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Physics News Update
Number 417, March 8, 1999 by Phillip F. Schewe and Ben Stein

WEIGHING VIRUSES. A "nanobalance," a vibrating carbon nanotube that can directly weigh microscopic organisms and particles piggybacked onto it, has been demonstrated by researchers at Georgia Tech (Walt de Heer, 404-894-7879). First discovered in 1991, a carbon nanotube is essentially a sheet of carbon atoms (arranged in a hexagonal pattern) rolled up into a nanometer-diameter tube that is capped at both ends with carbon hemispheres. In the present experiment, the researchers viewed their nanotubes, protruding from a carbon fiber, with an electron microscope. The fiber in turn was attached to a gold wire, mounted on an insulator. This setup allowed the researchers to send an electrical current through the nanotubes. Putting the whole assembly on a special sample holder put it only 5-20 microns away from an oppositely charged electrode. Applying an oscillating electrical voltage to the wire, opposed to this electrode, caused the tubes to vibrate. Tuning the oscillation frequency to the natural "resonance frequency" of a given nanotube would cause it to vibrate by a maximum amount. Attaching a particle to such a nanotube would change this resonance frequency, enabling researchers to deduce the mass of the particle. With this technique, the researchers measured a graphite particle to be 22 femtograms (22 quadrillionths of a gram). In general, this technique can determine the mass of particles with similar dimensions in the range of femtograms to picograms (trillionths of a gram) . This includes viruses. (Poncharal et al., Science, 5 March 1999; see figure at Physics News Graphics.)

PARITY NONCONSERVATION (PNC) IN ATOMS is an area of fundamental physics--- testing parity conservation, or the proposition that interactions are the same even if you view them in a mirror ---carried out not at an immense particle accelerator but on a tabletop. Atoms are chiefly governed by the electromagnetic force, an ally of parity conservation but, according to current theory, also feel a very faint tug from the weak nuclear force, a notorious abuser of parity. Evidence for this has been the observation of rare "forbidden" transitions between particular atomic levels. For example, Carl Wieman of the University of Colorado (303-492-6963, cwieman@jila.colorado.edu) sees such transitions in cesium by detecting the fluorescence from 6S atoms boosted into the 7S state (see Physics Today, April 1997). In chemistry class, one learns of S and P states, in which configurations of the atoms' electrons is such as to give the atom spherical and dumbbell shapes, respectively. (Other states, corresponding to even higher angular momentum levels, also exist.) A photon, with an angular momentum unit of one, cannot link to S states, but a Z boson (one harbinger of the weak force) can. Now new precision in theoretical calculations of the transitions have progressed to a point where the theory of the electroweak force can be put to the test. The Colorado comparison reveals a small but intriguing discrepancy between theory and observation. Added impetus for these studies is the fact that atomic PNC is sensitive to electron-quark interactions different from those explored in high energy experiments. (S.C. Bennett and C.E. Wieman, Physical Review Letters, 22 March 1999; see figure at Physics News Graphics).

THE AMERICAN PHYSICAL SOCIETY CENTENNIAL will be celebrated in two weeks in Atlanta. This largest physics meeting in history will feature gatherings of more than 50 Nobel laureates and representatives of physical societies worldwide, and an array of important physics announcements, which will be featured in future Physics News Updates. For more information see the APS Centennial Meeting website.