Number 100, October 23, 1992 by Phillip F. Schewe and Ben Stein
HIGH-VELOCITY PULSARS , neutron stars born in supernovas and flung out of the plane of the galaxy at velocities greater than 800 km/sec, may well be contributing to the gamma-ray burst emissions measured by the orbiting Gamma Ray Observatory. The recent GRO mapping of the burst population found an isotropic distribution, thus challenging an earlier supposition that gamma bursts came primarily from pulsars, which were believed to reside mostly in the galactic plane. Hui Li and Charles Dermer of the University of Texas suggest that gammas from high-velocity pulsars high above the galaxy disk could account for the burster pattern. Others have argued that some or all of the gamma bursters are extra-galactic. (Nature, 8 October 1992.)
LASER ACTIVITY WITHOUT POPULATION INVERSION has reportedly been achieved in a sodium vapor. Population inversion is the condition in which a majority of atoms in a laser's active medium is maintained in excited states. This prevents ground-state atoms, which readily absorb laser photons, from sabotaging the laser process. In a paper to be published in the journal Optics Communications, Jin-Yue Gao of Jilin University in China reports the use of microwave radiation to prevent ground-state atoms in a sodium vapor from absorbing photons produced in the vapor, thereby obviating the need for population inversion. The microwave radiation brings about an "induced transparency" in the medium by coherently linking two closely spaced ground states. The probability amplitudes for absorption from either of the two superimposed states cancel, and the result is no absorption at all; laser light passes through unimpeded. Marlan Scully of Texas A&M refers to materials in which atomic states are coherently linked as "phase controlled materials," or "phaseonium." The use of such materials has led to reduced quantum noise in some laser systems. Scully believes that it could also lead to highly refracting, non-absorbing, transparent lenses. These lenses would result in higher-resolution optical microscopes and perhaps even setups enabling laser light to boost electrons in accelerators. (Science, 2 October 1992.)
ASTRONOMY WILL BE THE FIRST ALL-DIGITAL SCIENCE , according to Larry Smarr of the University of Illinois. Charge-coupled devices are largely taking over as a means of making observations; CCD's are efficient (up to 80% of incoming photons are recorded as compared to 2% for photographic film) and the data is in a form that can be readily processed by computer. This will facilitate large-scale, semi-automated projects, such as the University of Chicago's 5-year plan to chart the positions of a million galaxies and 100,000 quasars. (The Economist, 17 October 1992.)
THE OPACITY OF MATTER under conditions like those in stellar interiors has been measured using Livermore's powerful Nova laser (L.B. Da Silva et al, Phys. Rev. Lett., 20 July 1992). The measurements of the photoabsorption of iron, heated by x rays to a temperature of 2.5 x 10**5 K, are in agreement with recent theoretical estimates made at Livermore (the so-called OPAL model) and disagree with an earlier model derived at Los Alamos. Many astronomers had already been rooting for the OPAL model since it had done a better job in predicting certain stellar features, such as the pulsation rate for Cepheids. (Nature, 15 October 1992.)
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