A Cool Nobel: Ice Cube
This post is about the 2026 Nobel physics prize to Francis Halzen, announced earlier today.
Background: The universe seems to be made up of many types of particles. Those that carry electric charge, like the electron, can be detected through their interactions with other charge-carrying particles (such as protons) and with electromagnetic fields.
Those that are neutral, like the neutron, are detected via their interactions through the strong interactions. Other particles, like the quarks and muons, are detected via weak interactions. When the particles have mass, they also couple to the world through the gravitational force.
The neutrino (its existence was first postulated by Pauli. The name was apparently given by Amaldi, it means "the little neutral one" in Italian) is electrically neutral and thus cannot be detected electromagnetically; it has a very small mass, and hence responds little to gravity; and the weak force is short-ranged. All this means that the neutrino hardly interacts with regular matter and is therefore exceptionally hard to detect.
The Neutrino Zoo: There are 3 types of neutrinos, associated with the electron, muon and tau particles, respectively. Each of these 'flavors' has an anti-neutrino version as well. Importantly, the neutrinos change (oscillate) from one flavor to another as they only exist in three distinct mass states, which do not line up one-to-one with their flavor types.
A Brief history of Neutrino Detection: I will summarize this timeline referring to the Nobels awarded.
3.1 1988 Nobel: To Lederman, Schwartz and Steinberger, for discovering muon neutrinos.
3.2 1995 Nobel: To Reines, for the discovery of the electron neutrino.
3.3 2002 Nobel: To Davis and Koshiba (I got to have lunch with him just before he got the prize!), for the detection of neutrinos from the sun and from a distant supernova.
3.4 2015 Nobel: Kajita and McDonald, for observing neutrino (flavor) oscillations.
Ice Cube: Francis Halzen is a Belgian-American physicist. He is a theorist, which might sound surprising, given that the prize cites the Ice Cube experiment.
But Ice Cube was his invention. He was the first to suggest, in 1988, using a cubic kilometer of ice for detecting neutrinos. He had realized that Koshiba's detector, which used about 3000 tons of water to see low-energy neutrinos from relatively close astrophysical source, would need to be a million times larger to see high-energy neutrinos from deep space (these neutrinos carry information about where cosmic rays bombarding earth come from). His great idea was to use already-present Antarctic ice. Halzen has since been the lead scientist of Ice Cube, supplying the vision, driving the strategy, and securing funding for the enterprise.
In some ways, his Nobel is like the one awarded to Kip Thorne (who incidentally is speaking on our campus day after), which was not given for any specific theoretical breakthrough as such, but for showing that gravitational wave sources should be putting out enough power, and for identifying ways to lower instrumental noise.
Accomplishments: Some of the results from Ice Cube, whose construction was finished in 2011, which played into the Nobel:
5.1 2013: First detection of high-energy neutrinos from outside of the solar system.
5.2 2018: First detection of high-energy neutrinos from a supermassive black hole (a blazar to be more precise).
5.3 2023: First neutrino-based image of the Milky Way.
Summary: This Nobel was for neutrino astronomy. It is now part of what is generally referred to as `multi-messenger astrophysics', where information about the universe reaches us via electromagnetic radiation, gravitational waves - and now neutrinos.
