To Professor Daniel Kleppner, one 19th century development marked the turning point in the precision of physics.

"For me, the breakthrough was the invention of the Michelson Interferometer by A.A. Michelson in 1883," says Kleppner, the Lester Wolfe Professor of Physics at MIT. "The interferometer made it possible to measure distances to a small fraction of the wavelength of light. Since a meter typically has a million wavelengths, the resulting precision is enormous."

Kleppner examines the evolution of precision measurements, from the King's thumb to the atomic clock and beyond, in "How Physics Got Precise," the Fermilab Colloquium presentation on Wednesday, January 19 at 4 p.m. in Wilson Hall's 1 West conference room. Kleppner sets the stage with the impressive accuracy of time measurement in early civilizations.

"The Babylonians, about 7000 B.C., and Hipparchus in Egypt, about 150 B.C., knew the length of the year to about 5 minutes," Kleppner says. "The length was found by telling the time of the day of the equinoxes over a long time. If you estimate it to one hour, which is pretty easy, and keep count of the days between equinoxes for twelve years, you have the length of the year to five minutes. Tycho Brahe, in 1600, knew the length of the year to about three seconds."

Kleppner also offers another surprise for post-modern time measurement, involving the re-introduction of human-made or human-related "artifacts."

"For a new generation of atomic clocks," he says, "time keeping could be so precise that the effects of the local gravitational potentials on the clock rates would be important. This would force us to re-introduce an artifact into the definition of the second-specifically, the location of the primary clock."

The classic example of such an artifact, of course, is the King's thumb. Or foot.

"Nobody seems to know which king's thumb defined the foot and inch," Kleppner says. "They probably changed whenever there was a new king."

Kleppner cites one of the "glories" the French Revolution as establishment of an international system of units, based on supposedly 'natural' standards.

"The meter was defined in terms of the circumference of the Earth," he says, "but this was difficult to determine in a practical fashion and in 1888 it was defined in terms of an artifact, a platinum bar which resided in Paris. Similarly, the unit of mass was a lump of platinum in Paris."

The second was defined then as a fraction of the time it took the Earth to rotate; today, the second is defined in terms of the natural frequency within an atom, which can be measured with high precision.

"However, when General Relativity is taken into account," Kleppner explains, "the relative rate of two clocks depends on the gravitational potential. The rate changes by about one part in 1018 for each meter of height near the Earth. This means that eventually the second will have to be defined in terms of the natural frequency of an atom at some convenient place. I suggest MIT, and the French would certainly propose Paris. But the real experts on this are at the National Institute of Standards and Technology in Boulder, Colorado, and they will probably opt for Boulder."