I. I. Rabi proposed the atomic clock in 1945

Person: Isidor Isaac Rabi
Lived: 1898–1988
Field: Physics
From: Polish-born American

The story

For centuries the best clocks relied on swinging pendulums and, later, vibrating quartz crystals, but their accuracy suffered from changes in temperature. Physicists such as James Clerk Maxwell and Lord Kelvin had speculated that the natural vibrations of atoms might keep better time. Isidor Isaac Rabi's magnetic resonance experiments at Columbia University in the 1930s measured those atomic frequencies with extraordinary precision, and he realised that they could serve as the ticking of a clock far steadier than any mechanical device.

In 1945 he proposed the idea publicly in the Richtmyer Memorial Lecture of the American Association of Physics Teachers. The science journalist William L. Laurence wrote it up for The New York Times under the headline "'Cosmic pendulum' for clock planned". The Jewish Telegraphic Agency later credited Rabi's discoveries as instrumental in the development of the atomic clock.

The idea moved quickly from lecture to laboratory. A prototype measuring transitions in the ammonia molecule was built in 1949, and before long Rabi's former colleagues Jerrold Zacharias and Norman Ramsey had built atomic clocks. In 1949 Ramsey invented the separated oscillatory field method, which greatly improved the accuracy of resonance measurements, and with his student Daniel Kleppner he developed the hydrogen maser. In 1955 Louis Essen and Jack Parry built the first practical caesium atomic clock at the National Physical Laboratory in Britain. Ramsey received the 1989 Nobel Prize in Physics for his method and its use in atomic clocks.

Since 1967 the international definition of the second has been based on a precise frequency of the caesium-133 atom, measured with atomic clocks that apply Ramsey's refinement of Rabi's resonance technique.

Atomic clocks also made satellite navigation possible. Systems such as GPS and Europe's Galileo work by comparing precise time signals broadcast from satellites, and a timing error of just one billionth of a second corresponds to a position error of about 30 centimetres. Precise atomic time now underpins navigation, telecommunications and scientific research around the world.

Why it matters

Every smartphone map, financial network and telecommunications system relies on the atomic timekeeping that grew from Rabi's idea.

About Isidor Isaac Rabi

Galician-born physicist who discovered nuclear magnetic resonance in molecular beams, won the 1944 Nobel Prize, and helped found Brookhaven and CERN, the root of today’s MRI and atomic clocks.

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Isidor Isaac Rabi

1898–1988 · Physics

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