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Physics News Update
The American Institute of Physics Bulletin of Physics News

Number 408, December 23, 1998 by Phillip F. Schewe and Ben Stein

A TWO-DIMENSIONAL ATOM GAS (2DAG) has been optically trapped for the first time. Two-dimensional arrays of particles are rare: 2-dimensional electron gases (2DEG) are at the heart of the quantum Hall effect, and planar sprinklings of atoms at the surface of a superfluid have been studied. But only now have physicists been able to trap atoms in a quasi-2-dimensional pancake only 200 nm thick about 800 nm above a gold substrate. Harald Gauck (, 011-49-7531-88-3837, -3752) and his colleagues at the University of Konstanz use a battery of lasers to cool and confine argon atoms in what is essentially a planar resonant cavity for atom waves. By shaping the local light fields, the researchers hope to fashion planar structures such as miniaturized atom interferometers and even one-dimensional atom waveguides. The planar gas of argon atoms is not dense enough to fall into a Bose-Einstein condensate, but the Konstanz optics setup might lend itself to achieving eventually a 2D condensate. (Gauck et al., Physical Review Letters, 14 December 1998; see figure at Physics News Graphics .)

THE NEOLITHIC TRANSITION IN EUROPE. Human behavior is much more complicated than the behavior of atoms, liquids, or planets. Nevertheless, physicists and mathematicians have attempted to apply their equations in the social sphere; recent examples recounted in Physics News Update include such topics as the arms race (Update 403), economics (Update 395), bird flocking (Update 395), and the making of group decisions (Update 385). Now two Spanish physicists have applied diffusion/reaction equations--- governing, say, the diffusion of one fluid through another with due allowance for chemical reactions along the way---to the diffusion of agricultural technology into Europe in the early centuries of the Neolithic epoch roughly 10,000 years ago. Such an effort had been tried before, but the model predictions poorly matched the observed anthropological, linguistic, and genetic data. According to Joaquim Fort of the University of Girona (, 011-34-972-418-490), a much better match can be achieved by using equations with additional "time-delay" terms of the type used successfully to model the spread of forest fires and epidemics. In the case of human migration a time delay factor would reflect the fact that generally the offspring of migrating farmers must grow to adulthood before they themselves "diffuse" outwards. Fort believes that mathematical modeling will become even more important to anthropology and history, but only in concert with high-quality data from fieldwork. (Fort and Mendez, Physical Review LettersScientific American, Oct 1990; see figure at Physics News Graphics.)

RELATIVISTIC SLEIGH RIDE. The December 11 issue of Fermi News seeks to answer the perennial question of how Santa Claus can, in the course of a single night, deliver gifts to each of the world's 2 billion children. Even if a full-scale quantum computer were to work out the optimum course plan St. Nick must still cover a flight path of some 160 million km and stop at 800 million homes along the way. How does he do it? By traveling at close to the speed of light, of course, which, incidentally, also explains why (thanks to time dilation) Santa never seems to age. The Fermi News article helpfully addresses such questions as to how it is that the fat fellow can fit into Lorentz-contracted chimneys in the first place and how one can determine the color of the Doppler-shifted light emitted by Rudolph-the-rednosed-reindeer at sleigh velocities approaching the speed of light.

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