By Sam Jarman

The scribble that started it all | Image: Big Ear Radio Observatory, NAAPO

Monday was dragging for Jerry R. Ehman, in a way that only a volunteer astronomer at the SETI project could truly understand. The frequency data printed out by the IBM 1130 computer was, as always, infuriatingly ordinary. The stream of paper he needed to analyse by hand was seemingly endless. Coffee in a Styrofoam cup was slowly growing lukewarm. Then… something different.


On August 15 1977, Ohio State University’s Big Ear radio telescope picked up an intense and unexpected burst of radio waves, which lasted for a full 72 seconds. Stunned by the break in the monotony of his task, Ehman couldn’t help but briefly forget his deeply engrained skills in data analysis. Taking a bright red marker, he drew a circle around the numbers and letters indicating the burst, and wrote one word next to it: “Wow!

Ehman wasted no time in letting his fellow members of SETI know about his discovery. The implications of the burst were not lost on any of them. The SETI volunteers knew there and then that the burst could not be explained by any current science. On each of their minds was the possibility that the purpose of their project had at long last been fulfilled: evidence of intelligent life beyond the Solar System.

 It doesn’t sound much like ‘We come in peace’, or ‘As you no doubt will be aware, the plans for the development of the outlying regions of the western spiral arm of the galaxy require the building of a hyperspace express route through your star system and, regrettably, your planet is one of those scheduled for demolition’, but the Wow! Signal could be the closest thing we have to a message from aliens

Hey, I know that frequency!

To understand why Ehman and his colleagues were so excited by the burst, we need to look at the characteristics of the particular frequency their telescope was picking up. The Big Ear radio telescope had been programmed to detect signals with the very specific frequency of 1420.406 Megahertz. That number might not roll off the tongue, but it’s incredibly important in astrophysics, and even has its own name: the Hydrogen Line frequency.

The red colours shows where hydrogen gas has been detected within the Milky Way. As it turns out, there’s a lot of it | Image: NASA

Hydrogen gas is extremely common in the wide expanses of space which lie between the stars. Atoms of the gas, consisting of an electron orbiting a single proton, are normally very stable and pretty uninteresting. But occasionally, the electron will flip over spontaneously due to quantum processes – a phenomenon known as spin flip transition, making the system unstable. To return to normal, the electron needs to flip back around, releasing a flash of light with a specific energy – a radio photon – in the process.

This process is incredibly rare for individual hydrogen atoms, but when hydrogen gas is gathered in clouds which span many light years, enough radio photons are given off by spontaneously flipping electrons that they can be easily picked up by radio telescopes on Earth. These photons make up the radio signal of the Hydrogen Line, which is important both to radio astronomers and to those hopeful that other technologically advanced civilisations could be out there somewhere.

The Hydrogen Line frequency is useful to astrophysicists, as they can use it to detect the exact locations of clouds hydrogen gas within our galaxy, and in those beyond. The clouds will always be densest in galactic arms, allowing radio astronomers to map out the distinctive structures of galaxies. They can also use the Doppler Effect to measure how fast hydrogen gas at various distances from the centre of galaxies is moving. From this, they can then create rotation curves for different galaxies, which currently give the clearest evidence we have for the existence of Dark Matter (but that’s a whole other story!)

So when astronomers point their radio telescopes at hydrogen gas clouds, it’s hardly surprising for them to observe radio waves at the Hydrogen Line frequency. But what if we observe them in places in the sky where we aren’t expecting them, or at higher-than-expected intensities? Any astronomer will agree that if this happens, something unexplained is going on.

“Huh, these aliens know about hydrogen spin-flip transition and have an in-depth understanding of their place in the galaxy… so why haven’t they invented clothes yet?” | Image: NASA

Astronomers at SETI became interested in the Hydrogen Line frequency because it is such a fundamental figure in astronomy. Clouds of hydrogen gas are so abundant in the galaxy that their signal can be picked up no matter where you go in space. SETI figured that if there are any other enlightened civilisations elsewhere in the universe, then they must realise this too. No matter how different their scientific units are to ours, if we asked them to create a radio signal at the Hydrogen Line frequency, it would be exactly the same as the signal we would create. So what better way to announce your presence to the galaxy than to send out a distinct, high-intensity transmission of the frequency?


