Number 369, April 29, 1998 by Phillip F. Schewe and Ben Stein
A DENSE NUCLEAR ENVIRONMENT CAUSES PARTICLES TO LOSE WEIGHT, or at least act as if they have a lower mass, new experiments have suggested. In the world of room-temperature semiconductors, an electron moves through silicon more slowly than it does through gallium arsenide, even when subjected to the same forces. Physicists therefore say that an electron has a lower "effective mass" in gallium arsenide. At the April APS/AAPT meeting in Columbus, Johann Peter Wurm of the CERES collaboration at CERN announced experimental evidence for a similar effect in particles produced inside collisions between heavy nuclei. One such particle is the rho meson, an object consisting of an up quark and antidown quark bound together. In a vacuum, the rho has a mass of 780 MeV/c2, but in the fireball conditions of the collision, it acts as if it has a significantly lower mass (perhaps as low as 400 MeV/c2). In turn, the rho's lower effective mass changes its lifetime and its rate of decay into pairs of leptons such as electrons and positrons which were observed experimentally. According to Volker Koch of Lawrence Berkeley Laboratory, this effect can take place for particles inside any nuclear environment, from the most common atoms to superdense neutron stars.
IN MAGNETIC SOURCE IMAGING(MSI), super-sensitive superconducting detectors sample the tiny magnetic fields that come from electrical signals flowing through the body. MSI has some advantages over more established imaging methods such as MRI or PET in that it has a sharper time resolution (it can produce more images per second) and does not base its imaging on local blood flow (which can lag behind the actual activity of interest, in the heart or brain, say). Sam Williamson of NYU (212-998-7692) has previously made detailed maps of the auditory cortex, showing where in the brain tones of different pitch are perceived. At the recent meeting of the American Physical Society in Los Angeles, Williamson reported on the behavior over time of signals received in the various stages of the visual cortex. In particular, he showed that when a subject is given a visual cue, such as the sight of a checkerboard pattern, parts of the visual cortex light up in a succession of stages, for periods as long as 30 seconds, suggesting hints of a "remembering" pathway in the brain. Each part in this pathway seems to have its own memory lifetime. (Science, 3 April 1998.)
THE MOST DISTANT GAMMA RAY BURST, at a red shift of 3.4, was discovered in December and observed at several wavelengths. Optical measurements (yielding a red shift) of the object made with the Keck Telescope and the Hubble Space Telescope were announced by Caltech astronomer Shrinivas Kulkarni at last week's APS/AAPT meeting. Optical astronomers were led to the burst's spot in the sky by x-ray observations made by the BeppoSax satellite. (Science, 24 April 1998.)
ARTIFICIAL VACUUMS ON EARTH. Inside his trap for antiparticles, Gerald Gabrielse of Harvard achieves a vacuum of 5 x 10-17 torr, corresponding to about 2 atoms per cm3. By comparison, the atoms between stars are spaced a centimeter apart; in the gaseous halo of our galaxy the spacing is about 10 cm; and for intergalactic voids, it's up to 10 m, the lowest density (or highest vacuum) ever measured, at least indirectly. (New Scientist, 25 April.)