The Wow! signal was one single impossibly strong radio signal picked up by the Big Ear telescope in 1977. We have never picked up a similar signal before or since, yet it remains the strongest possible evidence that we are not alone.
Back in the 70s computers didn’t have enough processing power to analyze all the information picked up by these telescopes, who would move back and forth across the sky 24/7. Instead SETI would get volunteers to go through mountains of printed-out data by hand and mark any numbers that stood out. One such volunteer was Jerry R. Ehman.
On the day Jerry was going through this data he’d mostly been marking 7′s and 9′s and 6′s - high numbers, likely from planets, comets, and other regular objects in space. Then all of a sudden, in-between a bunch of 1′s, there it was: 6EQUJ5. A radio signal so strong it broke the number scale and jumped straight to letters. 30 times stronger than background radiation. All he could do was write “Wow!” next to the observation.
Of course telescopes were immediately pointed in the direction of the signal, which was somewhere around the Sagittarius constellation. But it was never picked up again.
The signal was continuous in strength, and the only reason it seems to rise and fall in intensity is that the telescope was moving as it picked up the signal. When graphed it looks like this:
The observed rising and falling in intensity in time with the telescope’s movement means that the signal must have come from a fixed point in the sky. However, the fact that the signal was only heard once means that it can’t have come from a regular object like a star or a black hole, as we would’ve been able to hone in on that and pick it up again.
The signal was sent at a frequency of 1420 megaherz - the frequency of hydrogen. If you want to send a radio signal to a civilization that you know nothing about, you need to pick a frequency that the observers are likely to be looking for. You need to pick a benchmark, something that naturally emits radio waves at a set frequency, and that they will be familiar with. Hydrogen is the most abundant and the simplest element in the universe. Every scientist in the universe knows it. The frequency was and still is illegal to use on Earth because it’s considered significant to science.
Technically the signal was very slightly above the frequency of hydrogen - it was observed at 1420.4556 MHz. This could mean that the object that sent it is moving towards us (extremely slowly btw, we won’t make contact with it for another couple billion years at least). Or, more likely, it could mean that the source corrected the signal for red- or blueshift (a phenomenon where a wavelength increases/decreases due to various factors) before sending it. The latter explanation is most likely since the slight difference from hydrogen lines up with how the frequency would be corrected to line up with the galactic standard of rest.
Another reason we believe the signal was made by a civilization is that it’s narrow-banded, meaning it stays at its own frequency and doesn’t leak into other frequencies. The only known objects in space to emit narrow-band radiowaves are astrophysical masers - which, again, don’t behave like the Wow! signal did.
So in summary, this is a very powerful narrow-band radio signal that appears technological in origin and is sent at the exact frequency at which we would expect people to send a signal.
But like I said, the signal has never been picked up again. It might just have been a single burst. We still hope it might be periodic, but even if it is, we have no idea when it might repeat itself. The Big Ear was demolished in the 90s, and there are no projects exactly like it at the moment. We aren’t observing that single spot in the Sagittarius region 24/7. We simply can’t afford to.
If we can’t observe the signal, if we can’t study and verify the signal… the rule of thumb among astronomers is that it doesn’t really matter. It’s evidence of something, but it’s not proof of anything. If the signal is ever picked up again it will be proof that we are not alone. But until then, all we can do is speculate.
If the signal is extraterrestrial, it tells us that we aren’t alone. That somewhere on another planet there are scientists with a radio transmitter, who want us to know about them. Maybe the signal was their equivalent of the Voyager Golden Records or the Arecibo Message - we know no one can respond to those. But just the thought that someone else in the universe might know about us - might even be comforted by knowing that they’re not alone - is in itself comforting. All we can do is tell other people, over and over, that we share this universe, and just hope that they might one day respond.
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