Isidor Isaac Rabi (; born Israel Isaac Rabi, 29 July 1898 – 11 January 1988) was an American physicist who won the Nobel Prize in Physics in 1944 for his discovery of nuclear magnetic resonance, which is used in magnetic resonance imaging. He was also one of the first scientists in the United States to work on the cavity magnetron, which is used in microwave radar and microwave ovens.
Born into a traditional Jewish family in Rymanów, Galicia, in what was then part of Austria-Hungary, Rabi came to the United States as a baby and was raised in New York's Lower East Side. He entered Cornell University as an electrical engineering student in 1916, but soon switched to chemistry. Later, he became interested in physics. He continued his studies at Columbia University, where he was awarded his doctorate for a thesis on the magnetic susceptibility of certain crystals. In 1927, he headed for Europe, where he met and worked with many of the finest physicists of the time.
In 1929, Rabi returned to the United States, where Columbia offered him a faculty position. In collaboration with Gregory Breit, he developed the Breit–Rabi equation and predicted that the Stern–Gerlach experiment could be modified to confirm the properties of the atomic nucleus. His techniques for using nuclear magnetic resonance to discern the magnetic moment and nuclear spin of atoms earned him the Nobel Prize in Physics in 1944. Nuclear magnetic resonance became an important tool for nuclear physics and chemistry, and the subsequent development of magnetic resonance imaging from it has also made it important to the field of medicine.
During World War II he worked on radar at the Massachusetts Institute of Technology (MIT) Radiation Laboratory (RadLab) and on the Manhattan Project. After the war, he served on the General Advisory Committee (GAC) of the Atomic Energy Commission, and was chairman from 1952 to 1956. He also served on the Science Advisory Committees (SACs) of the Office of Defense Mobilization and the Army's Ballistic Research Laboratory, and was Science Advisor to President Dwight D. Eisenhower. He was involved with the establishment of the Brookhaven National Laboratory in 1946, and later, as United States delegate to UNESCO, with the creation of CERN in 1952. When Columbia created the rank of University Professor in 1964, Rabi was the first to receive that position. A special chair was named after him in 1985. He retired from teaching in 1967 but remained active in the department and held the title of University Professor Emeritus and Special Lecturer until his death.
He is also known as Israel Isaac Rabi.
Rabi died at his home on Riverside Drive in Manhattan from cancer on 11 January 1988. His wife, Helen, survived him and died at the age of 102 on 18 June 2005. In his last days, he was reminded of his greatest achievement when his physicians examined him using magnetic resonance imaging, a technology that had been developed from his ground-breaking research on magnetic resonance. The machine happened to have a reflective inner surface, and he remarked: "I saw myself in that machine... I never thought my work would come to this."