UW professor’s dream leads to breakthrough on cosmic rays
Particle detector in South Pole built after conversation in Poland
For more than a century, the origin of cosmic rays — fragments of atoms that rain down on the Earth at close to the speed of light — had been one of the great mysteries in science, thwarting the best minds in physics.
An international team of scientists reported Thursday that the likely solution arrived at just after 3:54 p.m. Central Time on Sept. 22, in a scene beyond anything special effects wizards in Hollywood could have imagined.
A mile beneath the South Pole ice, a ghostly highenergy particle zooming from a distant corner of the universe and from 4 billion years ago, when Earth was in its infancy, slammed into the nucleus of a single atom creating a new particle that raced away trailing a kilometer of flaring blue light.
The moment — a few millionths of a second — triggered months of observation and analysis, all described in a pair of papers just published in the journal Science. Yet it might have passed unnoticed were it not for the dream of a physics professor from the University of Wisconsin-Madison, Francis Halzen.
Thirty years ago, Halzen told a small group of physicists at a conference in Poland about his idea of building a powerful particle detector deep beneath
The discovery by IceCube, built at a cost of almost $280 million ... was especially sweet for Halzen. Since IceCube’s completion in 2011, he has worried that it might not make the spectacular discoveries needed to justify its price tag.
the South Pole, a project that came to be known as IceCube. And to his surprise, no one laughed.
Decades passed, the observatory was built and just 43 seconds after the September collision beneath the ice, an alert sped out across cyberspace to scientists monitoring more than 20 of the largest telescopes in the world, from the Canary Islands to Hawaii to the Fermi Gamma-ray Space Telescope orbiting 350 miles above Earth. The alert read:
“IceCube detected a track-like, very-high-energy event with a high probability of being astrophysical in origin.”
One by one, the telescopes turned toward the particle’s path.
The cosmic object that had traveled so far to reach Earth, a subatomic particle called a neutrino, shoots through the universe in a straight line — passing through stars and even people without shifting course. Formed in collisions involving cosmic rays, neutrinos can tell scientists where a cosmic ray has been.
In this case, the neutrino led like an arrow back to one of the universe’s most violent features, an enormous particle-firing galaxy called a blazar.
“We’re looking down the barrel of a gun,” said Jordan Goodman, distinguished physics professor at the University of Maryland, explaining the giant accelerator hurling particles toward Earth.
A blazar is distinguished by an enormous, rapidly spinning black hole at its center and twin poles that blast particles into space. Among the particles spewing forth from the poles: the mysterious cosmic rays.
Blazars could be the origin of the highest energy cosmic rays, which carry tens of millions times more energy than the particles accelerated through the man-made Large Hadron Collider near Geneva, Switzerland.
Goodman said the international team behind the two papers — the authors list includes more than 1,100 scientists — has established “strong evidence” implicating the blazar as a source of cosmic rays, though it will take more detections to achieve certainty.
The discovery by IceCube, built at a cost of almost $280 million (a fraction of the $10 billion collider), was especially sweet for Halzen, 74. Raised in Belgium, Halzen’s was not the stereotypical boyhood of a future physicist, hours spent gazing in wonder at the night sky; he daydreamed of becoming a professional cyclist.
Since IceCube’s completion in 2011, he has worried that it might not make the spectacular discoveries needed to justify its price tag.
“It cannot be described,” Halzen said recently, laughing and struggling for words sufficient to describe the joy of seeing the observatory achieve a breakthrough.
“It’s like when my son was born.”
A longstanding mystery
As with other fundamental breakthroughs in physics — quantum mechanics and Albert Einstein’s theory of relativity — scientists do not yet know, what, if any, practical applications will flow from identifying the origin of cosmic rays.
However, quantum mechanics paved the way for today’s economy with its dependence on computer chips. And we have relativity to thank for the accuracy of the Global Positioning Systems that allow us to call up directions on our cellphones.
The identification of blazars as a likely source for cosmic rays rounds out a story that began with one of the most daring discoveries in all of physics.
Early in the 20th century, many scientists believed background radiation levels on Earth diminished at higher altitudes. At great personal risk, Austrian physicist Victor Hess made a series of 10 ascents between 1911 and 1913 in a hydrogen balloon, carrying special equipment he’d designed.