SETI has already sent signals like this out into space, in the hope that others out there may be listening. In fact, they have a strict control over the transmission of the Hydrogen Line signal; it is now illegal to get anyone’s hopes up by transmitting the frequency yourself. But that isn’t the only method we have used to broadcast our knowledge of the Hydrogen Line. In 1972, under the efforts of Carl Sagan, Frank Drake and other journalists and astrophysicists, the famous Pioneer plaque was attached to probes Pioneer 10 and Pioneer 11.

In the top-left of the plaque is etched a diagram representing the distinctive electron flipping process which causes the Hydrogen Line. Between two representations of hydrogen atoms with electrons in the two different states is a horizontal line 21.106 cm long – the exact wavelength of a radio wave at the Hydrogen Line frequency of 1420.406 MHz. If an intelligent civilization ever finds one of the probes then it’s a safe bet that they will understand exactly what the diagram is representing, no matter their language.

 An explanation, or an alien-killing pretender?

Not long after Ehman’s bizarre discovery in 1977, the newly dubbed Wow! Signal, named after Ehman’s hasty scrawl in red pen, had taken the worlds of both science and the media by storm. While astrophysicists rushed to discover the astronomical source of the signal, journalists began to enthusiastically speculate that without any existing scientific explanation, it could have been more purposefully created in origin. For hopefuls of the existence of extra-terrestrial intelligence, the evidence was now more tantalising than ever.

Sorry it was me all along… or was it? | Image: NASA, ESA, J.-Y. Li

Yet for all the attention the Wow! Signal gained, the search for its origin proved fruitless for scientists and alien hunters alike. For over 40 years, the result sat there unexplained; frustrating some, and instilling hope in others. But in April 2017, astronomer Antonio Paris from St Petersburg College, Florida claimed in a paper to have solved the mystery once and for all.

Paris argued that on August 15, 1977, two comets inside the Solar System – 266P/Christensen and 355P/Gibbs – passed directly in front of the Big Ear radio telescope. Surrounding one of the comets was a cloud of hydrogen gas, which was given off by one of the masses of ice and rock. Naturally, the telescope picked up the Hydrogen Line signal of the cloud, but only as it passed through Big Ear’s field of view. For the first time, it looked like the mystery had been solved. But not everyone was satisfied with the new explanation.

Within weeks, Paris was receiving backlash from scientists, who had some strong criticisms of his paper. In June, Robert S. Dixon, director of the SETI project himself, published a rebuttal to Paris’ paper, claiming that the two comets weren’t in fact within Big Ear’s field of view on the day of the Wow! Signal. Other criticisms included claims that the comets were a long way from the Sun and therefore inactive, meaning neither of the comets could possibly maintain a hydrogen cloud around themselves. Some scientists even had many harsh words to say about Paris’ scientific methods in general. Paris stands by his theory, but he’s open to debate.

So for now, many astronomers still regard the Wow! Signal as a mystery, and hope remains that the true explanation of the burst could be another advanced civilization broadcasting its existence. But this isn’t the only case where strange signals in astronomy have gone unanswered for long periods, or where far-fetched and alluring explanation theories have been thought up.

Over the last century, the world of astronomy has been become famous for detecting mysterious signals, making bizarre discoveries, and throwing up seemingly unanswerable questions. Some of these questions have had mundane explanations. Others have led to new research which has come to revolutionize our understanding of the universe. And, like the Wow! Signal, still others remind us just how much we have yet to learn.

In this series, we will find out more about the signals which have both answered and created some of the most enticing questions in modern astronomy. Next time, we will turn back the clock to before 1977, when astrophysics was reeling at the revelation that our place in the universe was far less significant than we realized.

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