Every second, some hundred billion invisible neutrinos pass through each square centimeter of a human body, almost none of them noticed, and for most of the twentieth century they were close to a physicist's fantasy. This year's Nobel Prize in Physics, awarded to Francis Halzen for the IceCube observatory buried beneath the South Pole and for the discovery of neutrinos born beyond our solar system, honors the patient work of turning that ghostly presence into knowledge. It recognizes a long human effort to read the universe through the one messenger that nothing can easily stop. As the field r
What Is a Neutrino? The Science Behind the IceCube Nobel and the Ghostlike Particles It Detects
Neutrinos are truly ghostlike particles
Mark
Why do neutrinos keep winning Nobels?
Mimi
This is the fifth physics prize tied to detecting or discovering them. They're everywhere, yet nearly impossible to catch, so each advance in catching them opens something new.
Mark
What's special about Halzen's contribution?
Mimi
He saw in the 1990s that clear Antarctic ice, free of bubbles and contaminants, could serve as a detector. Then his team drilled into a cubic kilometer of it and strung photomultiplier tubes through it.
Luke
Let's be clear about scale. Only a few dozen astrophysical neutrinos are detected a year. That's a small sample for big claims.
Mimi
Fair, and that's why each one matters so much.
Mark
Fuller mentions a neutrino with 200 times the energy of IceCube's highest. Where did it come from?
Luke
Nobody knows. Fuller says so directly. His line that it may be saying something profound is his interpretation, not a finding.
Mark
And the mass question?
Luke
Also open. The source says scientists don't know the mass or where it comes from. The 10 million times lighter than an electron figure is a rough "maybe."
Mark
What about the Simons Observatory?
Luke
It's anticipated data. It "may" provide the most deep-space neutrino data. That's a hope, not a result yet.
Le Pouls
- Neutrinos slip through almost all matter, so catching even one is a rare and difficult event.
- Halzen's Nobel recognition has drawn fresh attention to a field long defined by near-invisibility.
- A cubic kilometer of Antarctic ice now serves as a detector, its strings of sensors waiting for a faint blue flash.
- Only a few dozen ultra-high-energy neutrinos are caught each year, and their origins remain unknown.
- The Simons Observatory in Chile may soon push deep-space neutrino data further, reaching toward the early universe.
Every second, some hundred billion invisible neutrinos pass through each square centimeter of a human body, almost none of them noticed, and for most of the twentieth century they were close to a physicist's fantasy. This year's Nobel Prize in Physics, awarded to Francis Halzen for the IceCube observatory buried beneath the South Pole and for the discovery of neutrinos born beyond our solar system, honors the patient work of turning that ghostly presence into knowledge. It recognizes a long human effort to read the universe through the one messenger that nothing can easily stop. As the field reaches toward the earliest moments of the cosmos, these tiny particles are becoming a window into where everything came from.
Francis Halzen has won this year's Nobel Prize in Physics for his decisive role in building the IceCube Neutrino Observatory and in discovering high-energy neutrinos that come from beyond our solar system. It is the fifth physics Nobel to honor work connected to neutrinos. George Fuller, a theoretical astrophysicist at the University of California San Diego who once wrote an early paper with Halzen on IceCube, helps explain why the achievement matters.
Neutrinos are elementary particles with no electric charge and almost no mass, and scientists still do not know their exact mass or where it originates. They interact so weakly with matter that they stream through nearly everything, from the sun's core to the human body. Their weak interaction is roughly twenty orders of magnitude feebler than electromagnetism, which is why detecting them demands extraordinary measures. The idea began in the 1930s, when Wolfgang Pauli proposed the particle to explain missing energy in radiation, admitting he had postulated something that could not be detected. Decades later, in 1956, Clyde Cowan and Frederick Reines confirmed it.
In the 1990s, Halzen recognized that the deep, clear ice at the South Pole could serve as a detector. His team drilled into a cubic kilometer of ice and lowered strings of photomultiplier tubes, which can catch the faint blue Cherenkov light produced when a rare high-energy neutrino collides with the ice. Because such events are so scarce, IceCube records only a few dozen each year, but the light patterns reveal each neutrino's direction and energy.
