Neutrinos, they are very small,
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids down a drafty hall….
—John Updike, from “Cosmic Gall”
In a world that seems increasingly complicated, there is some solace in knowing that certain physical concepts don’t waver. Energy conservation—the idea that what goes in must come out—is one of them. For example, when a balloon explodes, it does so with the force of the breath that filled it, no more or less. Likewise, when a cue ball moves across a pool table, it accelerates at a speed proportional to the force that hit it.
In 1930, however, the Austrian physicist Wolfgang Pauli discovered a problem: A small amount of energy seems to vanish—that is, it is not conserved—in the aftermath of a certain type of subatomic reaction. Pauli refused to believe that energy conservation was violated, and so he attributed the loss to an uncharged particle of nearly no mass that departs the reaction with the missing energy. It was a bit of a reach, and Pauli knew it. “I have done a terrible thing,” he wrote despairingly. “I have postulated a particle that cannot be detected.” On this count, he was wrong. In 1956, in an experiment that would later win a Nobel Prize, two American scientists were able to detect the existence of Pauli’s particle, named the neutrino by Enrico Fermi. Later experiments showed the neutrino to exist in three types, or “flavors”: electron, muon, and tau. Yet for each flavor, there remained a problem: Did the elusive neutrino have mass? If so, the mass is so small as to be undetectable by any conventional means.
That’s when things got interesting. Following Pauli’s work, theorists predicted how many electron neutrinos should reach the Earth as a result of the sun’s nuclear processes. But when that hypothesis was first tested in 1967, the results showed that only one-third of the expected electron neutrinos were arriving (more recent experiments have raised the observed total to more than half of the expected rate). Several explanations have been offered for this solar neutrino deficit, but the most compelling and likely one is that neutrinos oscillate. That is, they change flavor as they travel from the sun to the Earth (and elsewhere, for that matter). The MINOS experiment is designed to produce several results, of which the most important is a definitive and controlled demonstration of neutrino oscillation. But for something to oscillate, it must have mass. And so, if neutrinos oscillate, it follows that they also have mass.
Why does anybody need to know this? Currently, every cubic inch of the universe is filled with hundreds of neutrinos; they pass through all matter, animal, vegetable, mineral, in a constant, steady stream, and yet almost nothing is known about their properties. If neutrinos have mass—and most physicists believe that they probably do—then they could help cosmologists explain why the universe behaves as if there is a whole lot more mass to it than what is observable. For example, physicists are unable to explain the velocity and shape of galaxies based upon current physical knowledge. This is because the force of gravity is proportional to mass, and there is simply not enough detectable mass in a galaxy to exert the gravity necessary to hold it together. In fact, it is widely accepted that visible mass accounts for only ten percent of the total mass of the universe. Thus, over the past couple of decades, physicists have been searching for “dark matter” that would account for the other ninety percent. Neutrinos, if they have mass, might offer a partial solution to this problem.
Neutrino mass has other implications. For decades, scientists have relied upon a theoretical explanation of the elementary forces and particles called the Standard Model. It is a remarkably robust theory that has sustained numerous experimental attacks on its predictive powers. Yet for all of its successes, the Standard Model is widely acknowledged to be incomplete. Among its likely weaknesses is its suggestion that neutrinos lack mass (though it does not rule out the possibility). Proof that neutrinos have mass will shift science several steps toward a new physics. “Hey, it’s fundamental to our understanding of the universe,” exclaims Professor Earl Peterson, director of the Soudan lab. “And we should find that kind of stuff out.”
Peterson has been trying to find that stuff out for most of his career. His office, overlooking the mall on the University of Minnesota’s Minneapolis campus, is a repository of deep questions and occasional answers. A bulletin board displays data plots dating back to the early 1980s; the desk and tables are piled high with dust-coated journals and papers. The erased ghost of a drawing of the MINOS detector is barely discernible on the blackboard. Peterson is a wiry, graceful presence amid the chaos; he projects an orderly intelligence, the confident timbre of his voice suggesting an intellect accustomed to being consulted.
A native of Washington, he showed an early aptitude for science and math, which eventually led him to a Ph.D. in elementary particle theory from Stanford. In 1967, he began post-doc work at the University of Minnesota, but the topic did not hold him. “After two years I started looking for something else to do,” he says with a wry laugh, cigarette in hand. His search took him to renowned designers Charles and Ray Eames, to whom he offered himself as a “concept person.” “They were very nice to me,” Peterson remembers, even though they turned him down. In the end, he was drawn back to academic physics and, eventually, to Minnesota.
During an impromptu visit to Harvard in January 1979, Peterson got his initial ideas about proton decay from some friends there who were “developing the first of the grand unified theories.” He returned to Minnesota, hoping to conduct an experiment to observe the phenomenon. But there was a logistical problem: A proton-decay experiment could only be conducted in a place shielded from the rain of cosmic rays constantly hitting the Earth’s surface. That is, it had to be done underground.
“So we started looking for mines,” Peterson explains. “And it just so happened that Marvin [Marshak, professor of physics and astronomy at the University of Minnesota] and his family had been to Soudan and taken the historic mine tour. That’s where it started.” Peterson, Marshak, and their collaborators soon arranged for a small experiment to take place on an abandoned level of the old Soudan Mine, and on March 20, 1981, a teletype printer spit out the first data from the Soudan Underground Laboratory.