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Number 361, March 4, 1998 by Phillip F. Schewe and Ben Stein
A REPULSIVE FORCE IN THE UNIVERSE seems to be at work on a cosmic scale, at least partly neutralizing the attractive force of gravity. Astronomers have arrived at this conclusion after a series of observations of distant supernovas showed that the expansion of the universe has not only not slowed (through the mutual attraction of galaxies) but seems to be accelerating. The most recent data to support this view were reported by an LBL group in January at the AAS meeting in Washington (Update 355) and by the multi-national High-z Supernova Search Team two weeks ago in California at a symposium on dark matter (Science, 27 February 1998). Several theories attempt to explain the positive push; one popular model sees the effect as coming from a huge reservoir of energy hidden in the universal vacuum. An early precedent for this notion was Einstein's use of an antigravity fudge factor called the cosmological constant in his gravitational equations. (See also Science News, 28 Feb, and Updates 345 and 360.)
USING CHAOS TO ENCODE AND DECODE MESSAGES has been demonstrated in an all-optical system for the first time by Rajarshi Roy (404-894-5201) and colleagues at Georgia Tech. In 1994, Roy and co-workers showed that it is possible to synchronize two lasers even if their signals are chaotic, in which small differences in their starting conditions lead to outputs which quickly become unpredictable. (Update 170) In the present experiment, the researchers amplified an optical "message signal" (consisting of a square wave) in an erbium- doped fiber amplifier (EDFA) and combined it with a chaotic signal from an erbium-based laser. Transmitted by means of an optical fiber, the combined signal entered a second, nearly identical EDFA, which produced chaotic fluctuations synchronized with the first. Subtracting out this chaotic signal allowed them to recover the original message. (Science, 20 February 1998; see also Georgia Tech press release). Researchers at UC-San Diego and Cornell (Henry Abarbanel, UCSD, 619-534-5590) have shown that this technique can work even if the signal travels through a noisy communications channel. (Abarbanel et al., upcoming article in Physical Review Letters.)
A NEW LIMIT ON PHOTON MASS has been established using a tabletop apparatus. The mass of photons, the corpuscular manifestation of light, is usually assumed to be zero. A non- zero photon mass would have several implications, such as a frequency-dependent speed for light and the existence of longitudinal electromagnetic waves (in the conventional waves we know, the electric and magnetic fields oscillate perpendicularly to the line of travel). The best previous limit on photon mass (using a reasonably connected line of inference) comes from observations of Jupiter's magnetic field. The value, 6 x 10-16 eV, was determined in 1975. Now, Roderic Lakes of the University of Wisconsin (608-265-8697) has tightened the limit further---to below 7 x 10-17 eV---by carefully watching for anomalous torques in the motion of a Cavendish balance, basically a steel toroid wrapped in current-carrying coils. Essentially, Lakes' novel approach is to seek a cosmic vector potential. In electromagnetism, the change in the vector potential (denoted by the letter A) is related to the strength of the magnetic field in that part of space, just as an electric field can be calculated from a changing electric potential (voltage). Normally, A cannot be measured point-for-point in space, but it could be if the photon's mass were nonzero. Lakes did not find a nonzero A, but his method might be useful (beyond setting a new limit on photon mass) for searching out new features of short-range forces, such as the nuclear strong force. (Physical Review Letters., 2 March.) Physical Review Letters, 2 March 1998.)
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