The Breakthrough Listen project detected radio waves that seemed to be the best candidate yet for an alien signal, but it turns out it was just human technology
25 October 2021
A signal heralded as the best candidate yet for evidence of alien technology has finally been analysed, and it turns out that it was almost certainly just interference from our own radio technology. The Breakthrough Listen project, a $100 million programme funded by Russian billionaire Yuri Milner, has now scrutinised the unusual beam of radio waves spotted in 2019 and found that it wasn’t aliens after all.
What is this radio signal?
The signal, dubbed Breakthrough Listen Candidate 1 or simply BLC1, was detected by the Parkes Observatory in Australia, which observed the Proxima Centauri system over a huge range of wavelengths in 2019. Proxima Centauri is a particularly interesting star because it seems to have at least one planet orbiting in the habitable zone, where it is neither too hot nor too cold for liquid water on the planet’s surface.
Over the course of these observations, more than 4 million signs of radio emission were captured at various wavelengths. One of these so-called hits was what appeared to be a precise radio beam with a frequency of about 982 megahertz, meaning its wavelength was about 0.3 metres. It shone for around 2.5 hours on 29 April 2019, its frequency slowly increasing, and then disappeared.
What is so weird about it?
There are a series of properties that the researchers check for each hit they find. One is that if the signal is from a distant planet orbiting a star, the frequency we observe should slowly and smoothly change over time as that planet rotates and orbits, its motion modulating the frequency. Of the 4 million hits from the Parkes observations of Proxima Centauri, only about 1 million demonstrated this.
The second major criterion is that the signal should disappear when the telescope is pointed slightly away from the target star system. This eliminated most of the rest of the hits, narrowing the field down to 5160 promising signals. Of these, a few things made BLC1 special: the frequency band it covered was extremely narrow, ruling out all possible astrophysical sources of radio waves; there were no registered transmitters using that frequency within 1000 kilometres of the observatory; and it lasted longer than radio signals from aircraft or satellites passing above the telescope do. Out of all the millions of signals analysed by the Breakthrough Listen team so far, this was the only one that seemed like it really could be aliens.
How do we know it isn’t aliens?
After BLC1 was spotted and tagged as being interesting, a team of researchers led by Sofia Sheikh at the University of California, Berkeley, dug through archival observations of the Proxima Centauri system, looking for signals that were similar to this one. They found 60 other signals at varying frequencies that were otherwise nearly identical to BLC1. All of those signals were still detected when the telescope was pointing away from Proxima Centauri, indicating that they were produced by human technology near the observatory. While BLC1 was only detected while the telescope was pointed towards the target star system, the researchers found that it was probably a coincidence, and the signal was most likely produced by two interfering human-made radio transmitters.
“Given a haystack of millions of signals, the most likely explanation is still that it is a transmission from human technology that happens to be ‘weird’ in just the right way to fool our filters,” Sheikh said in a statement. We still can’t say with 100 per cent certainty that BLC1 isn’t a signal from alien technology – but the probability that it is alien are now extraordinarily low.
Journal reference: Nature Astronomy, DOI: 10.1038/s41550-021-01508-8
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