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Date: Thu, 18 Sep 97 10:35:28 EDT From: physnews@aip.org (AIP listserver) Subject: update.337

PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 337 September 18, 1997 by Phillip F. Schewe and Ben Stein

REAL PHOTONS CREATE MATTER. Einstein's equation E=mc^2 formulates the idea that matter can be converted into light and vice versa. The vice-versa part, though, hasn't been so easy to bring about in the lab. But now physicists at SLAC have produced electron-positron pairs from the scattering of two "real" photons (as opposed to the "virtual" photons that mediate the electromagnetic scattering of charged particles). To begin, light from a terawatt laser is sent into SLAC's highly focused beam of 47-GeV electrons. Some of the laser photons are scattered backwards, and in so doing convert into high-energy gamma ray photons. Some of these, in turn, scatter from other laser photons, affording the first ever creation of matter from light-on- light scattering of real photons in a lab. (D.L. Burke et al., Physical Review Letters, 1 September 1997.)

DNA-GOLD NANOPARTICLES, employing the talent of DNA strands for recognizing and attaching to complementary strands and gold's electronic and optical properties, operate as a new kind of biosensor. Scientists at Northwestern University glue various "probe" DNA segments onto tiny gold particles (13 nm wide). When a "target" single-stranded DNA introduced into the solution happens to be complementary to DNA already stuck to the particles, the probe and target strands link up, creating a sort of polymer network whose color is different from that of the original solution. Thus recognition of the target DNA is signaled by a color change. The researchers can already use this approach to detect single-strand DNA samples in 10-femtomolar amounts. (Science, 22 August 1997.)