Since 1998, astrophysicists at Australia's Parkes Observatory have been detecting mysterious bursts of radio waves (named "perytons") that seemed to emanate from all parts of the sky. They lasted for just fractions of a second and happened once or twice a year.
Most believed they came from somewhere quite nearby (perhaps resulting from lightning strikes), but the bursts had some characteristics that made it seem like they'd traveled millions of light-years from distant galaxies or other astronomical objects.
The truth, as recently discovered by graduate student Emily Petroff of Swinburne University of Technology, is decidedly more mundane. It turns out the signals were generated by someone at the facility opening the door of a microwave oven while it was still running.
How a microwave oven baffled astronomers for decades
Most astronomers had long been suspicious that the perytons were generated by something on Earth, because it'd explain why they appeared to emanate from all parts of the sky — they weren't coming from the sky at all, and instead were being generated right next to the telescope. Additionally, they almost always occurred during the daytime, when people were working at the facility.
But for years, scientists were unable to find a convincing explanation. The huge radio telescope at Parkes is in a radio quiet zone, which means that cellphones and other sources of radio emissions are banned nearby. And the perytons' frequency, 1.4 gigahertz (GHz), wasn't known to come from any obvious source of radiation.
Earlier this year, Petroff installed an interference monitor at the telescope. Fortuitously, three more perytons occurred in a single week. This monitor can detect a wider range of emissions than the telescope, and it found that along with each peryton at 1.4 GHz, there was a smaller spike of radio waves around 2.5 GHz.
"That led us to thinking, 'What emits at 2.5 gigahertz?'" Petroff told Popular Science. "And the obvious thing was microwaves."
Still, there was a mystery. Microwave ovens — like the one on site used by staff to reheat food — typically operate at around 2.5 GHz, not 1.4.
In a series of experiments, Petroff and others figured out that if you open the microwave's door while it's still running, the machine's magnetron emits a very brief spike of radiation at 1.4 GHz, as well as the standard 2.5 GHz. In all likelihood, the perytons were caused by a staff member with a habit of opening the microwave door before it was finished cooking. As National Geographic's Nadia Drake points out, this is why many other radio telescope facilities ban microwave ovens.
The scientific value of this discovery
But there's a bright side to this deflating finding. One of the reasons Petroff was looking into perytons was because of a related phenomenon, called "fast radio bursts" (FRBs), which continues to baffle scientists.
These are also short spikes in radio waves, but they genuinely seem to come from beyond the galaxy. And the microwave oven doesn't seem to have been causing them, too — so this work helps to rule out one possible terrestrial origin for FRBs.