Francis Halzen, neutrinos and the new window on the universe

For centuries, astronomy has essentially been the science of observing light. However, some of the universe’s most consequential events take place in environments so dense or violent that photons cannot escape. Could the universe be observed through something other than light? In 1930, when Austrian physicist Wolfgang Pauli proposed the existence of an elusive particle to explain a mystery in radioactive decay, he was laying the foundations for a new way of exploring the cosmos. The ghost particle called the neutrino was detected two decades later, and the quest to understand it has since been rewarded with a string of Nobel Prizes. On Tuesday, the Royal Swedish Academy of Sciences, which awards the Nobel Prize in Physics, recognised another pioneer in the field — Belgian-American astrophysicist Francis Halzen. His work has used neutrinos to probe some of the universe’s hitherto most inaccessible environments, helping open a new window on the cosmos.

Neutrinos have tiny masses, carry no electric charge and interact so weakly with matter that billions pass through the Earth — and our bodies — without leaving a trace. These particles can escape from the dense interiors of exploding stars and other extreme cosmic environments, carrying information about places that conventional telescopes cannot see. Halzen recognised this potential. His pioneering work led to the IceCube Observatory, an enormous neutrino detector buried deep beneath the Antarctic ice. Rather than building a conventional telescope, scientists effectively turned a cubic kilometre of natural ice at the South Pole into a detector, embedding more than 5,000 optical sensors in it. Almost every neutrino passes straight through the ice. But on the rare occasion when one sheds its shyness, the resulting flash of light can give information on the particle’s travel path.

In 2013, IceCube provided evidence for a flux of high-energy neutrinos originating beyond the solar system. Halzen’s work demonstrated that sometimes the way to see farther is not to build a more powerful eye, but to learn to detect what the universe is whispering through particles that pass silently through it. The next frontier is multi-messenger astronomy, in which neutrinos can be studied alongside light, gravitational waves and cosmic rays. Together, these cosmic messengers could reveal the workings of black holes, exploding stars and other extreme objects that remain invisible to any single form of observation.