The IceCube Neutrino Observatory, buried in Antarctic ice, has recorded neutrinos carrying energies roughly 10 billion times greater than those streaming from the Sun, according to Science Daily. The particles, which rarely interact with matter, arrived from distant cosmic sources. The report comes as physicist Francis Halzen received this year's Nobel Prize in Physics for his work on IceCube and for the discovery of high-energy neutrinos of astrophysical origin.
Why it matters
Neutrinos are everywhere and almost impossible to stop. Science Daily reports that about 100 billion of them pass through every square centimeter of the human body each second, and virtually none are noticed. Because they slip through matter so easily, they can carry information out of places that light cannot escape, such as the regions around collapsing stars and black holes.
That makes them a new kind of messenger for astronomy. According to Science Daily, work involving neutrinos has contributed to five Nobel Prizes in Physics, a sign of how central the particles have become to modern understanding of the universe.
A particle that barely interacts
Neutrinos are elementary particles, meaning scientists believe they have no smaller parts. They come in three types — electron, muon and tau — and carry no electric charge. They do have mass, but it is extraordinarily small, possibly around 10 million times less than an electron's, Science Daily reports. Their exact masses and the process that gives rise to them remain unknown.
Their near-total lack of interaction is what makes them so hard to study. Neutrinos do occasionally engage with matter through the weak force, which Science Daily describes as about 20 orders of magnitude weaker than electromagnetism. Those rare events are the only handle scientists have.
"Neutrinos are truly ghost-like particles," George Fuller, a distinguished professor of physics at the University of California San Diego, told Science Daily. "It's hard to stop them. It's hard to catch them."
Fuller, who worked with Halzen on an early paper about IceCube, said the particles could cross light years of lead depending on their energy.
The idea itself is old. In the 1930s, Wolfgang Pauli proposed an unseen particle to account for energy that seemed to vanish in certain radioactive decays, and he doubted it could ever be confirmed. Detection came in 1956, when Clyde Cowan and Frederick Reines observed neutrinos in an experiment called Project Poltergeist.
Why the South Pole
Halzen saw in the 1990s that Antarctic ice could serve as a detector medium. Deep below the surface, the ice is clear, with few bubbles or impurities to scatter faint signals. The result was IceCube, an instrument spanning about a cubic kilometer of ice, built by drilling deep holes and lowering strings of light sensors called photomultiplier tubes.
Those sensors look for brief blue flashes of Cherenkov radiation, which occurs when a neutrino strikes matter in the ice and produces a charged particle moving faster than light travels through that material. A muon, for instance, leaves a cone of such light. By reading the flashes, researchers can reconstruct events that would otherwise leave no trace.
The observatory is tuned to the rarest and most energetic arrivals. Solar neutrinos are abundant but low in energy; cosmic neutrinos of extreme energy turn up only a few dozen times a year, according to Science Daily.
The next step is pinning down where these particles originate, a problem Science Daily notes is difficult, with radio telescopes proposed as one way to help trace their sources.
