- Scientists investigate mysterious signals from Antarctic ice defying physics laws.
- The ANITA experiment detected anomalous radio waves, not explained by current particle physics.
- Larger detectors may solve the mystery, aiding the search for elusive neutrinos and cosmic rays.
ANTARTICA — Scientists are trying to solve a decade-long mystery by determining the identity of anomalous signals detected from below ice in Antarctica.
The strange radio waves emerged during a search for another unusual phenomenon: high-energy cosmic particles known as neutrinos. Arriving at Earth from the far reaches of the cosmos, neutrinos are often called "ghostly" because they are extremely volatile, or vaporous, and can go through any kind of matter without changing.
Over the past decade, researchers have conducted multiple experiments using vast expanses of water and ice that are designed to search for neutrinos, which could shed light on mysterious cosmic rays, the most highly energetic particles in the universe. One of these projects was NASA's Antarctic Impulsive Transient Antenna, or ANITA, experiment, which flew balloons carrying instruments above Antarctica between 2006 and 2016.
It was during this hunt that ANITA picked up anomalous radio waves that didn't seem to be neutrinos.
The signals came from below the horizon, suggesting they had passed through thousands of miles of rock before reaching the detector. But the radio waves should have been absorbed by the rock. The ANITA team believed these anomalous signals could not be explained by the current understanding of particle physics.
Follow-up observations and analyses with other instruments, including one recently conducted by the Pierre Auger Observatory in Argentina, have not been able to find the same signals. The results of the Pierre Auger Collaboration were published in the journal Physical Review Letters in March.
The origin of the anomalous signals remains unclear, said study coauthor Stephanie Wissel, associate professor of physics, astronomy and astrophysics at the Pennsylvania State University.
"Our new study indicates that such (signals) have not been seen by an experiment … like the Pierre Auger Observatory," Wissel said. "So, it does not indicate that there is new physics, but rather more information to add to the story."
Larger, more sensitive detectors may be able to solve the mystery, or ultimately prove whether the anomalous signals were a fluke, while continuing the search for enigmatic neutrinos and their sources, scientists say.
The search for neutrinos
Detecting neutrinos on Earth allows researchers to trace them back to their sources, which scientists believe are primarily cosmic rays that strike our planet's atmosphere.
The most highly energetic particles in the universe, cosmic rays are made up mostly of protons or atomic nuclei, and they are unleashed across the universe because whatever produces them is such a powerful particle accelerator that it dwarfs the capabilities of the Large Hadron Collider. Neutrinos could help astronomers better understand cosmic rays and what launches them across the cosmos.
But neutrinos are difficult to find because they have almost no mass and can pass through the most extreme environments, like stars and entire galaxies, unchanged. They do, however, interact with water and ice.
ANITA was designed to search for the highest energy neutrinos in the universe, at higher energies than have yet been detected, said Justin Vandenbroucke, an associate professor of physics at the University of Wisconsin, Madison. The experiment's radio antennae search for a short pulse of radio waves produced when a neutrino collides with an atom in the Antarctic ice, leading to a shower of lower-energy particles, he said.
During its flights, ANITA found high-energy fountains of particles coming from the ice, a kind of upside-down shower of cosmic rays. The detector is also sensitive to ultrahigh energy cosmic rays that rain down on Earth and create a radio burst that acts like a flashlight beam of radio waves.
When ANITA watches a cosmic ray, the flashlight beam is really a burst of radio waves one-billionth of a second long that can be mapped like a wave to show how it reflects off the ice.
An anomaly in the data
Twice in their data from ANITA flights, the experiment's original team spotted signals coming up through the ice at a much sharper angle than ever predicted by any models, making it impossible to trace the signals to their original sources.
"The radio waves that we detected nearly a decade ago were at really steep angles, like 30 degrees below the surface of the ice," Wissel said.
Neutrinos can travel through a lot of matter, but not all the way through the Earth, Vandenbroucke said.
"They are expected to arrive from slightly below the horizon, where there is not much Earth for them to be absorbed," he wrote in an email. "The ANITA anomalous events are intriguing because they appear to come from well below the horizon, so the neutrinos would have to travel through much of the Earth. This is not possible according to the standard model of particle physics."
The Pierre Auger Collaboration, which includes hundreds of scientists around the world, analyzed more than a decade's worth of data to try to understand the anomalous signals detected by ANITA.
The team also used their observatory to try to find the same signals. The Auger Observatory is a hybrid detector that uses two methods to find and study cosmic rays. One method relies on finding high-energy particles as they interact with water in tanks on Earth's surface, and the other tracks potential interactions with ultraviolet light high in our planet's atmosphere.
