2010 marked the completion of a bizarre telescope composed mainly of ancient ice. One billion tons of ice.

Buried a mile deep in the ice at the South Pole, IceCube is the world’s strangest telescope. Composed of water, it’s looking for the neutrino, nature’s most unusual particle. Eighty years after the neutrino was “invented” to balance a physics equation, it remains ultra-difficult to detect, measure and understand.

IceCube is focused mainly on particles that come all the way through the Earth. In other words, this telescope looks down.

Scientists say neutrinos can pass unscathed through a long bar of lead. How long? Say, one light year long — about 10 trillion kilometers. Because neutrinos can slip through everything in their path, including stars, galaxies and vast clouds of dust, they are unrivaled tattle-tales of ancient explosions in the deep universe.

The bad news is that the same property makes neutrinos extremely difficult to see.

But if you can somehow observe the neutrino’s insanely rare interaction with matter, you could learn something about the universe, and the gargantuan energy released by exploding stars.

Roots of a frozen telescope

That is the promise and the premise of IceCube, a $271-million project intended to solve a problem posed in 1930, when physicist Wolfgang Pauli proposed a new and rather odd particle. Tiny, energetic, with no electric charge and not necessarily any mass, it would be virtually undetectable.

Pauli himself admitted “I have done a terrible thing. I have postulated a particle that cannot be detected.”¹

The “now-you-don’t-see-it-and-you-never-will” neutrino was tailor-made for controversy; scientists detest what they can’t detect. Pauli’s idea was mocked² as “simply wrong” or “crazy.”

Today, scientists are sure nature is full of these shadowy characters: Rough calculations say a hundred trillion neutrinos whistle through your body every second.

Why make a big deal about neutrinos, which are, after all, less offensive than campaign ads? Because that ability to pass through all manner of interstellar crud allows neutrinos to carry messages from the far reaches of the universe.

Moreover, some neutrinos carry more punch than the wildest gamma ray. And just as you can’t pull a hot coal from a cold fire, you shouldn’t get “hot” neutrinos from “cool” sources like ordinary stars. These neutrinos, in other words, may deliver signals of some hip, blazingly hot stuff — neutron stars, active galactic centers, and exploding stars.

Finally, according to some scenarios, lower-energy neutrinos may comprise a small proportion of the mass — the stuff — of the universe, but they played a key role in the evolution of the universe.

In astronomy, as in love and antiques, “hard-to-get” translates into “most-wanted.” “The hope is that the particle that is almost nothing will tell us almost everything about the universe,” says Francis Halzen, a theoretical physicist at University of Wisconsin-Madison. Halzen directs IceCube, and did the same at IceCube’s predecessor, AMANDA, the Antarctic Muon and Neutrino Detector Array.

Search strategy for an elusive character

Neutrinos may be shy, but once in a great while, they actually hit an atom and produce a subatomic particle called a muon, which is easier to see.

Because the odds of a neutrino hitting anything are so dismal, physicists require bigger targets. It’s the same principle that lottery players use to “beat” the tiny odds of winning by buying hundreds of tickets.

Previous neutrino targets have included tubs of oil or dry-cleaning fluid and 5,000 tons of steel plates salvaged from battleships. To block spurious signals due to cosmic rays rather than neutrinos, these detectors have been sunk in the ocean or placed inside deep mines.

IceCube relies on a two-step detection sequence: First, the tiny percentage of neutrinos that interact with atomic nuclei in the ice produce muons. Second, these muons create Cherenkov light when they interact with matter.

When the detectors see Cherenkov light, they digitize the data and send it through electric cables to the surface for analysis. The detectors are housed inside 5,160 crush-proof glass spheres placed in holes drilled through the ice, and located 1450 to 2450 meters deep.

Another 324 detectors at the surface detect muons made by cosmic rays arriving from the Southern sky.

The Antarctic ice also has little radiation, and the detectors are so deep that air bubbles have been squeezed out, ensuring great optical clarity. Yet while the detectors are shielded from damage, they are under crushing pressure, and if they go bad, they will be busted forever.

IceCube will only look at muons that trigger at least eight detectors, says Halzen, and is most interested in muons moving upward — coming from the Northern Hemisphere. Downward signals can be confusing, as most of them are due to cosmic rays or lower-energy neutrinos, which Earth blocks.

Data from IceCube should suggest where the neutrinos originated and what sort of cosmic engine started them on their journey.

This desire to concentrate on neutrinos rather than cosmic rays explains why this frozen telescope, oddly but logically, looks downward.

What can these neutrinos tell us?

Neutrinos, “invented” to balance a physics equation, have grown to fascinate astrophysicists, galactic voyeurs seeking signals from astonishingly energetic structures and events in the deep universe. The direction and energy of neutrinos from each source should offer clues about the origin:

Image: NASA/JPL-Caltech/STScI/CXC/SAO

Located 10,000 light-years away in the constellation Cassiopeia, Cassiopeia A is the remnant of a massive star that died in a violent supernova 325 years ago. The dead star (turquoise dot in center) became a neutron star surrounded by a shell of junk blasted away in the explosion. Image is a composite from three orbital telescopes: Infrared data from the Spitzer Space Telescope is red; Visible light from the Hubble Space Telescope is yellow; Chandra X-ray Observatory data is green and blue.

Although supernova neutrinos have low energy and are hard to detect, a nearby supernova could light up IceCube enough to overwhelm the system. To prep for a supernova, Reina Maruyama, an assistant professor of physics at University of Wisconsin-Madison, is working to ensure that IceCube can handle this once-in-a-lifetime chance to get good data on a stellar explosion.

