Number 434, June 18, 1999 by Phillip F. Schewe and Ben Stein
MEASURING THE FREQUENCY OF LIGHT TO NEW LEVELS OF PRECISION is now possible, opening a new chapter in metrology which may lead to greatly improved determinations of fundamental constants and one way of making powerful optical versions of atomic clocks. Even the most advanced electronic equipment cannot directly measure electromagnetic frequencies higher than roughly 100 GHz (in the microwave range, where frequencies can be counted in terms of the number of oscillations induced in an electrical circuit). Now, researchers at the Max Planck Institute for Quantum Optics (Thomas Udem, 011-49-89-32905-257,Thomas.Udem@mpq.mpg.de) have shown that a femtosecond laser pulse can be used as a "ruler" for precisely determining the frequencies of visible light (which goes up to roughly a million GHz). A femtosecond pulse does not contain a single frequency; rather, its spectrum consists of many frequency peaks which give the appearance of a comb with the tips pointing upwards. The researchers have now shown that the very regular spacing of these peaks can potentially be used to measure differences of at least 20 THz (20,000 GHz) between two electromagnetic waves with a precision as high as 3 parts in 1017 (Udem et al., Optics Letters, 1 July 1999). For comparison, the best atomic clocks today, based on measuring radio-frequency atomic transitions, have accuracies of 2 parts in 1015. Locking the wave of interest to the low-frequency end of the femtosecond comb and locking a reference wave to the high-frequency end can determine the frequency difference between the two waves and ultimately allow one to reconstruct the frequency of the visible-light wave. Using femtosecond lasers, the researchers have already measured the frequency of visible light emitted by a cesium atom undergoing a specific transition (specifically, its "D1 line") to a precision of 120 parts per billion, almost 1000 times more precise than previous measurements of that light. (Udem et al, Phys. Rev. Lett., 3 May 1999). The D1 frequency can be plugged into a formula for precisely calculating the fine structure constant, which dictates the strength of the electromagnetic force.
HOW DO COMPLEX ORGANISMS FORM? A Darwinian mechanism of natural selection plus random mutation is not quite enough to explain the complex features of life on earth. For example, it does not predict or anticipate the fact that an ecosystem or a global community has a hierarchical structure, with interactions that take place at several size scales. For example, people communicate with each other in an organization; and organizations communicate with each other in a larger community. Barbara Drossel of the University of Manchester in England (011-44-161-275-4201, firstname.lastname@example.org) has introduced a simple mathematical model for describing how originally independent units may develop into a complex organism with a hierarchical structure. In her model hierarchy comes about because of the increase of a quantity she calls "productivity" (similar to "fitness" in biology and "utility" in economics). Individual units communicate with each other to increase productivity which leads, at the very least, to larger groups. Drossel's model incorporates the additional idea that the size of a group is restricted by the limited capacity of individuals to communicate and to travel. Therefore, she introduces a "communication cost" per partner and per unit distance to the partner. This encourages the formation of groups and ultimately the formation of supergroups and groups of supergroups which interact with each other. (Drossel, Physical Review Letters, 21 June 1999.)