American physicist Norman Foster Ramsey, Jr. (born 1915) was both an experimentalist and theoretician, who was awarded the Nobel Prize in Physics for his invention of the separated-oscillatory-field method and its use in the hydrogen maser.

Norman Foster Ramsey, Jr., was born in Washington, D.C., on August 27, 1915. His father, a West Point graduate, was an officer in the Army Ordnance Corps, and, as is characteristic of life in the military, the Ramsey family moved frequently from place to place. Norman, a gifted student, benefited from these moves as he was twice advanced a grade when he enrolled in a new school. As a high school student in Fort Leavenworth, Kansas, Ramsey became interested in science and won a scholarship to the University of Kansas; however, his father was transferred to Governor's Island, New York, so Ramsey entered Columbia University at age 16 and graduated in 1935 with a Bachelor of Arts in mathematics.

Ramsey majored in mathematics, but by the time he graduated from Columbia it was physics that aroused his curiosity. Thus, with a Kellett fellowship provided by Columbia University, Ramsey entered Cambridge University, England, as an undergraduate in physics. At that time the Cavendish Laboratory at Cambridge was a leading center of physics, and in this active environment it was an essay Ramsey wrote for his tutor that first stimulated his interest in molecular beams. When he obtained his second bachelor's degree, he returned to New York City in 1937 and joined I.I. Rabi's molecular beam group at Columbia University.

It was an auspicious time to join Rabi's research group. In 1937 Rabi's molecular beam research had evolved to the point where the magnetic resonance method was about to break on the scene and Ramsey, the first graduate student to work with the new method, shared in the discovery of the quadrupole moment of the deuteron. In 1940 Ramsey received his Ph.D. from Columbia University for his studies of the rotational magnetic moments of molecules. Ramsey's Ph.D. came during the early years of World War II, and for the duration of the war he was involved in the war effort: first with the development of radar at the MIT Radiation Laboratory (1940-1943) and later at Los Alamos with the Manhattan Project (1943-1945). When the war ended Ramsey returned to Columbia (1945-1947) and to molecular beam research. In 1947 he accepted a position at Harvard University where he founded an active research program in molecular beam physics, particle physics, and neutron-beam physics. By the time he retired as Higgins Professor of Physics in 1986, Ramsey had guided 84 graduate students through their theses research.

At Harvard, Ramsey began his own laboratory with the objective of carrying out accurate molecular beam magnetic resonance experiments. Plagued by difficulties in obtaining magnetic fields with sufficient homogeneity to achieve the desired accuracy, Ramsey created his separated-oscillatory-field method. In this method, the effective path length of the region in which quantum transitions of the beam particles are induced can be increased without maintaining a homogeneous magnetic field over the entire path length. This increase in path length means that beam particles spend more time in the resonance region, which results in a dramatic decrease in the width of the observed resonance peaks and, consequently, in increased precision. With the separated-oscillatory-field method, Ramsey and his students measured nuclear spins, nuclear magnetic dipole and electric quadrupole moments, rotational magnetic moments of molecules, spin-rotational interactions, spin-spin interactions, and electron distributions in molecules.

In Ramsey's molecular beam experiments, the time spent in the resonance region was determined by the spatial separation of the first and second oscillating fields. Ramsey's desire to increase still further this time factor and thereby to increase the precision of his magnetic resonance measurements led to the invention of the hydrogen maser in 1960. In this device, atoms of hydrogen were sent into an enclosure where they resided for approximately 10 seconds, which is 1,000 times longer than the time spent in a typical molecular beam apparatus; thus the line widths were reduced by a factor of 1,000. The hydrogen maser was used for extremely accurate measurements of the hyperfine separations of atomic hydrogen, deuterium, and tritium. Since its invention, the hydrogen maser has become one of the most accurate atomic clocks, and it has been used in applications ranging from sensitive tests of the theory of general relativity to the tracking of Voyager II in its encounter with Neptune.

Ramsey was primarily an experimental physicist; however, he was one of those rare physicists who was adept as both an experimentalist and a theoretician. In addition to developing theories of nuclear interactions in molecules that were directly related to his experimental work, Ramsey was the first to develop a successful theory of chemical shifts that has been central to the analysis of nuclear magnetic resonance spectra. He also published a paper providing the theory of thermodynamics and statistical mechanics at negative absolute temperatures.

In addition to his scientific work, Ramsey was an active leader in the world of physics. Together with Rabi, Ramsey initiated the discussions that led to the formation of the Brookhaven National Laboratory on Long Island, and he served as Brookhaven's first head of the Physics Department. On the 50th anniversary of the lab in 1995 he delivered the keynote speech. He held many administrative positions, including director of the Harvard Cyclotron; chairman of the MIT-Harvard committee in charge of the construction of the Cambridge electron accelerator; president of Universities Research Association, the governing body of Fermilab in Illinois; president of the American Physical Society; and chairman of the board of governors of the American Institute of Physics. Ramsey was also the first assistant secretary general for science in the North Atlantic Treaty Organization (NATO), where he initiated the NATO programs for advanced study institutes, fellowships, and research grants.

As might be expected, Ramsey was the recipient of many honors. In 1989 he won the Nobel physics prize for "the invention of the separated-oscillatory-field method and its use in the hydrogen maser and other atomic clocks." He has also received the Presidential Certificate of Merit, the E.O. Lawrence Award, the Davisson-Germer prize, the IEEE Centennial Medal, the IEEE Medal of Honor, the Rabi prize, the Rumford premium, the Compton medal, the Oersted Medal, and the National Medal of Science.

After his retirement from Harvard, Ramsey continued to receive recognition and honors for his contributions to physics and science. In 1995 he was selected by the National Science Board to receive the Vannevar Bush Award. That same year the country of Guyana issued a stamp in his honor. Ramsey actively supported the advancement of scientific research. He was one of sixty Nobel Prize winners signing a letter sent to President Clinton and Congress, on June 19, 1996, asking for increased federal funding to support university-based research. Ramsey is Harvard University's Higgins Professor of Physics, emeritus.

Further Reading

• Information on his work can be found in Norman F. Ramsey, "The Method of Successive Oscillatory Fields," Physics Today (July 1980). There is a biography of Ramsey in Volume I of the McGraw-Hill Modern Men of Science and Engineering (1980). Additional information on his work can be found in John S. Rigden, Rabi: Scientist and Citizen (1987). Press releases on Ramsey have been issued by the National Science Foundation. On the World Wide Web, there is extensive information on the scientists at the Nobel e-Museum (http://www.nobel.se/physics/laureates/1989/ramsey-autobio.html) and the Harvard University Department of Physics (http://www.physics.harvard.edu/fac_staff/emeriti.html).