Science: How to Keep Time

A millionth of a second is a long, long time in modern science; many spectacular things, such as atomic bomb explosions, happen much faster. Therefore scientists are forever striving for more accurate measurement of time. In the latest Physical Review Letters, Harvard Physicists H. Mark Goldenberg. Daniel Kleppner and Norman F. Ramsey tell about a new electronic clock that they hope will keep time with an accuracy of one part in i million billion. This is equivalent to making an error of only one second in 30 million years.

In the Harvard clock a thin trickle of hydrogen gas flows through...