In 2018 a team of physicists linked to the massive IceCube Observatory detector buried under the South Pole, and to a command center at Penn State University, advanced satellites, and several land-based observatories, pinpointed the first known cosmic source of a neutrino, a ghost-like particle that passes through virtually all matter on Earth. It’s estimated that each second there are about 100 billion neutrinos passing through your body.Some 3.8 billion light-years away, in the Constellation Orion, the Archer, exists a galaxy that is being sucked into a supermassive black hole with a mashup of galaxy’s gas, dust, stars, and possible planets blasted out as jets of x-rays, radio waves, and ultra-high energy particles known as a blazar.
The phenomenon was detected on Sept. 22, 2017, when a single high-energy neutrino from that blazar smashed into detectors buried more than a kilometer underground at the Ice Cube Neutrino Observatory at the South Pole. The signal was transmitted to the University of Wisconsin, then routed to a command center at Penn State which alerted more than a dozen other observatories and satellites around the world, each hoping to catch further signals that would help pin down the source.
“It’s like a crime scene investigation. The case involves an explosion, a suspect, and various pieces of circumstantial evidence,” said Matthias Kadler, astrophysicist at the University of Würzburg in Germany about the event that occurred on Sept. 22, 2017, when a ghostly particle ejected from a distant supermassive black hole sneaked through the ice of Antarctica at just below the speed of light, with an energy of some 300 trillion electron volts, nearly 50 times the energy delivered by the Large Hadron Collider at CERN, the biggest particle accelerator on Earth. The ghostly invader set off a cacophony of code-red detectors in the observatory, ultimately solving one of the enduring mysteries of physics and the cosmos.
NASA’s Fermi telescope soon followed, prompting a prolonged “observation campaign,” confirmed that the blazar was active, and gave the researchers confidence that they were looking at a true source, said astrophysicist Azadeh Keivani, “We have found the first source of cosmic rays,” said Francis Halzen, of the University of Wisconsin, Madison, and IceCube’s director.
Meanwhile two very strange events occurred in 2016 and 2018, when scientists reported that something mysterious –some sort of cosmic ray, a high-energy particle that’s blasted its way through space, into the Earth, was shooting up from the the ice-bound surface of Antarctica– that could possibly transform physics as we know it. But high-energy neutrinos, as well as other high-energy particles, have “large cross-sections” that will crash into something soon after piercing the planet and never make it out the other side.
These bizarre events were detected by scientists at the ANITA experiment (Antarctic Impulsive Transient Antenna) started in 2006 in the South Pole.searching for ultra-high-energy cosmic rays and neutrinos coming from deep in outer space, all tracked by an array of radio antennas attached to a balloon floating roughly 23 miles above the South Pole. Neutrinos are exceedingly small particles, created in a number of ways, including exploding stars and gamma ray bursts. They are everywhere within the universe and are tiny enough to pass through just about any object, from people to lead to buildings and the Earth itself.
Scientists remain baffled by the activity, with some 40 papers so far giving wildly different answers –from the ghostly pulses are neutrinos that passed unencumbered through the entire core of Earth and came out of the ground, to he pulses are the long sought-after “fourth” neutrino known as the sterile neutrino, to the mysterious “dark matter” of space, or this is an entirely unknown frontier of particle or astrophysics physics.
Not an Unknown Frontier?
Ian Shoemaker, an assistant professor in the Virginia Tech Center for Neutrino Physics, has a different, simpler explanation. In a recent paper published in the journal Annals of Glaciology, Shoemaker and several colleagues posit that the anomalies are not from neutrinos, but are merely unflipped reflections of the ultra-high-energy cosmic rays that arrive from space — miss the top layer ice — then enter the ground, striking deep, compacted snow known as firn.
“We think sub-surface firn is the culprit,” said Shoemaker, adding that “firn is something between snow and glacial ice. It’s compacted snow that’s not quite dense enough to be ice. So, you can have density inversions, with ranges where you go from high density back to low density, and those crucial sorts of interfaces where this reflection can happen and could explain these events.”
A More Earth-Bound Physics
“Whatever ANITA has found,” says Shoemaker, “it is very interesting, but it may not be a Nobel prize-winning particle physics discovery.” But he’s not discounting that the so-called anomalies have no scientific merit. “ANITA still could have discovered something interesting about glaciology instead of particle physics, it could be ANITA discovered some unusual small glacial lakes.”
Sub-glacial lakes were another consideration by Shoemaker and his team for the reflections. These lakes, deep underground, though, are too far spread apart according to current research, and hence are not the most likely explanation. But if there are far more lakes than previously known, this discovery would be a big win for scientists who study the landscape and interior of Antarctica. Shoemaker and his team suggest scientists purposefully blast radio signals into the areas where the anomalies occurred.
Buried Lakes of Antarctica –the Source?
“I didn’t know anything about them, but they really do exist,” Shoemaker said of sub-glacier lakes in Antarctica. “There are lakes under the ice in Antarctica, and those would have the right reflective properties, but they’re not widespread enough. Our idea is that part of the radio pulse from a cosmic ray can get deep into the ice before reflecting, so you can have the reflection without the phase flip. Without flipping the wave, in that case, it really looks like a neutrino.”
“When cosmic rays, or neutrinos, go through ice at very high energies, they scatter on materials inside the ice, on protons and electrons, and they can make a burst of radio, a big nice radio signal that scientists can see,” added Shoemaker. “The problem is that these signals have the radio pulse characteristic of a neutrino, but are appear to be traversing vastly more than is possible given known physics. Ordinary neutrinos just don’t so this. But cosmic rays at these energies are common occurrences and have been seen by many, many experiments.”
Image credit: DESY, Science Communication Lab/TNS