Ramsey, Norman (Foster)

(1915– ) US physicist: developed the caesium clock, and the hydrogen maser.

Educated at Columbia New York and Harvard, Ramsey became an associate professor of physics at Harvard in 1947. He received a share of the Nobel Prize for physics in 1989 for inventing a technique which greatly improved the accuracy of caesium atomic clocks, our current time standard. He also developed its application in the hydrogen maser. Quantum theory indicates isolated atoms will emit light when electrons drop from an excited to the ground state. Had developed the use of characteristic radiation to induce such emission, by passing a beam of particles through a magnetic field with a superimposed electromagnetic field. Ramsey knew that accuracy was limited by the time the atoms spent in the fields. He introduced two separate oscillatory electromagnetic fields, which produce an interference pattern, which is no longer so sensitive to the uniformity of the magnetic fields. He showed that more than two fields can be used if separated in time as well as space. In the caesium clock a transition between two closely spaced levels is used, and Ramsey could now do this far more accurately. Later he and D Kleppner developed the hydrogen maser, which is similar to a laser but emits microwaves.