If something like the 1987 supernova exploded nearby in our galaxy, Maruyama says, “there would be so many neutrinos, the whole ice would glow. We expect that a few supernovas will occur each century in the galaxy, if one goes off, IceCube has to be ready. We stand to learn a whole lot about how they explode, and about the particle nature of neutrinos.”

Dark matters

Even weirder than neutrinos, IceCube may explore dark matter, a type of, well, something, that comprises 23 percent of the overall universe. A measly 4 percent of matter, including the galaxies, stars and planets, is visible. The balance is an even stranger quantity called dark energy.

The first inkling that some matter is invisible came in the 1930s, when a physicist noticed that galaxies rotate too fast: their visible mass would create too little gravity, and thus they should spin themselves into oblivion.

The explanation for that increased gravity is now called dark matter, and the race is on to detect it.

Since dark matter affects gravity, Maruyama says it must gather in the sun and the galaxies. When dark matter particles collide, they are expected to release a type of neutrino called muon neutrinos. But IceCube found no muon neutrinos coming from the sun and the Milky Way, using a technique that was 1,000 times more sensitive than previous ones.

Does absence make the heart grow fonder?

It depends on your perspective whether that’s good or bad, says Halzen. “There was a big celebration when we published, because we placed limits on that particular type of dark matter, but I looked at it another way: We had gone 1,000 times deeper, and it was very disappointing not to see dark matter.”

However, an experiment in Italy may have seen dark matter interacting with a hunk of sodium iodide, based on an annual variation in the signal. If Earth indeed orbits through a cloud of dark matter, the detector would register alternating downstream and upstream motions that could account for that annual cycle.

The cycle could, however, be due to something unrelated to dark matter.

To answer that riddle, Maruyama wants to place a similar detector deep in the Antarctic ice, and has already piggybacked two prototypes onto IceCube strings. The prototypes are working well enough to justify a larger, more expensive detector, Maruyama says.

If and when the experiment is replicated in Antarctic Ice, Maruyama says, “A positive result would be interesting, and a negative result would be interesting. If we can see a signal with the same timing, that confirms the [Italian] results. If we don’t see a signal, the source must be something aside from dark matter.”

Lurking behind the IceCube project is the tantalizing prospect of learning more about the bizarre particle it detects — the neutrino. We already know that neutrinos have a tiny amount of mass, and that they range in energy through at least 30 orders of magnitude — an unimaginable range of energies. There have been recent — and controversial — reports that neutrinos can travel faster than light — breaking a basic law of physics.

Why so weird?

That’s another indication that neutrinos exist at the edge of the standard model that attempts to explain everything by gravity, electromagnetism, and two nuclear forces, Halzen says. “We are measuring the properties of neutrinos any way we can, and extrapolating to see what the standard model predicts, and looking for variations. The simple way to describe the experiment is that we collect muons and neutrinos, and everything you don’t understand is a discovery, either it’s physics beyond the standard model, or it’s new astrophysics.”

Halzen anticipates spotting an extremely high-energy particle called the GZK neutrino. “These are predicted by theory, and if one hits the detector, we won’t have to do any analysis, we will be able to look at the event display and know that we have made the discovery.” GZK neutrinos are, according to theory, made by cosmic rays that strike photons in the microwave background, Halzen says, and thus could finally reveal the origin of the cosmic rays, one century after their discovery.

Neutrinos are slippery characters; shy, coming in incomprehensible numbers, being emitted by sources we cannot pinpoint. Maruyama notes that neutrinos seemingly change to a different “flavor” without any apparent cause, and says this “oscillation” from one state to another is the strangest part of the neutrino story. “Oscillation could have implications on how the universe evolved to have matter, and not anti-matter,” she says. “These tiny particles could have such an influence on the universe.”

So what?

Why should non-scientists worry about neutrinos? Halzen, who has answered this question many times, says “I have a personal answer. The reason we know our place in the universe is not because of French philosophers, it’s because of physicists. With dark matter and dark energy, we know most of the universe is not made of the same material we are made of. … Is that important to know? I think so.”

IceCube is not intended to produce technology or solve today’s problems, Halzen acknowledges. “This is total curiosity-driven science, and you are allowed not to care. But if you don’t do fundamental research, we’re going to be a developing country, that is clear.”

Particle physics proves that theoretical pursuits can have results that are unpredictable, yet practical and profitable, Halzen says. “My previous job was at CERN [the European particle-physics lab], where people discovered the Web in 1989, to enable collaboration among remote scientists. I think we have paid for all theoretical physics with that one discovery.”

Super Kamiokande detector

Buried 1000 meters under the city of Hida, Japan, and packed with 11,146 photomultiplier tubes, the Super Kamiokande detector is set up to capture the presence of what may be the universe’s most bashful particle. The ghost-like neutrino has no electrical charge and a near zero mass. That means it doesn’t like to play with most other particles. No, the shy “little neutral one,” as the translation of neutrino from Italian goes, prefers to stick to itself. In fact, neutrinos sail right through all that rock over Super Kamiokande and leave most of the interacting particles that accompanied them on their journey through space to face plant in the dirt. With most of the excess noise from other particles filtered out above, researchers just have to wait for neutrinos to slip through the ceiling and dive into the detector’s 50,000-ton water tank (pictured here empty).

Despite their Olympic-caliber dive in, neutrinos do occasionally bump into an H2O molecule in the tank. The splash results in the release of a charged particle, either an electron or muon. Those particles shoot out from the neutrinos’ rare belly flops faster than the speed of light, in water, in which light travels more slowly than it does through the emptiness of space. The result of breaking that aquatic speed limit is the creation of a subtle cone of light, a kind of optical sonic boom. It’s this light cone that all those silver photomultipliers on the walls were installed to detect, and all sorts of information can be gathered by analyzing the cone’s precise shape. Astronomers have used the detector to see deep into our sun, collapsed stars, and the center of our galaxy.

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