Calling it, “a particle that cannot be detected,” Physicist Wolfgang Pauli 1st proposed this in 1930; it was detected in 1956

On the October 17, 2025 episode of Jeopardy!, the Final Jeopardy clue in the category “20th Century Science” highlighted a pivotal moment in modern physics. Contestants were asked to identify a particle first hypothesized by Wolfgang Pauli in 1930 and later confirmed experimentally in 1956.

What is the neutrino?

The correct response was the neutrino, one of the most elusive yet fundamental particles in the known universe.

The neutrino’s story begins during investigations into beta decay, a type of radioactive decay in which a neutron decays into a proton and emits an electron. Physicists noticed something odd: the energy of the emitted electron did not add up to the total energy expected. This violated the conservation of energy, one of the foundational principles of physics. To solve the puzzle, Pauli proposed that a second, neutral particle must also be emitted—one that was virtually undetectable with the instruments available at the time.

Pauli’s Hypothesis: Theoretical Boldness

In a letter to colleagues in 1930, Pauli referred to this proposed particle as a “desperate remedy.” He suggested that the unknown particle carried away the missing energy and momentum, preserving the conservation laws. Because it had no electric charge and seemingly no mass, it became known as the neutrino, Italian for “little neutral one,” a term later coined by physicist Enrico Fermi.

Fermi expanded on Pauli’s idea and incorporated the neutrino into his theory of beta decay in 1934. Despite this theoretical support, the neutrino remained undetected for decades. Its lack of electric charge and extremely low mass meant it could pass through matter virtually undisturbed, making it extremely difficult to observe experimentally.

Detection at Last: The 1956 Breakthrough

The breakthrough came in 1956, when physicists Clyde Cowan and Frederick Reines successfully detected neutrinos emanating from a nuclear reactor at the Savannah River Plant in South Carolina. They used a large tank filled with water and cadmium chloride, waiting for the rare interaction between a neutrino and a proton that would produce a telltale double-flash of gamma rays.

This landmark experiment provided the first direct evidence of the neutrino’s existence, confirming Pauli’s theory a full 26 years after it was proposed. For this achievement, Reines would go on to receive the Nobel Prize in Physics in 1995, nearly four decades later. Cowan, unfortunately, had passed away in 1974 and was not eligible for the prize posthumously.

The Neutrino’s Role in Modern Physics

Neutrinos have since taken on a major role in particle physics and cosmology. There are three known types—or “flavors”—of neutrinos: electron, muon, and tau. These particles are produced in vast quantities in nuclear reactions, including those in the sun, nuclear reactors, and during supernova explosions. Despite their abundance, neutrinos interact so weakly with matter that billions pass through every human being each second without leaving a trace.

In the late 20th and early 21st centuries, experiments revealed that neutrinos have a small, but non-zero mass and can change from one flavor to another—a phenomenon known as neutrino oscillation. This discovery required a modification of the Standard Model of particle physics and opened up new questions about the fundamental properties of matter and the evolution of the universe.

From Theoretical Ghost to Scientific Cornerstone

The journey of the neutrino—from Pauli’s theoretical “invisible” particle to a cornerstone of modern physics—underscores the enduring power of bold scientific ideas backed by evidence. What began as a proposal to preserve the laws of energy conservation eventually led to a deeper understanding of the building blocks of the universe. The 1956 detection marked not just a scientific triumph but also a turning point that would lead to decades of research into the most mysterious particles in the cosmos.

The Jeopardy! clue for October 17 served as a reminder of the patience, creativity, and determination that drive major discoveries in science—and how ideas once thought impossible can eventually reshape our understanding of reality