A Baffling Signal Shot Up From Deep Within Earth’s Ice—and No One Knows What Caused It

Here’s what you’ll learn when you read this story:

Anomalous radio signals detected in Antarctica by the ANITA experiment were coming upward from Earth rather than down from the sky.
Researchers used digital and mathematical simulations to test whether they could be subatomic particles known as neutrinos, but it is unlikely.
ANITA is retired, but the upcoming neutrino detector PUEO might finally identify what the signals are.

Strange signals from outer space are constantly reaching Earth, hailing from sources like pulsars, quasars, and supernovas. In Antarctica, a signal like this recently seemed to materialize out of nowhere, but it wasn’t coming from the sky above—instead, it was coming from the depths of the ocean.

Two decades ago, far above the vast expanse of ice and snow, instruments aboard the Antarctic Impulsive Transient Antenna (ANITA) experiment (now retired) were picking up radio waves from cosmic rays shooting through the atmosphere. When those rays collide with particles in said atmosphere, they scatter into secondary particles—a hundred of which pass right through our bodies every second without our realizing it. By detecting these particles, ANITA was giving insight into distant cosmic events. But then, things got weird.

ANITA detected anomalous signals coming from below the horizon, at unusually sharp angles some 30 degrees below the icy surface. What appeared to be radio pulses had to have somehow penetrated thousands of miles of rock that they should have been absorbed by. How these ultrahigh-energy cosmic ray (UHECR) pulses could have been detected at all was a mystery. Physicist Stephanie Wissel of Penn State University, who had been scouring ANITA data for signals from neutrinos (which are already mysterious subatomic particles) had never seen anything remotely like it.

“Given the difficulties in interpreting the anomalous ANITA events, they have attracted a lot of attention,” Wissel and her researcher team said in a study recently published in Physical Review Letters. “Theoretical interpretations involving [exotic physics] have been put forward invoking new particles that induce upward-going showers in the atmosphere.”

The researchers thought it was possible that the signal could have been coming from those elusive neutrinos, which are blasted out by objects and phenomena that generate extreme amounts of energy. Most neutrinos interact with nothing, but tau neutrinos will interact with ice. This flavor of neutrino is associated with the tau lepton, another subatomic particle that starts to decay after it is released from ice, losing energy as it breaks down. Tau neutrino decay ends up giving off signals known as air showers, and the electromagnetic radiation released by these showers is detectable by a radio antenna such as ANITA’s. But, despite that, the signal turned out not to be coming from neutrinos or tau neutrinos.

Whatever was creating these signals was incongruent with computer simulations and mathematical models of both cosmic rays and hypothetical upward air showers, and the signals themselves were unlike anything detectors similar to ANITA—such as the IceCube Neutrino Observatory and Pierre Auger Observatory—had ever seen. The signals didn’t even fit into the Standard Model of Particle Physics, which describes how matter is made up of subatomic particles. They could only be called anomalous.

“The results of this search do not support the interpretation that the anomalous pulses detected […] were caused by air showers sourced from particle interactions or decays, such as by decays of upward-going taus, the latter being the basis of proposed explanations based on physics beyond the Standard Model,” the researchers said.

Could the signals be coming from particles previously unknown to physics? Possibly. They might also, however, be from neutrinos that went undetected. Wissel is now working on ANITA’s successor—the larger and more sensitive PUEO neutrino detector. Maybe this anomaly will tell us something we would have never otherwise imagined.

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