The mysterious signal looked like a sign of alien technology, but it turned out to be radio interference.

Credit: CSIRO, provided by author

Last December, the media we Breakthrough Listen A project found in the data of our radio telescope. This signal, called BLC1, did not appear to be the result of recognizable astrophysical activity or the well-known Earth-based interference.

The problem was that I wasn’t ready to discuss it. When you are looking for signs of extraterrestrial life, you will want to be very careful to get it right before making an announcement. Last year I just started the secondary verification test and there were too many unanswered questions.

Today, we are ready to report that BLC1 is sadly not a signal from intellectual life beyond the Earth. Rather, it is radio interference that closely mimics the type of signal we were looking for.Our results are reported in 2 paper of Nature Astronomy..

Look for solar flares and signs of life

The story of BLC1 begins in April 2019 with Andrew Zic, who had a PhD at the time. Students at the University of Sydney began observing the nearby star Proxima Centauri with multiple telescopes in search of flare activity. 4.22 light-years away, Proxima Centauri, our closest stellar neighbor, is too thin to see with the naked eye.

Flares from stars are bursts of energy and hot plasma that can affect (and possibly destroy) the atmosphere of planets in their path. The sun produces flares, but they are neither strong nor frequent enough to disrupt life on Earth. Understanding when and how stars flare tells us a lot about whether those planets are suitable for life.

Proxima Centauri hosts an Earth-sized exoplanet called Proxima Centauri b, and Andrew’s observations suggest that the planet is being hit by intense “space weather.” Bad weather does not preclude the life that exists in the Proxima Centauri system, but it does mean that the surface of the planet is likely to be difficult to live in.

Still, as our closest neighbor, Proxima Centauri b remains an attractive target for the search for extraterrestrial intelligence (or SETI). Proxima Centauri is one of the only stars we may visit in our lifetime.

At the speed of light, a two-way trip takes 8.4 Light year.. Spacecraft can’t be sent that fast, but it’s hoped that a small lightsail camera will reach it within 50 years and beam back photos.

For this reason, working with Andrew Zic and his collaborators, CSIRO Parks Telescope Performs SETI observations in parallel with flare activity search (also known as Muryan in Willajuri).

Intriguing summer project

Searching for these observations would be a great project for summer students. In 2020, Shane Smith, an undergraduate student at Hillsdale University in Michigan, USA, participated in Berkeley SETI’s undergraduate research experience. program And I started sifting the data. BLC1 popped out towards the end of his project.


BLC1 signal. Each panel of the plot is an observation to Proxima Centauri (“on source”) or reference source (“off source”). BLC1 is a yellow drift line and is only present when the telescope is pointed at Proxima Centauri. credit: Smith et al. Nature Astronomy, Author courtesy

The Breakthrough Listen team was immediately intrigued by BLC1. However, the burden of proof for claiming the detection of life across the globe is so high that we do not get too excited until we have applied all possible tests. The analysis of BLC1 was led by Sofia Sheikh at the time of her PhD. A student at Pennsylvania State University, he conducted a thorough series of tests, many of which were new.

There was plenty of evidence that BLC1 was a true sign of extraterrestrial technology (or “technical signature”). BLC1 has many of the features you would expect from a techno signature.

  • BLC1 was only seen when heading to Proxima Centauri, not elsewhere (“off-source” observations).Interference signals are generally seen in all directions because they “leak” into the telescope receiver.
  • Signals occupy only one narrow band of frequencies, but signals from stars and other astrophysical sources occur in a much wider range.
  • The frequency of the signal slowly drifted over 5 hours. Frequency drift is expected for transmitters that are not fixed to the surface of the earth. This is because the relative movement of the transmitter causes the Doppler effect.
  • The BLC1 signal lasts for several hours, unlike other previously observed satellite and aircraft interference.

Nevertheless, Sophia’s analysis concluded that BLC1 is most likely to be radio interference from here on Earth. Sophia was able to show this by searching the entire frequency range of the Parks receiver and finding a “look-alike” signal whose characteristics are mathematically related to BLC1.

Similar to BLC1 NS Appears in off-source observations. Therefore, BLC1 is guilty of radio interference.

Not the technical signature we were looking for

It is not known exactly where BLC1 came from or why BLC1 was not detected by off-source observations such as similar signals.Our best guess is that BLC1 and analogs are produced by a process called Intermodulation, The two frequencies mix to create new interference.

If you’ve heard blues or rock guitar, you’re probably familiar with intermodulation. If your guitar amp is intentionally overdriven (up to 11), intermodulation adds pleasing sound distortion to your clean guitar signal. Therefore, BLC1 is probably just an unpleasant distortion from a device with an overdriven radio frequency amplifier.

Regardless of the cause of BLC1, it wasn’t the technical signature we were looking for. However, it was a good case study, showing that the detection pipeline was working and picking up anomalous signals.

Proxima Centauri is just one of the hundreds of billions of stars in the Milky Way. Searching for all of them requires maintaining momentum, continually improving tools and validation tests, and training next-generation astronomers such as Shane and Sophia to continue searching with next-generation telescopes.

Huge flare from the sun’s closest neighbor breaks the record

For more information:
Sofia Z. Sheikh et al, Analysis of Breakthrough Listen Target Signal blc1 Using Techno Signature Verification Framework, Nature Astronomy (2021). DOI: 10.1038 / s41550-021-01508-8

A search for radio technology signatures for Shane Smith and others, Proxima Centauri, gave us a signal of interest. Nature Astronomy (2021). DOI: 10.1038 / s41550-021-01479-w

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The mysterious signal looked like a sign of alien technology, but it turned out to be radio interference.

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