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Number 419, March 19, 1999 by Phillip F. Schewe and Ben Stein
NANOMETER-SCALE IMAGES OF SOUND WAVES on surfaces have been achieved by several groups, enabling researchers to observe oscillations of single atoms and promising ordinarily difficult-to-obtain nanoscopic information on material properties beneath the surface. At this week's international joint meeting in Berlin of the Acoustical Society of America, the European Acoustics Association, and the German Acoustical Society (ASA/EAA/DAGA '99), Ute Rabe of the Frauenhofer Institute for Nondestructive Testing in Germany (rabe@izfp.fhg.de) showed that such images are possible by generating ultrasound waves on a surface and detecting them with a scanning probe microscope. Eduard Chilla of the Paul Drude Institute in Berlin (e.chilla@pdi-berlin.de) used atomic force microscopy techniques to image insulators and a scanning tunneling microscope to image conductors. The STM in particular could record oscillations of single atoms responding to a sound wave. Using an AFM in various operating modes, Andrew Kulik of the Federal Polytechnic Institute of Lausanne, Switzerland (root@igahpse.epfl.ch) showed images of sound waves coursing through a tin sample. Different features in the image corresponded to regions of increased stiffness or flexibility, and likely were the sites of grain boundaries in the tin sample. In general, these techniques can potentially provide nanoscopic details on the elastic properties of a material and other subsurface information, such as the stress between different layers of a material.
SPACE-TIME FUZZINESS, the notion that space is like an irregular foam at the smallest of size scales (the Planck scale, 10-35 m) foreseen in current theories, should be detectable with gravity-wave detectors now under construction. So says Giovanni Amelino-Camelia of CERN, who believes that high-precision instruments like the Laser Interferometer Gravitational Observatory (LIGO), being built to detect the infinitesimal distortions of space caused by a passing gravitational wave, would also be able to probe the fundamental "noise" of the Planck froth. (Nature, 18 March 1999.)
THE ANTARCTIC MUON AND NEUTRINO DETECTOR ARRAY (AMANDA) watches the sky for TeV neutrinos. It does this by looking inward: using the whole of the earth to screen out all other particles, the detection scheme counts on the fact that only neutrinos can navigate safely through our planet. Emerging into the south polar ice mass, the high energy neutrinos (coming from uncharted violent astrophysical processes) will occasionally interact with atoms, creating muons whose potent energy is partly converted into cones of (Cerenkov) light that can be seen by strings of photodetectors buried in the ice. (The holes for the detectors, stretching down as far as 2.4 km, are the deepest ever carved with hot water.) Neutrino interactions are rare under any circumstance; 20 unambiguous neutrino scattering events have been fully analyzed so far, but up to 100 per year are expected shortly. These events are in a neutrino energy range, above 50 GeV, far different from that of detectors such as Super Kamiokande, in which oscillations of lower-energy neutrinos (less than 10 GeV) were observed in 1998. With AMANDA the muon trajectory (and essentially that of the parent neutrino) can be determined to within about three degrees. Neutrinos with energies below 1 TeV would probably come from cosmic ray events in our atmosphere. For energies above 1 TeV, the neutrino sources are expected to be gamma-ray bursters and active galactic nuclei. (Physics Today, March 1999.)
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