Number 55, November 7, 1991 by Phillip F. Schewe and Ben Stein A SEMICONDUCTOR FOR LIGHT WAVES , a material in which certain photon wavelengths would be excluded---creating, in effect, a photon band gap analogous to the forbidden electron energy bands in semiconductors---is being developed by scientists at Bellcore and the Polytechnic University, Brooklyn (Physical Review Letters, 21 October 1991). Eli Yablonovitch (Bellcore, 908-758-2805) and his colleagues drilled three sets of holes into the top surface of a slab of dielectric material, creating a criss-cross, face-centered-cubic structure whose unit cell is a rhombic dodecahedron. Theorists had predicted that such a honeycomb geometry would result in the exclusion of light at certain wavelengths. Indeed, when the Bellcore scientists sent microwaves into one sample (which, because of the holes, was 78% empty) they discovered a forbidden gap. The researchers believe that by proper tailoring of the holes optical-wavelength bands can also be induced and that this will make these "photonic crystals" useful in a variety of research areas, such as atomic physics and microelectronics. (Science News, 2 Nov. 1991.) THE MOST LUMINOUS GAMMA-RAY SOURCE in the sky, the quasar 3C279, was discovered recently by the orbiting Compton Gamma Ray Observatory. According to Kenneth Brecher of Boston University, the luminosity of 3C279 in gamma rays (assuming it radiates isotropically) is about 1048 ergs/sec, about 100 million times the gamma-ray flux of the Milky Way galaxy. 3C279 had not been detected by earlier missions, such as SAS-2 and COS-B, indicating that it is probably a variable source (turning on and off) of gammas. (Article by Brecher in Physics News in 1991, to be published in December 1991.) MERCURY'S POLES MAY BE COVERED WITH ICE. Radar studies by Duane Muhleman and others at Caltech and JPL reveal a brightness indicative of water ice; the radar signals also resemble those from known ice deposits on Mars. Despite its broiling nearness to the Sun (Mercury's equatorial temperatures reach 800 degrees Fahrenheit), the planet's rotational axis is vertical, an orientation which leaves the poles cold enough to foster ice. (The New York Times, 7 November 1991.) NEWTON'S GRAVITATIONAL CONSTANT measured underwater is consistent with laboratory determinations to within less than 1 part in 1000. A Scripps/AT&T Bell Labs/Los Alamos team used a submersible craft to measure gravity in the oceans at depths up to 5000 m. The point here was to use the ocean as attracting mass in the search for deviations from Newton's inverse-square law for several distance scales. Assessing their own results and those emerging from measurements carried out by others in mines and boreholes, the scientists (M.A. Zumberge et al., upcoming article in Physical Review Letters) assert that "Collectively, these studies offer no compelling evidence for deviations from Newtonian gravity in the scale range 10 m to 1200 m."