American physicist who received one-half of the Nobel Prize for Physics in 1989 for his development of a technique to induce atoms to shift from one specific energy level to another. (The other half of the prize was awarded to Wolfgang Paul (Paul, Wolfgang) and Hans Georg Dehmelt (Dehmelt, Hans Georg).) Ramsey's innovation, called the separated oscillatory fields method, found application in the precise measurement of time and frequency.

Ramsey studied physics at Columbia University, N.Y., and received a Ph.D. degree there in 1940. He also earned a D.Sc. degree from the University of Cambridge in 1954. After teaching at various American universities in the 1940s, he taught at Harvard University from 1947, becoming Higgins professor of physics there in 1966.

In 1949 Ramsey perfected a method to study the structure of atoms by sending them through two separate oscillating electromagnetic fields. The rapid energy-level transitions thereby induced in a beam of atoms produced an interference pattern that could provide important data about the structure and behaviour of atoms. When synchronized with a microwave oscillator, the atoms' oscillations could also be used to measure the passage of time with extreme accuracy, thus providing the basis for the modern cesium atomic clock, which sets present time standards. In the 1950s Ramsey helped develop the hydrogen maser, a microwave-emitting relative of the laser.