Posted on behalf of Chaz Firestone
Say the word “telescope” at the South Pole and you’ll be directed to one of two large dishes at the Martin A. Pomerantz Observatory, each of which searches the sky for cosmic microwave background radiation left over from the Big Bang: The South Pole Telescope and BICEP, the brawny name for Background Imaging of Cosmic Extragalactic Polarization. But there is a third telescope at the Pole, though it doesn’t really look like one. That’s because it’s pointed downward.
IceCube is an astrophysics project at the Pole that looks for traces of neutrinos, invisible particles churned out by nuclear reactions. They can originate from distant supernovae, from our own Sun and even from man-made nuclear reactors, and they are around us all the time, passing through matter with ease. This last property, owing to their lack of charge and tiny mass, makes them notoriously elusive, and neutrino collisions are rare events — so rare, in fact, that you have to go all the way to the South Pole to get the best view of them.
When a neutrino collides with an atom, a byproduct of the collision is a muon, which emits a faint blue light that can be detected by a sensitive enough instrument. Traditionally, neutrino collisions are detected in liquid water, which is used as a medium by labs in Japan and Canada. But one of the insights of IceCube was to realize that neutrino detection would work just as well in ice — and there’s plenty of that in Antarctica.
Yesterday, I met with Mark Krasberg, a physicist at the University of Wisconsin who works at IceCube, the largest neutrino detector in the world. IceCube searches for high-energy neutrinos with a more interesting source than our modest Sun: violent astrophysical events like exploding stars and colliding galaxies. Here’s how it works:
With a hot water drill, technicians bore a hole (pictured, above right) 2.4 kilometers deep. In it, they place a string of digital optical modules (DOMs) in the bottom kilometer of the hole. The DOMs (pictured, right) will remain in those holes for tens of thousands of years, and are built to detect that faint blue light from muons in the crystal-clear Antarctic ice. (With that long a shelf life, researchers like to make themselves a part of history by signing their names on the DOMs, which I had the opportunity to do.) After a neutrino collision, the resultant muon travels along the same course as the neutrino that produced it, so astrophysicists can retrace the trajectory of the muon to determine the source of the neutrino, much as a forensics specialist might do ballistics work. Even though only a few neutrinos from cosmogenic events (maybe just two or three!) will collide with an atom of ice each hour in a block 1 cubic kilometer in volume, the equipment at IceCube is sensitive enough to capitalize on the few collisions it observes.
Though the telescope is located just a few hundred meters from the South Pole itself, it actually surveys the northern sky for these violent astrophysical events. As mentioned above, neutrinos can originate from all kinds of sources, but the high-energy neutrinos IceCube is after (reaching energies of a peta electron volt!) have a better chance at passing through the Earth than lower-energy, garden-variety neutrinos. By searching southern ice for neutrinos originating from the north, scientists use the Earth itself as a filter, isolating the neutrinos of interest.
What’s all the trouble for? As Krasberg explained, certain astrophysical events and bodies aren’t easily detectable by traditional optical and radio telescopes. But neutrinos, which can pass through the interstellar medium with even less attenuation than photons, allow astrophysicists the unique opportunity to “see” these cosmogenic events. The end goal, Krasberg said, is to build on work done by IceCube predecessor, the Antarctic Muon And Neutrino Detector Array (AMANDA), to construct a detailed map of (half) the sky’s distant astrophysical bodies.