Daniel Kleppner, an experimental physicist who helped to develop an atomic clock that became an essential part of global positioning systems, or GPS, and who also helped to discover a rare fundamental state of matter predicted by Albert Einstein and his fellow theoretical physicist Satyendra Nath Bose, died on June 16 in Palo Alto, Calif. He was 92.
His wife, Beatrice, confirmed the death. She said he collapsed while visiting their daughter, Sofie Kleppner, and her son, Darwin, who was graduating from high school.
It was in the mid-1950s, while he was doing a fellowship at the University of Cambridge in England, that Dr. Kleppner learned something surprising: It was possible, a tutor told him, to build a clock precise enough to detect the effects of gravity on time. Curious, he went in search of more information and read Norman Ramsey’s 1953 book “Nuclear Moments.”
After his fellowship, he went on to do graduate work at Harvard University, where he discovered that Dr. Ramsey was on the faculty. He immediately applied for Dr. Ramsey’s research group and was accepted.
Dr. Ramsey would eventually share the 1989 Nobel Prize in Physics for research he had done in the 1940s, when he discovered a way to measure the frequencies of electromagnetic radiation absorbed by atoms and molecules. His experimental technique laid the groundwork for nuclear magnetic resonance, a precursor to the M.R.I. technology used in medicine today.
The atoms of each element vibrate at a unique frequency, like the signature call of a bird. Dr. Ramsey’s work made it possible for scientists to build what is known as an atomic clock — a device that measures those vibrations, using the information to keep incredibly precise time. (The official measure of a second, for example, is 9,192,631,770 oscillations of a cesium atom.)
The first atomic clock was made in 1954 by Jerrold R. Zacharias, a physicist at the Massachusetts Institute of Technology. When Dr. Kleppner joined Dr. Ramsey’s lab, he had an idea for how to improve its accuracy.
The key, he believed, was to observe atoms for as long as possible; the longer their oscillations could be tracked, the more accurate a clock would be. Dr. Ramsey suggested confining them in some sort of vessel, rather than letting them fly freely. That idea provided the basis for Dr. Kleppner’s groundbreaking contributions.
In a 2011 interview for the history project InfiniteMIT, Dr. Kleppner explained the idea.
“At first glance that sounds kind of nutty,” he said, “that here’s this atom, you’re trying to look at its frequency with very high precision. You have to treat them very delicately and you are going to let them hit walls and rattle around. It’s like trying to watch alarm clocks by batting them back and forth. But it turns out for the hydrogen atom, this might work, and this was the idea that we pursued.”
The timing device that they built, along with H. Mark Goldenberg, was called a hydrogen maser. The scientists published the results of their work in 1962 in the journal Physical Review.
The hydrogen maser had many practical uses: It allowed for the precise timing of communication signals, which made it possible to measure the distances between global positioning satellites and calibrate them; it enabled high-resolution imaging in radio astronomy; and it improved communication with deep-space probes.
After earning his Ph.D. in 1959, Dr. Kleppner was appointed an assistant professor at Harvard. He stayed there until 1966, when, after failing to gain tenure, he moved across Cambridge, Mass., to M.I.T., where he remained until his retirement in 2003. He lived in Belmont, Mass.
In the mid-1970s, Dr. Kleppner became interested in trying to produce a rare state of matter called Bose-Einstein condensation, whose existence was predicted by the scientists for whom it was named.
If the electrons in atoms have the same orientation, or spin, the atoms cannot form molecules — they bounce off each other instead of bonding. But Bose and Einstein theorized that if atoms were cooled to extremely low temperatures and compressed, they would enter their lowest energy state and undergo what is called a phase transition. The spin of their electrons would have the same orientation, and they would stop behaving as individual particles, instead acting as a single big one.
When Dr. Kleppner read about this theory in 1976, he said in the M.I.T. interview, he dismissed the idea as “totally absurd.” But a conversation with his colleague Tom Greytak, a low-temperature physics expert, changed his mind. Drawing on his earlier work with the hydrogen maser, Dr. Kleppner began experimenting with ways to create the elusive state of matter.
He and his team chose hydrogen as their test element, a decision that slowed their progress. “We realized,” he later said, “that at the densities we were aiming for, hydrogen would not be stable.”
As word of their effort spread, other laboratories joined the race. The first scientists to succeed, in 1995, were Eric Cornell, Wolfgang Ketterle and Carl Wieman at JILA (formerly known as the Joint Institute for Laboratory Astrophysics), a research institute in Colorado. (Dr. Ketterle had studied with David Pritchard, who himself was a former student of Dr. Kleppner’s.) The three shared the 2001 Nobel Prize in Physics for their accomplishment.
Finally, in 1998, Dr. Kleppner’s group was also able to create the condensate. When he announced that success at a conference in Verona, Italy, he received a standing ovation.
“I’ve had a sort of a love affair with hydrogen,” he said in a video honoring him for winning the 2006 National Medal of Science. “But this affair with Bose-Einstein condensation left me fairly bruised.”
His persistence paid off: In 2000, he helped found the MIT-Harvard Center for Ultracold Atoms and became its first director, further cementing his role in the field.
Daniel Kleppner was born on Dec. 16, 1932, in New York City, the second of three children of Otto and Beatrice (Taub) Kleppner. His father, who had emigrated from Vienna in 1906 and grew up “pretty much in poverty,” Dr. Kleppner said, founded his own advertising agency. His mother grew up in New Jersey and went to Barnard College.
Growing up in suburban New Rochelle, N.Y., Daniel liked to build things out of wood, including rowboats, which he learned to sail. He also liked to tinker with electronic devices.
His love affair with physics started in high school, just after World War II, when “physicists were considered heroes for having brought the war to a quick conclusion with the atom bomb,” he said in the M.I.T. interview.
He attended Williams College in Massachusetts, graduating in three years, and in 1954 set off for the University of Cambridge.
Aboard the ship to England, he met Beatrice Spencer. For him, it was love at first sight. She evidently needed more persuading, but they married in 1958.
In addition to his wife, his daughter and his grandson Darwin, Dr. Kleppner is survived by two sons, Paul and Andrew; three other grandchildren; and a sister, Susan Folkman.
The National Medal of Science was not the only honor Dr. Kleppner received. He was also given the Wolf Prize, considered the most prestigious award in physics outside of the Nobel Prize, in 2005; the Benjamin Franklin Medal in 2014; and the Oersted Medal, for notable achievements in teaching physics, in 1997.
Dr. Kleppner felt there were not enough physics teachers in high schools, so in 2002 he founded a summer program called Teaching Opportunities in Physical Sciences to help recruit teachers.
He also helped to create an advanced physics course for M.I.T. freshmen with more than a rudimentary knowledge of the subject. He and Robert J. Kolenkow wrote a 2013 textbook for the course, “An Introduction to Mechanics.” The course, which is still taught, is so difficult that it gained the nickname “Mechanics for Masochists.”
At the end of the M.I.T. interview, Dr. Kleppner was asked what advice he had for students.
“This question of choosing problems, that’s the crucial problem in physics,” he said. “But that’s why the advice I gave to young people, it’s just follow your interest and do something. And if you’re lucky, it’ll lead someplace interesting.”
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