How astronomers traced a puzzling signal to a lunchtime mistake
Australian scientists solved the mystery of the peryton, a perplexing radio wave with an Earthly origin.
In the series I Made a Big Mistake, PopSci explores mishaps and misunderstandings, in all their shame and glory.
Astronomers can’t help but be enchanted with fast radio bursts, or FRBs, thanks to their mysterious nature. These humongous pulses of radio waves blast toward Earth from outer space, often from beyond the Milky Way. But these bursts were almost thrown aside as noise almost 10 years ago, all because of a lunchtime blunder. FRBs are real signals from space, but a very similar radio wave, known as a peryton, originates from an Earthly mistake.
When the first FRB was discovered in 2006, researchers knew they’d found something unexpected—but they didn’t know what it was. West Virginia University astronomers Duncan Lorimer and Maura MacLaughlin were trawling through old troves of radio telescope observations, hunting for signals from pulsars, the rapidly spinning husks of dead stars. Pulsars pulse because they have bright jets that sweep across Earth, like an interstellar lighthouse. One day, a student working on this project came in with a bizarre finding: a pulse more than a hundred times brighter than expected.
The team’s first thought was that it could be interference from Earth-based radio transmissions, but this burst had all the usual fingerprints of something coming from space—it was definitely something new and strange the universe had produced. They published this detection in the prestigious journal Science in 2007, and this first FRB discovery became known as the “Lorimer burst.”
The Lorimer burst spurred more searches, with teams of astronomers scouring radio data to see whether they missed any FRBs in past observations. FRBs were elusive. Years went by without discovering new ones.
Astronomers did, however, find another type of signal in 2007: the peryton. No one knew exactly what it was, but it showed up in radio telescope data for decades, looked kind of like an FRB, and was clearly coming from Earth—not space, like the Lorimer burst had claimed to be.
These perytons “basically cast doubt on the original event,” says Lorimer. Even its name evokes this doubt—the mythological peryton, created by Argentine writer Jorge Luis Borges, is an elk-bird creature that casts a misleading human shadow. “Many people just moved on.”
But not everyone. At the time, a graduate student in Australia named Emily Petroff was writing her PhD thesis on FRBs. Her advisor, though, was so concerned about perytons that he asked her to get to the bottom of the mystery. “The line between the two [perytons and FRBs] was blurry enough to be concerning,” she says. During her PhD work, she’d present new results to her research group, only to be met with the same question, she recalls: “That’s great, but have we figured out the perytons yet?”
[Related: Astronomers spot repeating radio burst patterns from deep space]
Petroff and her collaborators collected all the hints about perytons observed at their local facility, Parkes Observatory. Perytons only showed up when the telescope was pointed in particular directions, so the scientists deduced it had to be something at the observatory. Their monitoring systems showed a spike in energy at the same time a peryton was observed around 2.5 gigahertz, a common frequency that WiFi, Bluetooth, kitchen appliances, and other electronics employ. Looking through old data revealed these spikes had happened since 1998, so the cause had to be decades-old technology. And most damning—they happened much more often around lunchtime.
All signs pointed to microwaves, which were in the two buildings where perytons appeared to come from. But what exactly about the microwaves made this signal? The observatory staff tried everything: microwaving water, microwaving different foods, using different settings, and more. As they experimented, one engineer would stand by the microwave, communicating on a walkie-talkie to another engineer at the telescope. Eventually, they tried breaking the major rule of microwaves—opening the door while it’s still running.
And voila, the peryton appeared.
[Related: Two bizarre stars might have beamed a unique radio signal to Earth]
The mystery was solved with a clear-cut, satisfying answer. Response to the result, published in the journal Monthly Notices of the Royal Astronomical Society in 2015, was electric. “I’ve heard from university teachers, high school teachers, that they teach this paper as how science works,” adds Petroff, who now works as support staff at the Perimeter Institute in Canada. “I don’t think I’ve ever had as satisfying of a moment in my research.”
With the peryton mystery solved, astronomers could devote more time to the puzzle of FRBs. They finally detected a second batch of FRBs in 2013, six years after the initial Lorimer burst, and their count reached almost a dozen by 2015. The momentum shows no signs of stopping, either, as the Canadian CHIME instrument discovers multiple FRBs daily. Radio astronomers have a plethora of other telescopes on the case, too: the Green Bank Telescope in West Virginia, precursors to the Square Kilometer Array in South Africa, the Deep Synoptic Array in California, and ASKAP in Australia. “Ten years from now, we’ll have probably well over 50,000 FRBs,” Lorimer says.
Astronomers also finally have a clue to what these things actually are. The leading theory traces FRBs to magnetars, spinning dead stars (similar to pulsars) with extremely strong magnetic fields. “When you’re spinning around on a carousel, you have some rotational energy due to the fact that you’re spinning,” explains Alice Curtin, astronomy graduate student at McGill University. But magnetars also store energy in their magnetic fields. “We think that it’s something having to do with the possible release of energy from their magnetic fields that could be powering FRBs.”
FRBs have also proven to be an extraordinary resource for exploring the universe. “FRBs are encoded with information about all the stuff between us and them,” adds Curtin. Armed with their newly-expanded catalog of FRBs, astronomers can track hard-to-see dust and gas filling the spaces between the stars. When the FRB travels through matter in space, parts of it are slowed down, smearing out the FRB across frequencies. By looking at the amount of smear, scientists can approximate the amount of stuff. Just earlier this year, a team used FRBs to explore the Milky Way, finding our galaxy actually has less matter than expected.
Perytons are a thing of the past: Radio observatories now have stricter rules for using microwaves. But other sources of radio interference are ramping up, threatening astronomers’ ability to observe the night sky. SpaceX’s infamous Starlink satellites, for example, are ruining so-called radio-quiet zones around major telescopes. The future looks shaky for some ground-based astronomy, and it won’t be as easy to solve as turning off a microwave.
But, for now, the tale of the peryton-producing microwave is a great example of a mistake with a satisfying scientific conclusion—and a fun story. “When I talk to people about FRBs, or even just radio astronomy,” Petroff says, “someone will almost always mention microwave ovens.”