraterrestrial radio transmissions are all wrong. We imagine a constant beacon shining toward Earth from a distant planet. But the energy required to sustain such a broadcast — in all directions, at all times, across millions of light years — is equal to thousands upon thousands of our biggest power plants.

What if the alien civilization isn't a hyper-advanced race with limitless resources, but instead is more like us? The more economical approach would be to broadcast the signal from a type of radio "lighthouse" that transmits its message in only one direction at a time. If that's the case, then our current system of searching for alien life — focusing on one patch of sky for 20 minutes before moving on to the next — would require tremendous luck to catch the signal as it briefly flashes our way [source: Andersen].

Perhaps Gray was onto something. in 2021, amateur astronomer and founder of The Exoplanets Channel, Alberto Caballero, used that same hypothesis and focused exclusively on the constellation Sagittarius, knowing that the Big Ear telescope's two receivers were pointing in the direction of the constellation on the night of the Wow! signal. Using the European Space Agency's database of stars from the Gaia satellite to look for stars in that specific area, Caballero found one remarkably like our sun and thinks it could be the source of the Wow! signal.

The star, which is named as 2MASS 19281982-2640123, is about 1,800 light-years away and has a temperature, diameter and luminosity nearly identical to our sun. Caballero published his findings May 6, 2022, in International Journal of Astrobiology. "2MASS 19281982-2640123 could be, therefore, the only sun-like star found among the thousands of stars located in the Wow! signal region," the study says.

Case closed? Not even close. 2MASS 19281982-2640123 is located much too far to send any type of radio or light transmission reply, though the study says it could be a good target for further observations when searching for techno-signatures such as artificial light or satellite transits.

So, the Wow! signal mystery continues — for now.


Originally Published: Feb 17, 2015

Lots More Information

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  • Andersen, Ross. "The '"Wow!"' Signal: One Man's Search for SETI's Most Tantalizing Trace of Alien Life." The Atlantic. Feb. 16, 2012 (Feb. 4, 2015)
  • Caballero, Alberto. "An approximation to determine the source of the WOW! Signal." International Journal of Astrobiology. May 6, 2022. (May 27, 2022)
  • Gray, Robert H. and Marvel, Kevin B. "A VLA Search for the Ohio State 'Wow.'" The Astrophysical Journal. Jan. 10, 2001 (Feb. 4, 2015)
  • Kawa, Barry. "'Big Ear' designer a pioneer in field." Sept. 18, 1994. (May 27, 2022)
  • Krulwich, Robert. "Aliens Found in Ohio? The "Wow!" Signal." NPR. May 28, 2010 (Feb. 4, 2015)
  • NASA. "Exoplanet Exploration Program." (May 27, 2022)
  • NASA. "NASA's Kepler Discovers First Earth-Size Planet In The 'Habitable Zone' of Another Star." April 17, 2014. (May 27, 2022)
  • NASA. "NASA's Kepler Marks 1,000th Exoplanet Discovery, Uncovers More Small Worlds in Habitable Zones." Jan. 6, 2015 (Feb. 5, 2015)
  • National Radio Astronomy Observatory. "Welcome to the Very Large Array!" (Feb. 4, 2015)
  • Selle, Don. "The Wow! Signal." August 2012. (May 27, 2022)
  • "Humanity Responds to 'Alien' Wow Signal, 35 Years Later." Aug. 17, 2012 (Feb. 4, 2015)
  • Vergano, Dan. "Kepler Telescope Discovers Most Earth-Like Planet Yet." National Geographic. April 17, 2014 (Feb. 4, 2015)