Brian Remlinger

Brian graduated from MIT this June with a Bachelor of Science degree in physics and a minor in history. During his time at MIT, he enjoyed acting and "teching" in the theater community, spending time with his hallmates, and drinking far too much coffee.

Detecting neutrinos is hard. Of the trillions and trillions of neutrinos that pass through the Earth, the atmosphere, and you and me every year, scientists observe only a few thousand because neutrinos rarely interact with everyday matter. These particles might unlock some of the Universe's most puzzling secrets, but observing a neutrino is like finding a needle in a haystack, if the haystack were a planet and the needle were invisible. Worse, just seeing the neutrino doesn't necessarily tell an astrophysicst much—to answer the really interesting astrophysical questions, you have to know where that neutrino comes from.

It turns out that identifying a neutrino's source is even more difficult than detecting that neutrino in the first place. Neutrino astrophysicists only really care about extrasolar neutrinos—neutrinos created by supernovae or other astronomical events far removed from the solar system. Unfortunately, the vast majority of neutrinos that pass through the Earth are produced by other sources, for example, the fusion that makes the sun shine, the decay of elements like uranium in the Earth's crust, or from other particles, like electrons, protons, or photons hitting the atmosphere. For many neutrino astrophysicists, those neutrinos hold little interest—they say nothing about the Universe outside the solar system. Unfortunately, scientists can't turn off the sun, stop nuclear decays, or shield the earth from incoming particles. Neutrinos don't come with a handy "Fusion-formed in the Sun" tag. Between the difficulty in seeing any neutrinos at all and the difficulty in figuring out where observed neutrinos were produced, is this problem just too hard to solve?

Naoko Kurahashi Neilson says no. Kurahashi Neilson, an astrophysicist at the University of Wisconsin-Madison, is a member of the IceCube collaboration, a group of scientists and engineers that operates perhaps the strangest telescope in existence.

The IceCube Neutrino Observatory is located deep beneath the Antarctic ice. The observatory is no normal telescope—there are no lenses, no eyepieces, and it isn't exactly pointed at a target. Instead, the observatory consists of dozens of "strings" of detectors, 86 in all, which are frozen in the ice 1,450 to 2,820 meters below the surface. The detector covers a cubic kilometer of ice—very clear, very dark ice. When a neutrino hits the detector and interacts with particles in that ice, a flash of light called Cherenkov radiation is created. In any reasonable place, this light would be undetectable—it'd be blotted out by a firefly. The world a kilometer beneath the Antarctic, though, is pitch black, so this tiny flash of light can be seen. In retrospect, this almost sounds straightforward. Where would you find cubic kilometers of ice if not the South Pole? But IceCube is cutting-edge science at the ends of the Earth, and there was nothing straightforward about it.

In use since 2005, and fully operational since 2010, Ice Cube has recently announced major discoveries putting it at the forefront of neutrino science—but it was not always obvious that the observatory would be a success. "It took visionaries," says Kurahashi Neilson of the researchers who originally proposed IceCube. The leaders had to be charismatic and forward-looking to head such an audacious project. It wasn't just the location, which is inaccessible from the outside world nine months out of the year, but that the drilling techniques for making three-kilometer-deep holes was unproven. Even if everything worked perfectly, there was no certainty that IceCube would see anything that other neutrino observatories—observatories in old mine shafts and deep caves only a few miles from civilization—had not. "It was scary," says Kurahashi Neilson, but as of 2013, after six years of construction and three years of full operation, it's all starting to work out.

What makes IceCube so special? Yes, the ice is very dark, but so is the air in your local abandoned nickel mine. The mine is already drilled, and is accessible all year. Why go to the ends of the Earth instead? One reason is the great depth to which they can drill in ice. This rivals and surpasses the deepest mines. This depth protects IceCube from cosmic rays which constantly bombard the earth. These particles can produce the same flashes of light IceCube is using to detect neutrinos. Since the fraction of "events" that IceCube really wants to detect is almost negligible—ten or twenty out of millions—this protection is needed. Another reason is the large volume of target material surrounding the detector. A clear material, like water, must be brought into a mine to act as the neutrino target. IceCube is located in the Antarctic because the clear-ice target is already there.

In order for particles to create Cherenkov radiation, they must be moving very, very fast, usually very close to the speed of light. That means that the Cherenkov light isn't produced all in one place—it's not like a flashlight turning on. Instead, it's like a police car with lights flashing speeding across a parking lot. If you take a photo, it comes out blurry because of the car's motion, and a single blurry photo can't tell you where the car came from. If you set up cameras all over the parking lot, though, and took many blurry photos, you'd be able to reconstruct the car's path. Like the lights on that police car, the Cherenkov radiation from particle interactions is only useful when lots of pictures are taken.

Of course, neutrinos are moving quickly with lots of energy, so the blur of light created by high-energy neutrino interactions will be very long, roughly half a kilometer in length. This is where IceCube shines—it is a neutrino observatory with enough sheer volume to detect the Cherenkov radiation patterns produced by high-energy particle interactions in their entirety. Different particles will leave different blurs of light in the same way that an ambulance's lights would look different from a fire engine's, and IceCube can use those differences to throw out non-neutrino signals.

In 2013, the IceCube collaboration published two ground-breaking papers. The first announced the discovery of the two most energetic neutrinos ever observed. Because of their enormous energy, these neutrinos could only have been produced by massively powerful astronomical events. In other words, these were the first independently confirmed high-energy extrasolar neutrinos and an important milestone in neutrino astrophysics. The second paper, on which Kurahashi Neilson was a primary author, was even more exciting. It used reconstructions of the light blurs created by particle interactions to extrapolate where these neutrinos may have come from—like a car's light trail, the smears should point straight back to the neutrino source. Using 28 high-energy events collected over the course of two years, the collaboration was able to match each of the neutrinos to a fairly small region of the sky with more certainty than any previous analysis. Soon, with more data, there's hope that IceCube can identify specific neutrino sources.

With these discoveries under its belt, IceCube is off to the races. In the next few years, the collaboration hopes to increase the volume covered by IceCube from one to ten cubic kilometers with a new set of sensors. Additionally, a separate set of new detectors sensitive to lower energy neutrinos is proposed to allow IceCube to examine neutrino mass. Even without these upgrades, the IceCube team is very happy with the work they've done this far and are optimistic for the future. For an instrument in the middle of the desolate desert of the Antarctic, Kurahashi Neilson says, "We have a beautiful detector."