Icy telescope “reads” ancient message from distant universe
The surprise, after 15 years of planning and development, was that the ghostly particles from outer space were so easy to see. After all, “The neutrino is a tough particle to deal with,” says Francis Halzen, principal investigator of IceCube at the University of Wisconsin–Madison, where the detection was made.
Scientists have produced neutrinos in laboratories, measured them outside nuclear reactors, and detected them streaming from the sun.
This week, in Science, the IceCube Collaboration reports the detection of 28 high-energy neutrinos, including energetic Ernie at 1.14 (see portrait at right). These neutrinos were so energetic that they must have come from outside the solar system.
Because many of them did not line up with the plane of our galaxy, they likely came from beyond the Milky Way.
But what’s a neutrino, and why should we care? For starters, they are elusive. Neutrinos almost never interact with matter. That makes them almost invisible. Billions of neutrinos have passed through your body since you clicked over to this page.
In 1930, physicist Wolfgang Pauli, who first proposed the neutrino to balance an equation, said, “I have done a terrible thing. I have postulated a particle that cannot be detected.”
His postulation was, as they say, close enough for horseshoes and hand grenades. But a vanishingly small percentage of neutrinos do strike other particles, creating a particle called the muon that is much easier to detect.
But that vice is also a virtue. That failure to interact allows neutrinos to travel unscathed from the most distant reaches of the universe, where they are (presumably) born in catastrophic collisions or titanic explosions — or something even weirder.
Searching for the source
Finding that something is the ultimate goal of IceCube, an upside down telescope at the South Pole, where scientists have converted a cubic kilometer of pure ice into the world’s largest astrophysics experiment.
The project is headquartered at Madison and funded by the National Science Foundation. The IceCube detector was completed in December 2010 after seven years of construction. It was built on time and on budget and in its first two years has exceeded design specifications.
The IceCube collaboration is interested in high-energy neutrinos, rather than the lower-energy type created in our sun. “This is the first indication of very high-energy neutrinos coming from outside our solar system, with energies more than one million times those observed in 1987 in connection with a supernova,” says Halzen. “It is gratifying to finally see what we have been looking for. This is the dawn of a new age of astronomy.”
The bad news is that it’s too soon to know any specifics about the source of these ultra-high energy particles, which must be something amazingly energetic.
Dealing with data
At IceCube, a digital data dance begins the moment a photon of light triggers eight spherical detectors deep in the ice, and a digital notice is sent to the surface. The computers weed through gobs of data: From 6,000 “events” every second, it sifts out one neutrino every six minutes or so.
But most of those neutrinos come from the sun… so further winnowing leaves about one high-energy neutrino per month, says Halzen.
To see a neutrino, scientists generally look for the muons that are created after the rare collision between a neutrino and a particle in the target. Because such collisions are rare, bigger detectors rule.
In the 1960s and ’70s, scientists went to great lengths (and depths) to find neutrinos, burying large masses of water, oil or iron plates deep underground, where the target was sheltered from spurious, lower-energy signals.
In 1987, Halzen had a brainstorm. Why not use the Antarctic ice-cap as your target, and freeze detectors inside an essentially unlimited mass of ice?
It was an unlikely scheme: A telescope that looked downward. A gadget designed to see things that are phenomenally shy. And an astrophysics experiment built around a target of unknown purity.
“Usually you study the material first,” says Halzen, “but we had no access to our material. We had to take what was given to us by nature.”
The blue light created by muons will only travel eight meters in distilled water, but the muons must travel much further to be seen by at least eight IceCube detectors. Eight is the borderline that makes an event energetic enough to be worth further study.
In the ultra-pure ice-cap, muons can travel 100 meters or more. “You cannot filter water this clear,” says Halzen. “It’s the clearest solid in the universe, as far as we know.”
Easy does it
Ironically, after all the difficult, expensive and dangerous work of drilling holes and setting globes in the ice, identifying the highest-energy neutrinos turns out to be eyeball-easy. “I have looked at thousands of neutrino tracks, but when I see one of these tracks, there is no doubt, none,” says Halzen. “It’s a needle in a haystack, sure, but these events are so spectacular that it’s a needle that is sticking out of the haystack.”
While an intriguing map shows where in the universe these neutrinos originated, it’s too soon to pinpoint the sources, Halzen says. But he intimates that could quickly change. “Now that we know what we are looking for, we can expand the search and find more events.”
– David J. Tenenbaum