Norman Foster Ramsey Jr. (August 27, 1915 – November 4, 2011) was an American physicist who was awarded the 1989 Nobel Prize in Physics for the invention of the separated oscillatory field method (see Ramsey interferometry), which had important applications in the construction of atomic clocks. A physics professor at Harvard University for most of his career, Ramsey also held several posts with such government and international agencies as NATO and the United States Atomic Energy Commission. Among his other accomplishments are helping to found the United States Department of Energy's Brookhaven National Laboratory and Fermilab.

Norman Foster Ramsey Jr. was born in Washington, D.C., on August 27, 1915, to Minna Bauer Ramsey and Norman Foster Ramsey. His mother was the daughter of German immigrants and an instructor at the University of Kansas. His father, who was of Scottish descent, was a 1905 graduate of the United States Military Academy at West Point and an officer in the Ordnance Department who rose to the rank of brigadier general during World War II, commanding the Rock Island Arsenal. 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. This allowed him to skip a couple of grades along the way, so that he graduated from Leavenworth High School in Leavenworth, Kansas, at the age of 15.

Ramsey's parents hoped that he would go to West Point, but at 15, he was too young to be admitted. He was awarded a scholarship to the University of Kansas, but in 1930 his father was posted to Governors Island, New York. Ramsey therefore entered Columbia University in 1931 and began studying engineering. He became interested in mathematics and switched to this as his academic major. By the time he received his BA from Columbia in 1935, he had become interested in physics. Columbia awarded him a Kellett Fellowship to Cambridge University, where he studied physics at Cavendish Laboratory under Lord Rutherford and Maurice Goldhaber, and encountered notable physicists, including Edward Appleton, Max Born, Edward Bullard, James Chadwick, John Cockcroft, Paul Dirac, Arthur Eddington, Ralph Fowler, Mark Oliphant and J. J. Thomson. 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 BA degree by Cambridge.

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 PhD 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 was part of Rabi's team that also included Jerome Kellogg, Polykarp Kusch, Sidney Millman and Jerrold Zacharias. Ramsey worked with them 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 PhD 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 married Elinor Jameson of Brooklyn, New York, and accepted a teaching position at the University of Illinois at Urbana–Champaign. The two expected to spend the rest of their lives there, but World War II intervened. 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. Ramsey was one of the scientists recruited by Rabi for this work.

Initially, Ramsey was in Rabi's magnetron group. When Rabi became a division head, Ramsey became the group leader. The role of the group was to develop the magnetron to permit a reduction in wavelength from 150 centimetres (59 in) to 10 centimetres (3.9 in), and then to 3 centimetres (1.2 in) or X band. Microwave radar promised to be small, lighter and more efficient than older types. Ramsey's group started with the design produced by Oliphant's team in Britain and attempted to improve it. The Radiation Laboratory produced the designs, which were prototyped by Raytheon, and then tested by the laboratory. In June 1941, Ramsey travelled to Britain, where he met with Oliphant, and the two exchanged ideas. He brought back some British components, which were incorporated into the final design. A night fighter aircraft, the Northrop P-61 Black Widow, was designed around the new radar. 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, working for Edward L. Bowles in the office of the Secretary of War, Henry L. Stimson.

In 1943, Ramsey was approached by Robert Oppenheimer and Robert Bacher, who asked him to join the Manhattan Project. Ramsey agreed to do so, but the intervention of the project director, Brigadier General Leslie R. Groves Jr., was necessary in order to prise him away from the Secretary of War's office. 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. There were only two Allied aircraft large enough: the British Avro Lancaster and the US Boeing B-29 Superfortress. The United States Army Air Forces (USAAF) wanted to use the B-29 if at all possible, even though it required substantial modification. Ramsey supervised the test drop program, which began at Dahlgren, Virginia, in August 1943, before moving to Muroc Dry Lake, California, in March 1944. Mock-ups of Thin Man and Fat Man bombs were dropped and tracked by an SCR-584 ground-based radar set of the kind that Ramsey had helped develop at the Radiation laboratory. Numerous problems were discovered with the bombs and the aircraft modifications, and corrected.