Bondi-Sachs metrics and Photon Rockets

We study the Bondi-Sachs rockets with nonzero cosmological constant. We observe that the acceleration of the systems arises naturally in the asymptotic symmetries of (anti-) de Sitter spacetimes. Assuming the validity of the concepts of energy and mass previously introduced in asymptotically flat spacetimes, we find that the emission of pure radiation energy balances the loss of the Bondi mass in certain special families of the Bondi-Sachs rockets, so in these there is no gravitational radiation.Comment: 12 pages, to appear in General Relativity and Gravitatio

Expanding, axisymmetric pure-radiation gravitational fields with a simple twist

New expanding, axisymmetric pure-radiation solutions are found, exploiting the analogy with the Euler-Darboux equation for aligned colliding plane waves.Comment: revtex, 5 page

Pure-radiation gravitational fields with a simple twist and a Killing vector

Pure-radiation solutions are found, exploiting the analogy with the Euler- Darboux equation for aligned colliding plane waves and the Euler-Tricomi equation in hydrodynamics of two-dimensional flow. They do not depend on one of the spacelike coordinates and comprise the Hauser solution as a special subcase.Comment: revtex, 9 page

In-situ investigations of the reversible hBN-cBN-hBN-transformation in the Li3N−BN\mathrm{Li_3N-BN} catalyst system using synchrotron radiation

The kinetics of the transformation of boron nitride's hexagonal form (hBN) into the polymorphic cubic high pressure phase (cBN) were studied in the Li3N-BN catalyst system under in-situ pressure and temperature conditions. Energy-dispersive X-ray experiments were performed in a MAX 80 high pressure cell at HASYLAB, DESY, Hamburg using synchroton radiation. The transformation of hBN into cBN and the reverse transformation of cBN into hBN within the same experimental run were examined. Simultaneously the kinetics of the transformation were determined in the pressure range 0.65-6.5 GPa and at temperatures between 600–1400 °C. It could be shown that, in most cases, the transformation was rather fast and it was completed in less than 5 min. The observed data confirm the experimental results and the phase diagram as given by Bundy and Wentorf, but are in contrast to the experimental results and thermodynamic calculations by Solozhenko and by Maki