PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 768 March 9, 2006 by Phillip F. Schewe, Ben Stein, and Davide Castelvecchi
LIGHT-LIGHT INTERACTIONS IN VACUUM will be possible soon. Vacuum---the very name suggests emptiness and nothingness---is actually a realm rife with potentiality, courtesy of the laws of quantum electrodynamics (QED). According to QED, additional (albeit virtual) particles can be created in the vacuum, allowing light-light interactions. Physicists from Umea University (Sweden) and the Rutherford Appleton Lab (UK) hope to explore the vacuum by aiming three powerful laser streams at each other. The laser light is not aimed at any material target and is not trying to initiate any nuclear fusion. Instead the three beams will merge to produce a fourth stream with a wavelength shorter than any of the input beams. This idea of mixing beams has been broached before but the earlier proposals had the beams all in a single plane. The Swedish-British proposal (contact Mattias Marklund, 46-90-786-7717, [EMAIL PROTECTED]), by contrast, foresees a fully three-dimensional wave mixing process. The actual experiment is planned to be carried out over the next year at the Rutherford Appleton Lab. By carefully polarizing the incoming light beams, the number of photons in the output beam can be controlled, providing valuable information about the interactions that took place in the vacuum. What is this "four-wave mixing" good for? For studying QED itself, but also for testing theories that propose the existence of minor departures from Lorentz invariance, which is the proposition (essential to special relativity) that there is no preferred frame of reference. Light-light interactions might also be used to explore various hypotheses related to dark energy. (Lundstrom et al., Physical Review Letters, 3 March 2006) USING FIBER OPTICS TO PINPOINT STRUCTURAL PROBLEMS EARLY. At this week's OFC/NFOEC optical fiber meeting in California, University of Ottawa physicists reported an optical system that detects problems in important structures, including natural-gas pipes and concrete columns, more precisely and potentially earlier than before. Already being considered for commercial production, the new system can catch much earlier signs of costly and dangerous structural failures than previously possible. Called the Distributed Brillouin Sensor (DBS), the system uses fiber optics to detect deformation, cracks, and bending in two structures under real-world conditions. In one demonstration, conducted with Edmonton-based engineering firm C-Fer Technologies and Trans-Canada Pipeline Limited, the Ottawa researchers (Xiaoyi Bao, [EMAIL PROTECTED]) deployed the DBS system on a section of steel pipe designed to transport natural gas. Laying 10 lines of optical fiber along the axis of the pipe, they created one pulse of laser light and one continuous light wave, each traveling in opposite directions in the fiber. When the researchers applied tension and compression to the pipe to mimic real-world disturbances, it produced vibrations (sound waves). Through a phenomenon called the Brillouin effect, these sound waves then slightly changed the speed of light in the affected part of the fiber and consequently altered the frequency difference between the two propagating light waves, providing precise information on the mechanical strains that were applied to the pipe. Unlike present structural health analysis, which is done on a spot-by-spot basis, DBS can detect problems over all points in the entire structure and pinpoint the location of a structural deformation to within 5 centimeters, while measuring mechanical strains as low as 20 microstrains (20 millionths of a strain, a dimensionless property that generally reflects the structure's change in length over its original length). This exceeds the 1-meter resolution and 50 microstrains that the construction industry has wanted and expected. In another demonstration, conducted with civil engineers at the University of Ottawa, the researchers tested the DBS system on a concrete column encased with fiber-reinforced rods and sheets. Subjecting the column to simulated seismic forces such as those that would occur in an earthquake or tsunami, the researchers could detect signs of debonding (in which the concrete detached from the fiber casing) and the cracks (crushing) of concrete as a result of compression forces. Unlike competing techniques, the system could readily tell the difference between debonding and crushing. The Ottawa researchers say that DBS can prevent potentially life-threatening and environmentally damaging accidents and multimillion-dollar repairs. In addition, the technique can improve the testing of structures and materials by providing valuable information during the testing process. (Paper OTuL7 at meeting, www.ofcnfoec.org ) *********** PHYSICS NEWS UPDATE is a digest of physics news items arising from physics meetings, physics journals, newspapers and magazines, and other news sources. It is provided free of charge as a way of broadly disseminating information about physics and physicists. For that reason, you are free to post it, if you like, where others can read it, providing only that you credit AIP. Physics News Update appears approximately once a week. AUTO-SUBSCRIPTION OR DELETION: By using the expression "subscribe physnews" in your e-mail message, you will have automatically added the address from which your message was sent to the distribution list for Physics News Update. If you use the "signoff physnews" expression in your e-mail message, the address in your message header will be deleted from the distribution list. Please send your message to: [EMAIL PROTECTED] (Leave the "Subject:" line blank.)
