Using Nuclei To Tell Time

Most research in physics relies on precise measurements, and Higgins Professor of Physics Emeritus Norman F. Ramsey is responsible for developing the procedure that yields the most exact measurements that have ever been made.

As a result, the research for which he won the Nobel Prize has had a broad impact on physics in areas ranging from radio astronomy to particle physics.

The most famous application of Ramsey's work is in the atomic clock, which uses the stable internal properties of an atom to define more accurately the units of time people use every day.

Ramsey's procedure is so precise that the number of times the nucleus of a cesium atom rotates now can be measured with an accuracy of one part in a trillion.

It was Ramsey's increased precision that enabled scientists to use the atomic clock for the official measurement of world time. In 1967, his method was used to redefine the second as the length of time it takes a cesium nucleus to rotate 9,192,631,770 times.

Previously, the second had been based on a much less accurate measurement, the motion of the earth about the sun.

To make these measurements, a beam of atoms, usually cesium, is sent through an oscillating magnetic field, which reverses the spin on the atoms' electrons.

After the atoms are deflected by another magnet, the change in its path caused by the oscillating field can be detected, and from this the frequency of the atom's internal rotation can be measured.

This general procedure was created initially by I.I. Rabi, a Nobel laureate who was Ramsey's Ph.D adviser at Columbia. Ramsey improved on Rabi's method by separating the oscillating field into two smaller fields, which increased the precision of the experiment and the accuracy of the measurement.

A significant portion of Ramsey's work at Harvard dealt with developing this technique for hydrogen atoms, in what is called a hydrogen maser.