March 2001 Volume 54, Number 3
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Cover: For measuring time, atomic ion clocks compete with neutral atom clocks for accuracy. In this example--built by Dana Berkeland and John Miller at NIST in Boulder, Colorado--a string of laser-cooled 199Hg+ ions is confined between and parallel to the trap rods in the center of the photo. On the right is the microwave horn, which drives the ions' hyperfine clock transition. The ions' state is detected by observing their fluorescence with the lens on the left. To learn more about the technology of measuring time, turn to the article by James Bergquist, Steven Jefferts, and David Wineland, which begins on page 37.

  Special Focus: Celebrating NIST's Centennial

Articles from the Physics Today Archive
We are proud to present a collection of articles from our archives that are associated with this issue. Updated throughout the month.

Adjusting the Values of the Fundamental Constants
The best values of the fundamental constants can rarely be determined by a direct measurement. Instead, they are usually found at the end of a chain of experimental observations and theoretical relationships -- Peter J. Mohr and Barry N. Taylor

Time Measurement at the Millennium
The latest clocks use a single ion to measure time with an anticipated precision of one part in 1018 -- James C. Bergquist, Steven R. Jefferts, and David J. Wineland

Joseph Loschmidt, Physicist and Chemist
When atoms and molecules were still quite hypothetical, Loschmidt used kinetic theory to get the first reasonable estimate of molecular size -- Alfred Bader and Leonard Parker

  Departments

Physics Update

Reference Frame
On the matter of the meter -- Daniel Kleppner

Letters
Diffusion Waves and Their Uses (Part 1)
Diffusion Waves and Their Uses (Part 2)
Szilard's Inventions Patently Halted
Elegance in Crystal Symmetry
Authors Amend Article on Strontium Ruthenate
Correction

Search and Discovery
Researchers stop, store, and retrieve photons--or at least the information they carry
Ultrafast x-ray diffraction tracks molecular shape-shifting

Issues and Events
Lane Leaves White House for Rice University
Electron Holography Lab Pushes Resolution Limit
Iridium Satellites Kept Aloft, Piggyback Magnetic Measurements Continue
Spy Station Retooled into Astronomy Institute
News Notes
Web Watch

Books
The Road since Structure: Philosophical Essays, 1970-1993, with an Autobiographical Interview, T. S. Kuhn, edited by J. Conant and J. Haugeland, and Thomas Kuhn: A Philosophical History for Our Times, S. Fuller (reviewed by K. G. Wilson)
The Physics of Foams, D. Weaire and S. Hutzler (reviewed by P.-G. de Gennes)
An Introduction to Chaos in Nonequilibrium Statistical Mechanics, J. R. Dorfman (reviewed by G. Gallavotti)
Introduction to Quantum Mechanics in Chemistry, M. A. Ratner and G. C. Schatz (reviewed by H. F. Bettinger and G. E. Scuseria)
Methods of X-Ray and Neutron Scattering in Polymer Science, R.-J. Roe (reviewed C. J. Glinka)
New Books

New Products
 Focus on test and measurement

We Hear That
International Science Prize Goes to Two
Gemant Award Bestowed on Trefil
Leal and Others Receive SOR Prizes
OSA Announces Engineering Excellence Awards
Carter Will Lead ACA in 2002
In Brief

Obituaries
Herbert Friedman
Louis Goldstein
Lawrence Marvin Langer

Job Opportunities