Ultimately soaring above 17,000 feet, Hess was able to determine that after a brief decrease, radiation levels climbed the higher his balloon rose. By taking the balloon out during a near-total eclipse of the sun, he was able to rule out the sun as the source for the higher levels of radiation.
For his discovery of high-energy radiation originating far from Earth — now known as cosmic rays — Hess shared the 1936 Nobel Prize in Physics with Carl D. Anderson, an American particle physicist.
“We’ve been looking at this for more than 100 years to put our finger on where cosmic rays come from,” said Goodman, the University of Maryland professor.
In Halzen’s memory, it was around 1987 that he first presented his idea for an observatory in the ice at a small break-out session at a physics conference. “I didn’t have the courage to present it to the (whole) conference,” he said.
Other neutrino detectors have been built a mile and a half beneath the Mediterranean Sea off the French coast and more than a kilometer below Lake Baikal in southeastern Russia.
Ice makes a good location for a neutrino observatory because it is transparent and does not experience the waves that make ocean detectors a challenge.
A UW-maintained project
It took seven years, from 2004 to 2011, to build IceCube, which is maintained by UW and was largely paid for by the National Science Foundation.
The observatory spans one cubic kilometer of ice and consists of 5,160 sensors strung along cables and sunk deep into the ice. Builders drilled the holes for the cables using jets of near-boiling water.
“It’s a very special place,” said Albrecht Karle, a UW physics professor who has made more than a dozen trips to the observatory. “It’s also a demanding place. It takes a couple of weeks to adjust.”
He said the blast of cold air — 30 degrees below zero in the summer — hits you as soon as you open the airplane door. The station sits at an elevation of 11,000 feet. The air is dry and thin.
There are no birds or animals of any kind. Just 500 miles of ice dominating the horizon in all directions.
The scientists live in dormitory-style rooms with small bunks, and work in a room that looks like a high computing facility with rack upon rack of computers and cables.
Just two people stay the winter maintaining IceCube.
Despite the alert sent out after the Sept. 22 event, it was days before scientists outside the observatory began gathering evidence supporting the significance of what had taken place beneath the ice. Similar alerts had gone out before, but fizzled in importance upon close examination.
“It was potentially exciting,” Karle said, “but I was experienced enough to know not to get too excited.”
The investigation became an example of both “Big Science,” in which breakthroughs increasingly require large collaborations, and also what is called “multimessenger astronomy,” in which discoveries depend on observing the interactions of several different kinds of particles.
Soon after the initial alert was sent, the Fermi Gamma-ray Space Telescope and the telescope known as MAGIC in the Canary Islands reported a flare of highenergy gamma rays linked to the specific blazar thought to be the source of the high-energy particle detected by IceCube.
Halzen says he began to see the event’s importance after a scientist in Japan contacted him about data from the Fermi Gamma-ray Space Telescope.
Still, confirming the implications of the event beneath the Polar ice became a massive task of assembling observations and evidence.
Joshua Wood, a 30-year-old post-doctoral researcher at UW, found himself working through weekends, canceling vacations and falling asleep on the couch as he analyzed nine years worth of data from IceCube.
He and three other scientists examined what had been happening in the blazar’s location over those nine years. They found that during a 100-day period between 2014 and 2015 — a period when they would expect to see only one or two neutrinos — IceCube detected 13 that all appeared to have come from the same blazar.
“When I’m thinking about a problem of this intensity I have trouble falling asleep. I have to work until all of the questions I wanted to answer for that day have been answered,” Wood said, adding that it was a sacrifice worth making.
“There is a sense of adventure to this type of field.” While Hollywood movies have brought many of the wonders of outer space seemingly within the grasp of our imagination, blazars will be another matter entirely, Halzen said.
Our own sun is used as a unit of measurement in the Cosmos called a solar mass. Very large stars are about 20 solar masses.
Blazars can be 100 million, or even 1 billion solar masses.
“It’s something neither you or I can imagine,” Halzen said, “yet these things are as real as the cup of coffee on my desk.”