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Number 446, September 1, 1999 by Phillip F. Schewe and Ben Stein
DARK MATTER ANNIHILATION AT THE GALACTIC CENTER. How does the presumed massive black hole at the center of our galaxy shape the distribution of the presumed halo of dark matter in its vicinity? Paolo Gondolo of the Max Planck Institute of Physics (Munich, Germany) and Joseph Silk of Oxford (UK; also UC Berkeley) suggest the black hole sculpts the dark matter into a dense spike where particle annihilation (or, more to the point, self-annihilation, since one of the leading hypothetical dark-matter particles is the "neutralino", which is its own antiparticle) would be enhanced. Of all the annihilation products (e.g., electrons, positrons, protons, etc.) issuing from the galactic center (a region half a light year wide) neutrinos would be the most serviceable since they can travel to Earth undeflected by magnetic fields. Gondolo and Silk have calculated how present and future neutrino telescopes can probe the density of inner halo dark matter. (Physical Review Letters, 30 August; gondolo@mppmu.mpg.de, 011-49-893-235-4427.)
TO MEASURE LOCAL GRAVITY with an uncertainty of 3 parts per billion, Steven Chu uses an atom interferometer, in which cesium atoms are treated like waves, split apart into two wavelets, each of which takes a separate path. When the wavelets are brought back together they produce an interference pattern which depends sensitively on the local force (gravity) tugging on the atoms. Not only is this an improvement (by a factor of a million) in accuracy over previous atom interferometers but represents, according to Chu, "the best confirmation of the equivalence principle between a quantum and macroscopic object." (Peters et al., Nature, 26 August 1999.)
NERVE CELLS MAY HOLD THEIR FIRE to allow their neighbors to send electrical signals, researchers have proposed, potentially explaining how interconnected networks of nerve cells send information with high fidelity, and providing insights into how to design better signal-processing devices for electronic equipment such as CD players. At a cocktail party, many people talk simultaneously, and one is able to hear several nearby conversations at once. In a conference call, on the other hand, people generally take turns to speak. Researchers (Doug Mar, Boston University, 617-353-5463) have proposed that an interconnected network of nerve cells is similar to a conference call: when a nerve cell fires, its neighbors are inhibited and do not fire until it is done. One consequence is that the nerve cells fire in rapid succession, permitting the network to transmit signals at higher frequencies. Moreover, the pattern of random firings of nerve cells, corresponding to noise, is smoothed out, enabling the cells to convey information with higher fidelity. Finding direct evidence for these phenomena in biological systems will be challenging, because it is currently difficult to measure accurately the firing patterns of several interconnected neurons simultaneously. In the meantime, the researchers are working with Analog Devices, Corp. in Massachusetts to apply the lessons from the theory to creating biologically inspired networks of interconnected electric circuits with improved characteristics, such as an extended range of operating frequencies. (Mar et al., Proceedings of the National Academy of Sciences, 31 August.)
US HIGH-SCHOOL PHYSICS ENROLLMENTS AT A POSTWAR HIGH. The percentage of US high-school students taking physics has risen by eight percent in the last decade, reaching an all-time high of 28% since the end of World War II. In the late 1990s, girls now represent almost half (47%) of students taking high-school physics (as opposed to 39% in 1987). However, African-American and Hispanic students remain underrepresented in physics classes; the same holds true for women and non-white physics teachers. These statistics come from a new AIP report entitled "Maintaining Momentum: High School Physics For A New Millennium." (Contact Mark McFarling at mmcfarli@aip.org. Full report at www.aip.org/statistics/trends/reports/hsreport.pdf.)
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