University of Rochester

Alumnus Honored for Solving Great Physics Puzzle

February 18, 2000

A University of Rochester alumnus will share the 2000 Wolf Prize in physics, a prize second only to the Nobel in prestige, for his discovery of the mass of the neutrino, a fundamental particle that has ramifications for our understanding of fusion and the birth of the universe. Masatoshi Koshiba, who earned his doctorate from the University in 1955, will accept the prize from the president of Israel on May 21. Nearly half of all previous winners of the Wolf Prize have subsequently won the Nobel.

"This is a great honor to have one of our alumni recognized this way," says Arie Bodek, chair of the Department of Physics and Astronomy. "His work lays a cornerstone of physics for the next century."

The $100,000 prize will go to Koshiba, professor emeritus at the University of Tokyo, and Raymond Davis Jr. of the University of Pennsylvania, who collaborated on the discovery. In 1967, Davis built the first experiment to detect neutrinos produced by the sun, and Koshiba recently built a much larger detector in Japan which showed that neutrinos have mass. The findings were announced in 1998 to great excitement in the scientific community.

Thus they solved one of the great puzzles of the 20th century: whether or not neutrinos, tiny particles smaller than atoms, have mass. First detected in 1956, scientists saw neutrinos as a way to study the inner workings of the sun because they are a byproduct of the sun's fusion. Unfortunately, neutrinos are so insubstantial that most which reach the Earth just fly right through and out the other side---so finding a way to catch a few proved enormously difficult.

"The search for the neutrino's mass became even more important when new theories of physics began hinging on it," explains Bodek. Scientists theorized that the Big Bang, the explosion that gave birth to the universe, created an enormous number of neutrinos that should still be around today. Neutrinos might also compose part of the mysterious "dark matter" of the universe---an invisible mass that might someday force the universe to collapse on itself. And perhaps most importantly, knowing that a neutrino has mass could help scientists find the "holy grail" of physics---the Grand Unified Theory. Physicists have tried for decades to develop such a theory to describe how every aspect of physics works, resolving the inconstancies between Einstein's theory of relativity and the strange world of quantum physics.

To catch that one-in-a-trillion neutrino, Koshiba used thousands of gallons of ultra-pure water in a mine deep below the surface of the Earth. There, protected from other types of radiation, they waited for one of the trillions and trillions of neutrinos that pass through the Earth every second to strike a molecule of water and release a telltale burst of energy.

Koshiba noticed a change depending on whether the neutrinos entered the water from above or below, and from that small detail physicists could tell that the neutrino was changing its form in a way that meant it must have mass.

"Their observations of the elusive neutrinos of astrophysical origin have opened a new window of opportunity for the study of astronomical objects, such as the sun and exploding stars, and the study of fundamental properties of matter," said the Wolf jury.