Nearly two decades ago, an experiment floating high above Antarctica caught a weird signal.
Designed to capture the radio spurts of cosmic rays falling from above, in 2006 the Antarctic Impulsive Transient Antenna (ANITA) recorded a short pulse of radio waves from below – an event that looked like an upside-down shower of cosmic rays, not bouncing off the surface, but emanating from under the ice sheet.
The balloon-borne suite of instruments recorded a similar event in 2014, and scientists have been scratching their heads ever since. No explanation quite fits, suggesting that the culprit could be a particle unknown to science.
“The radio waves that we detected were at really steep angles, like 30 degrees below the surface of the ice,” explains astrophysicist Stephanie Wisselof Pennsylvania State University.
Scientists thought that such a neutrino may come from a supernova that then tunnels its way right through Earth and comes out the other side. However, only the 2014 detection coincided with a supernova that could be responsible – no such event was found for the 2006 detection.
“You have a billion neutrinos passing through your thumbnail at any moment, but neutrinos don’t really interact,” Wissel says. “So, this is the double-edged sword problem. If we detect them, it means they have traveled all this way without interacting with anything else. We could be detecting a neutrino coming from the edge of the observable Universe.”