Norman F. Ramsey may not be a household name, but he was a giant of 20th-century experimental physics. His basic-science work earned him the 1989 Nobel Prize in Physics and laid the foundation for technologies now used by millions of people. He died last Friday at the age of 96.

In exploring how atoms and molecules absorb and emit light in magnetic fields, Ramsey paved the way for studies of nuclear magnetic resonance, the basis of MRI machines. His investigations also led to the development of atomic clocks. In his July 1993 article in Scientific American, which I had the pleasure of editing, Ramsey and co-author Wayne Itano discussed the need for ever more accurate time-keeping:

Few people complain about the accuracy of modern clocks, even if they appear to run more quickly than the harried among us would like. The common and inexpensive quartz-crystal watches lose or gain about a second a week—making them more than sufficient for everyday living. Even a spring-wound watch can get us to the church on time. More rigorous applications, such as communications with interplanetary spacecraft or the tracking of ships and airplanes from satellites, rely on atomic clocks, which lose no more than a second over one million years.

There might not seem to be much room for the improvement of clocks or even a need for more accurate ones. Yet many applications in science and technology demand all the precision that the best clocks can muster, and sometimes more. For instance, some pulsars (stars that emit electromagnetic radiation in periodic bursts) may in certain respects be more stable than current clocks. Such objects may not be accurately timed. Meticulous tests of relativity and other fundamental concepts may need even more accurate clocks. Such clocks will probably become available. New technologies, relying on the trapping and cooling of atoms and ions, offer every reason to believe that clocks can be 1,000 times more precise than existing ones. If history is any guide, these future clocks may show that what is thought to be constant and immutable may on finer scales be dynamic and changing. The sundials, water clocks and pendulum clocks of the past, for example, were sufficiently accurate to divide the day into hours, minutes and seconds, but they could not detect the variations in the earth's rotation and revolution.

A key application of atomic clocks is in the Global Positioning System, which relies on accurate timing and frequency signals among satellites so that they know where they are—and thereby tell you where you are.

Ramsey may be best known for the creation of the hydrogen maser clock, which he described in his article (and was illustrated this way):



In this instrument, a radio frequency discharge first splits hydrogen molecules held in a high-pressure bottle into their constituent atoms. The atoms emerge from a small opening in the bottle, forming a beam. Those in the higher energy level are focused by magnetic fields and enter a specially coated storage bulb surrounded by a tuned, resonant cavity. In the bulb, some of these atoms will drop to a lower energy level, releasing photons of microwave frequency. The photons will stimulate other atoms to fall to a lower energy level, which in turn releases additional microwave photons. In this manner, a self-sustaining microwave field builds up in the bulb—thus the name "maser." The tuned cavity around the bulb helps to redirect photons back into the system to maintain the stimulated emission process.

The maser oscillation persists as long as the hydrogen is fed into the system. A loop of wire in the cavity can detect the oscillation. The microwave field induces a current in the wire, which leads out of the cavity to a series of circuits. The circuits convert the induced current to a lower frequency signal suitable for generating timing pulses.

The full article, titled "Accurate Measurement of Time," reviews other kinds of atomic clocks and their pros and cons.

You can read a personal reminiscence by scientist and author Boulent Atalay on National Geographic's News Watch page.

Image of Norman Ramsey from his Nobel Prize announcement.