Fuller finds the ultra-high-energy events the most compelling. Some carry billions of times the energy of a typical solar neutrino, and their sources remain a mystery. "The fact that it got here at all may be saying something profound about the universe," he said. The work continues elsewhere as well. The Simons Observatory in Chile, where UC San Diego is a founding institution, may soon provide the most deep-space neutrino data yet, potentially illuminating the Big Bang and the early universe.
Francis Halzen has won this year's Nobel Prize in Physics "for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin." It is the fifth time the physics prize has gone to work tied to detecting or discovering neutrinos. George Fuller, a distinguished professor of physics at the University of California San Diego, helped explain what that work means. He specializes in theoretical nuclear and elementary particle astrophysics and cosmology, and he worked with Halzen on an early paper about IceCube.
Start with the particle itself. Neutrinos are elementary particles, which means that, as far as anyone knows, they cannot be broken into smaller parts. They come in three flavors: electron, muon and tau. They carry no electric charge. They do have mass, but it is tiny, perhaps 10 million times lighter than an electron. Scientists do not know the actual mass of a neutrino, and they do not know where that mass comes from.
The particles interact only weakly with matter, so they travel through space at nearly the speed of light. Some come from the sun, some from deep space, and some from sources nobody has identified. The current estimate is 10^89 of them, roughly one billion neutrinos for every atom in the universe. About 100 billion pass through each square centimeter of your body every second.
"Neutrinos are truly ghostlike particles," Fuller said. "It's hard to stop them. It's hard to catch them. How do we even know they exist or where they are?" The answer is that neutrinos can occasionally scatter or be absorbed through the weak interaction, which is about 20 orders of magnitude weaker than electromagnetism. That makes detection possible but requires extraordinary measures. Depending on their energy, Fuller said, neutrinos would pass through light-years of lead. The same property lets them escape the dense surroundings of collapsing stars and black holes.
The idea dates to the 1930s, when Wolfgang Pauli proposed the particle, which he then called a "neutron," to explain energy that seemed to vanish in certain kinds of radiation. He remarked, "I have done a terrible thing, I have postulated a particle that cannot be detected." For a time he was right. In 1956, Clyde Cowan and Frederick Reines detected neutrinos in an experiment called Project Poltergeist.
In the 1990s, Halzen realized that deep Antarctic ice at the South Pole could be clear of the contaminants and bubbles that would interfere with detection. He and his team drilled holes through a cubic kilometer (0.24 cubic miles) of ice and lowered strings of photomultiplier tubes into them. The array can pick up the blue Cherenkov light that signals a high-energy cosmic neutrino.
IceCube is built for ultra-high-energy neutrinos from deep space, which are far rarer than solar ones. Only a few dozen are detected each year. When one strikes the ice, it produces a fast-moving electron or muon. The muon throws off a cone of Cherenkov light, which lets scientists trace the neutrino's direction and estimate its energy.
Fuller finds these rare events the most tantalizing. Some ultra-high-energy neutrinos seen at IceCube carry roughly 10 billion times the energy of a typical solar neutrino. A similar experiment detected one with 200 times more energy than that. "We don't really know where that neutrino came from," Fuller said. "The fact that it got here at all may be saying something profound about the universe." In that sense, he said, Halzen is at the frontier of experimental fundamental particle physics.
UC San Diego is pursuing neutrinos elsewhere too. The Simons Observatory, high in Chile's Atacama Desert, collects data on the cosmic microwave background, the universe's oldest light. UC San Diego was a founding institution, and distinguished professor Brian Keating is principal investigator. The observatory can also probe the universe's oldest neutrinos and may supply the most data on deep-space neutrinos yet. Fuller says that data could help illuminate the Big Bang and the early universe.
Citations marquantes
Neutrinos are truly ghostlike particles. It's hard to stop them. It's hard to catch them.— George Fuller, UC San Diego physicist
I have done a terrible thing, I have postulated a particle that cannot be detected.— Wolfgang Pauli, 1930s
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