Francis Halzen discusses decades-long quest to bring IceCube to fruition
Posted December 4, 2009
“Big science, as often as not, hinges on small moments. Once the grants have been secured and the politics navigated, the ground broken and the visionary promises made, when banks of computers flicker to life and fingers curl above keyboards, ready to flash the first discoveries via e-mail, a decade’s work can still come crashing down in the final hour – a castle erected in the thin air of theory, too weak to withstand true gravity.”
— Francis Halzen, “Antarctic Dreams,” The Sciences, New York Academy of Sciences (1999)
About a block from the State Capitol square in Madison, Wis., stands a nondescript office building, its façade all glass, with a noonday autumn sun glinting off the exterior.
Only one business occupies the fifth floor. Cubicles crowd the interior while window-side offices ring the perimeter with a wall of glass. The hum of computers, clatter of fingers on keyboards, and hushed tones of conversation provide white noise en route to the corner office.
But Francis Halzen is no power-suited CEO, even if his corner office looks out over downtown. He dresses simply in black slacks and in a black sweater, a pen clipped to the neck. Round, professorial glasses sit on a prominent nose. His Belgian accent is still thick even after nearly 40 years of living in the United States and working in the physics department at the University of Wisconsin-Madison.
Halzen is also no ordinary academic, even though his background is in theoretical physics, a realm of abstract reality where E=mc2 and Star Trek scriptwriters concoct warp drives and transporters. From this corner office the 65-year-old scientist leads the largest single experiment on the continent of Antarctica.
Or, more accurately, in the ice itself — an altogether different telescope that when completed will span a cubic kilometer of area in the ice sheet below the geographic South Pole. Naturally, it’s called IceCube.
“IceCube is basically a big computer,” explains Halzen, taking a break from editing one of the dozens of scientific papers the experiment has generated, even though construction is still two years away from completion and big discoveries yet made.
IceCube is really big, looking for something really small — a subatomic particle called a neutrino. The most abundant particle in the universe aside from photons (light particles), high-energy neutrinos result from violent events in the far-flung corners of the universe, such as exploding stars or by the formation of black holes.
Physicists believe neutrinos contain information about such intergalactic events thanks to their ability to make a straight beeline through the universe without changing course. IceCube can help scientists trace the neutrinos back to their place of origin by capturing the incredibly brief interaction of the particles with other atoms as they speed through ice.
The collision produces another particle called a muon, which leaves a trail of blue light in its wake. It’s that flash of light that IceCube captures using strings of digital optical modules (DOMs) frozen in the ice sheet.
“Each module is like a satellite you launch, and it detects light. It tells you each photon it detects and when it detects it to within two nanoseconds,” Halzen says. “The rest is just a computer that collects information and makes sense out of it.”
Halzen and the other 250 scientists around the world with a stake in IceCube are intensely interested in what that $272 million computer will eventually tell them about the processes that shape the universe.
Even now, with only 59 of the 80 planned strings of DOMs entombed in the ice, IceCube is capturing about 100 high-energy neutrino events per day, according to Halzen. That’s an amazing rate considering the rarity of a muon occurrence.
“It’s incredible what’s flowing in here,” Halzen says, waving back to the cubeland behind him where engineers and technicians hunch over computer screens.
He anticipates the next question. “When will we discover something?” he asks. “I don’t know,” he adds, laughing at his moment of self-reflection. “It’s not that I don’t care, but even if we don’t discover anything, it’s been so exciting.”
Consider that it’s been 10 years since the proposal to build IceCube went through the peer-review gauntlet. Funds came from the National Science Foundation’s Major Research Equipment and Facility Construction program, which requires special allocation from Congress every year. The epic effort to build the ice-cold neutrino detector goes back even further, to 1987.
That’s when Halzen wrote the first paper on the idea of using Antarctic ice as a medium to ensnare neutrinos. Other physicists had been trying to use DOM-like instruments in water, including a group in Hawaii that almost succeeded with the Deep Underwater Muon and Neutrino Detector (DUMAND), led by John Learned.
Halzen figured the ice’s clear, inviolate nature would work better than the ocean, which is susceptible to storms and waves, as well as biological process that could interfere with the detector. (However, a Mediterranean-based detector like IceCube is also under construction today called ANTARES.)
It then took about 10 years to make the now-famous AMANDA experiment work. Construction of the Antarctic Muon and Neutrino Detector Array was a sort of proof of concept for IceCube. The first detector string for AMANDA went down into South Pole ice in 1993. Four years later, AMANDA recorded the first-ever precision map of a high-energy neutrino event.
Halzen waxed eloquently about the event in an essay he wrote for the New York Academy of Science in 1999. The anthology series Best American Science Writing re-published the story in 2000. Halzen wrote:
“At five in the morning on October 12, 1997, the diagram told us, a neutrino — one of nature’s smallest and most elusive elementary particles — had entered the earth in the middle of the Pacific Ocean, between Midway Island and the Aleutians, hurtled straight through the planet, and collided head-on with a proton on the underside of the Antarctic ice. Two kilometers beneath the surface, our grid of photomultipliers had picked up a subatomic spark from that collision as it flew upward through the ice and flared past them for about a microsecond.
