Bengaluru: On 15 August, 1977, US astronomer Jerry Ehman was using the Ohio State University’s Big Ear radio telescope to scan the sky for possible signals from extraterrestrial intelligence. He received one that looked very much like it was from aliens. Mystified, Ehman circled it on the readout and wrote: “Wow!”.
This powerful blast of radio waves had arrived from the direction of the constellation Sagittarius and bore all the hallmarks of extraterrestrial origin. The incident came to be known as the ‘Wow! Signal’ and has since been used to support the argument to search for alien civilisations.
The signal has not been decoded, or detected again. But it remains the strongest candidate for alien radio transmission that scientists have received so far.
The Big Ear
One of the reasons why this signal has been such a fixation among science enthusiasts is because the Big Ear radio telescope was specifically built to function on a frequency speculated to be the one in which aliens would communicate. The 1.42 GHz radio frequency is naturally emitted by hydrogen. And since hydrogen is the most common element in our universe, it was considered highly likely that intelligent life outside of the earth would be familiar with it.
The Wow! Signal was recorded for 72 seconds, an incoming signal’s maximum duration of detection. As the earth rotated, the telescope’s antenna swept through the night sky, listening for signals coming from various directions.
The antenna was pointed at the same angle for several days and then titled slightly so that it could scan the next band of sky around the earth.
Any point in the sky would be in the telescope’s range for 72 seconds only because that’s how long signals can still be accessible through reflection from the antenna’s giant dish.
All data received that night was printed on paper since there was no digital storage facility back in 1977.
The Big Ear telescope has since been decommissioned and disassembled.
For an alien to contact humans, they would have to transmit their signals at frequencies they know will be heard.
As mentioned above, the most obvious choice of frequency would be to use hydrogen as it had the highest probability of being recognised.
Then there’s the physical hurdle of interplanetary dust and planetary atmosphere. Our atmosphere acts as a thick cover filtering out a large selection of frequencies. It is transparent to only two frequencies of electromagnetic radiation: the visible spectrum, enabling us to see sunlight and starlight directly, and the 1 to 10 Ghz gap.
It turns out that below 1Ghz, there is a lot of noise in the galaxy emitted by astronomical bodies, making it hard to identify a particular signal. Above 10 GHz, there is greater noise from the tiniest of sub atomic particles, whose whizzing becomes loud.
Also, there are ‘waterholes’ in space, where there is abundance of hydrogen and hydroxyl (OH). A proton, as it spins and wobbles around its axis, would draw a circle in the air if extended. This is called precession and occurs in planets as well. This precession can be heard for each interstellar hydrogen and interstellar hydroxyl, at 1.42 GHz and 1.66 GHz respectively.
A waterhole is significant because of the presence of hydrogen and hydroxyl, both components of water, whose presence increases the probability of habitability.
The Wow! Signal had arrived at 1.42 GHz.
Decoding and explanation
This signal still doesn’t have an explanation, although several theories have been put forth (and rejected). Firstly, the frequency that the signal was transmitted in was a rarely used one. It was not used for human transmission, and didn’t come from a known planet or asteroid in that position.
Secondly, Ehman had said that it was likely a signal emitted from earth reflected off a bit of space debris. But this would have to mean that the space debris was still in space relative to the telescope.
Thirdly, the twinkling of astronomical bodies, called interstellar scintillation, was thought of as an explanation, but other more sensitive telescopes that were built to detect such signals have not.
None of the numerous other theories could prove conclusively that this signal wasn’t of intelligent alien origin either.
However, the alien theory won’t gain any credibility unless the signal can be replicated, and so far, efforts to detect the signal again have been unsuccessful.