Norman Ramsey and the Oscillatory Field Method

Norman Foster Ramsey Jr. (1915-2011)

Norman Foster Ramsey Jr. (1915-2011)

On August 27, 1915, American physicist and Nobel Laureate Norman Foster Ramsey Jr. was born. Ramsey was awarded the 1989 Nobel Prize in Physics, for the invention of the separated oscillatory field method, which had important applications in the construction of atomic clocks.

Norman Foster Ramsey Jr. was born in Washington, D.C., to Minna Bauer Ramsey, an instructor at the University of Kansas, and Norman Foster Ramsey, an officer in the Ordnance Department who rose to the rank of brigadier general during World War II. He was raised as an Army brat, frequently moving from post to post, and lived in France for a time when his father was Liaison Officer with the Direction d’Artillerie and Assistant Military Attaché. He graduated from Leavenworth High School in Fort Leavenworth, Kansas, at the age of 15.

Ramsey went to Columbia University in 1931, and began studying engineering. He became interested in mathematics, and switched to this as his academic major and received his B.A. from Columbia in 1935, when he switched his interest to physics. Columbia awarded him a Kellett Fellowship to Cambridge University where he studied physics at Cavendish Laboratory under Lord Rutherford and Maurice Goldhaber. At Cambridge, he took the tripos in order to study quantum mechanics, which had not been covered at Columbia, resulting in being awarded a second B.A. degree by Cambridge in 1937.

A term paper Ramsey wrote for Goldhaber on magnetic moments caused him to read recent papers on the subject by Isidor Isaac Rabi, and this stimulated an interest in molecular beams, and in doing research for a Ph.D. under Rabi at Columbia. Soon after Ramsey arrived at Columbia, Rabi invented molecular beam resonance spectroscopy, for which he was awarded the Nobel prize in physics in 1944. Ramsey worked with Rabbi’s team on the first experiments making use of the new technique, and shared with Rabi and Zacharias in the discovery that the deuteron was a magnetic quadrupole. This meant that the atomic nucleus was not spherical, as had been thought. He received his Ph.D. in physics from Columbia in 1940, and became a fellow at the Carnegie Institution in Washington, D.C., where he studied neutron-proton and proton-helium scattering.

In 1940, he accepted a teaching position at the University of Illinois at Urbana–Champaign. In September 1940 the British Tizard Mission brought a number of new technologies to the United States, including a cavity magnetron, a high-powered device that generates microwaves using the interaction of a stream of electrons with a magnetic field, which promised to revolutionize radar. Alfred Lee Loomis of the National Defense Research Committee established the Radiation Laboratory at the Massachusetts Institute of Technology to develop this technology and Ramsey was one of the scientists recruited for this work.

The role of the Ramsey’s group was to develop the magnetron to permit a reduction in wavelength to 3 centimetres Microwaves. Microwave radar promised to be small, lighter and more efficient than older types. The Radiation Laboratory produced the designs, which were prototyped by Raytheon, and then tested by the laboratory. After exchanging their research with the British and incorporating new British components to improve the device, Ramsey returned to Washington in late 1942 as an adviser on the use of the new 3 cm microwave radar sets that were now coming into service.

In 1943, Ramsey was approached by Robert Oppenheimerr, who asked him to join the Manhattan Project. A compromise was agreed to, whereby Ramsey remained on the payroll of the Secretary of War, and was merely seconded to the Manhattan Project. In October 1943, Group E-7 of the Ordnance Division was created at the Los Alamos Laboratory with Ramsey as group leader, with the task of integrating the design and delivery of the nuclear weapons being built by the laboratory. The first thing he had to do was determine the characteristics of the aircraft that would be used. The United States Army Air Forces (USAAF) wanted to use the B-29 if at all possible, even though it required substantial modification. Plans for the delivery of the weapons in combat were assigned to the Weapons Committee, which was chaired by Ramsey. Ramsey went to Tinian, where he was involved in the assembly of the Fat Man bomb.

At the end of the war, Ramsey returned to Columbia as a professor and research scientist. Rabi and Ramsey picked up where they had left off before the war with their molecular beam experiments. With Rabi, he helped establish the Brookhaven National Laboratory on Long Island. In 1946, he became the first head of the Physics Department there. His time there was brief, for in 1947, he joined the physics faculty at Harvard University, where he would remain for the next 40 years. During the 1950s, he was the first science adviser to NATO. On moving to Harvard, his objective was to carry out accurate molecular beam magnetic resonance experiments. However, the accuracy of the measurements depended on the uniformity of the magnetic field, and Ramsey found that it was difficult to create sufficiently uniform magnetic fields. He developed the separated oscillatory field method in 1949 as a means of achieving the accuracy he wanted. Ramsey and his PhD student Daniel Kleppner developed the atomic hydrogen maser, looking to increase the accuracy with which the hyperfine separations of atomic hydrogen, deuterium and tritium could be measured, as well as to investigate how much the hyperfine structure was affected by external magnetic and electric fields. He also participated in developing an extremely stable clock based on a hydrogen maser. Ramsey was awarded the Nobel Prize in Physics in 1989 “for the invention of the separated oscillatory fields method and its use in the hydrogen maser and other atomic clocks” shared with Hans G. Dehmelt and Wolfgang Paul.

Ramsey retired from Harvard in 1986. However, he remained active in physics, spending a year as a research fellow at the Joint Institute for Laboratory Astrophysics (JILA) at the University of Colorado. Ramsey died on November 4, 2011, aged 96.

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