“The end result, abstracted on a computer screen, would have seemed unremarkable to most people. But to us it was the sole trace of a particle that had traversed vast distances to reach us, perhaps a remnant of one of the most spectacular events in the universe. More to the point, it was the first concrete proof that AMANDA worked — that it could help map the distant depths of space, thereby perhaps resolving some of the most heated controversies in physics. Later, when I e-mailed the diagram to our collaborators, one of them wrote back: ‘This is why I’ve spent five years of my life on this project.’”
Halzen has devoted more than two decades of his life to chasing the elusive neutrinos through polar ice, with two years to go before the IceCube construction phase is complete. It’s only a matter of time at this point before the long struggle is over. It wasn’t always a fait accompli.
“I thought this project was over many times over the last 20 years,” Halzen confides.
The science is the relatively easy part. IceCube digitizes the signals from the high-tech DOMs, which makes it easier to analyze the data. In theory, Halzen could run the experiment from a laptop in his office. But technical glitches, software meltdowns, will always require a couple of people to be on the Ice even after the detector is finished in 2011.
One of the biggest obstacles was the technology to construct the detector. Specifically, the challenge was building the hotwater drill, which burns a hole about 2.5 kilometers deep. The hose alone weighs more than 12,000 tons. It’s not something you find sitting on the shelf at the corner hardware store.
“The drilling was the most exciting part, I must say. You realize that you don’t go buy a drill somewhere. Nobody builds that stuff. You can’t start your own business and do this. You have to go to experts and convince them,” Halzen explains.
In the end, only one company in the world, in Italy, had the expertise to build a hose for that drill. The Physical Sciences Laboratory at the University of Wisconsin-Madison built the drill itself.
“It’s exciting working on a practical problem like this when you spent your life working on particle physics,” Halzen adds. That was another hurdle in convincing the NSF and peers that the project could work. Some balked at putting a theoretician in control of such an unwieldy experiment. Halzen admits he had his own doubts.
“It’s the biggest project ever done out of a university. In fact, a lot of people thought we couldn’t do it.
“They thought I was crazy. Who can blame them? … I was actually looking for someone who could actually do this,” he recalls. A mathematician turned theoretical physicist — this was the 1960s and the thing to do at the time — Halzen says today he would have gone into cosmology.
“I always thought it was much more exciting to do the experiment than theoretical work,” he says, “so it’s not a total accident that I am sitting here today. … What I didn’t anticipate is that here I am sitting 20 years later doing it. My intention was always to hand this off to somebody else. That somehow never happened.”
The smartest thing he did, Halzen says, was hire the right people to manage the project on the ground. “That’s why this project has succeeded,” he notes. Two years into the project, a new project director, Jim Yeck, was hired. “When I say that the smartest thing I did was hiring the right people I was mostly thinking of that position.”
The big discoveries — finding the origins of cosmic rays or of dark matter, the latter more of a European fetish — are still ahead. But that doesn’t mean IceCube hasn’t already proven its worth in other unexpected ways.
“You really build these experiments to have surprises. The real surprise we got was that someone plotted the sky with IceCube. Not through the Earth, which is what we typically do with neutrinos, but looking from above,” Halzen says.
There are also projects of opportunity with IceCube. Some researchers have done glaciology with the detector, sending down an instrument to measure the concentration of dust and other particles frozen in the ice. This is the sort of work normally reserved for ice cores.
“We can see the dust of individual volcanoes. We have mapped the ice impurity with better resolution than people have done with cores,” Halzen says.
Then there’s the fact that scientists have instrumented a cubic kilometer of ice, something no one has ever done. The ice sheet moves about 10 meters per year. How does the flow rate change with depth? “It flows pretty uniformly, so it is the small deviations that we’re trying to measure,” Halzen says.
The galaxy itself may provide a different opportunity not too far into the future. Scientists predict a supernova explosion occurs once every 30 years. Such a stellar explosion, which can radiate as much energy as the sun could emit over its entire lifetime, would justify the existence of IceCube in about 10 seconds, Halzen enthuses.
“If we now had a supernova explosion in our galaxy, we would measure it with incredible precision,” he says, the words almost a purr of intellectual satisfaction.
What would scientists learn about such an event? Many things: The neutrinos would record information about the explosion itself, as well as how stars collapse and explode. What’s left behind in the debris is a neutron star or black hole.
“The neutrino basically sends you a movie of what’s happening,” Halzen says.
The experiment is designed to last 20 years or more, and the last supernova occurred in 1987, so there is an excellent chance IceCube will be around with a front row seat to that rare show.
In the meantime, while waiting for those small moments of opportunity made possible by big science, Halzen and his team of scientists, engineers, technicians and drillers will continue to bore into the Antarctic ice sheet. Completion is a foregone conclusion. Difficulties working in the world’s harshest environment are a foregone fact.
“I have no idea how boring it will be once all the construction is over,” Halzen says.
NSF-funded research in this story: Francis Halzen, University of Wisconsin-Madison, Award Nos. 0236449, 0639286 and 0636875.
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