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| Issue 4, 2024 |
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Extracts from the Internet | ||
Gravitational waves1 January 2001In the very near future, large-scale gravitational-wave detectors such as LIGO and VIRGO will be put into operation (for reviews, see Uspekhi 170 743 (2000) and the paper on page 3 in this issue). The much less sensitive measuring devices of the past thirty years, while failing to detect gravitational waves, have yielded an important negative result - the upper limits on the wave amplitude and the number of their sources. The best limits to date come from the IGEC project, a network of five cryogenic resonant detectors located in America, Europe, and Australia. The sensitivity of IGEC would be sufficient, for example, to detect a gravitational signal from the centre of our galaxy had the energy equivalent of 0.1 of the solar mass gone to gravitational waves. Measurements performed in 1997-1998 failed to separate any signal from the instrument noise background, and reducing noise is currently one of the key problems the LIGO and VIRGO projects have to overcome. Gravitational waves predicted by A Einstein in 1918 are deformation of the space-time geometry and may be generated by masses performing variable-acceleration motion. Researchers hope to detect gravitational bursts from a merger of binary neutron stars or black holes in other galaxies, or from supernova explosions. The detection of gravitational waves from mergers of black holes would provide a test of general relativity in the high field range. It is also assumed that the background of fossil gravitational radiation that came into being immediately after the Big Bang may be filling the whole of the Universe. Source: Phys.Rev.Lett. 85 5046 2000 Anapole moment1 January 2001The SAMPLE experiment at the MIT/Bates Linear Accelerator Center reports having discovered an anapole (toroidal dipole) moment in the proton. A proton with such a moment interacts differently with positive- and negative-circularly polarized electrons. The SAMPLE researchers studied the scattering of a highly energetic electron beam from hydrogen and deuterium targets with the aim, originally, of assessing the relative contributions of u-, d-, and s-quarks to the proton magnetic moment. For the s variety, a less-than-expected value of 6% or less was found. Source: http://unisci.com/ Superconductivity in fullerenes1 January 2001The fullerenes C60 are insulators under normal conditions, but when doped by alkali metal ions they start conducting a current and at low temperatures become superconductors, with a maximum transition temperature Tc=40K. Since alkali metals are donors for (i. e., donate electrons to) C60, a theoretical prediction has been made that a rise in Tc might be achieved via acceptor doping. Such doping, however, is complicated by the fact that C60 is strongly electronegative and so pushes charged holes away. Bel Lab researchers overcame this problem without relying on doping but instead injecting holes to the crystal by means of an electrical field between a pair of electrodes attached to the crystal surface. In this way, a transition temperature of 54 K - a record high for non-copper oxide superconductors - was achieved. This increase is presumably due to the deformation of the crystal lattice and the associated change in the nature of the interaction between the electrons and phonons (vibrational excitations of the lattice). See Uspekhi 170 113 (2000) for a recent review. Source: http://physicsweb.org/ Magnetar1 January 2001Anomalous x-ray pulsars, cosmic lone rangers avoiding being involved in a multiple star system, produce regular x-ray bursts whose source of energy is not yet known. Two models have been put forward. In one, the energy comes from the accretion disk left over from the moment the neutron star was created; in the other, from the decay of a magnetic field of 1015 Gauss, 100 times that of an ordinary neutron star (the `magnetar' model). Evidence in favor of the magnetar now comes from the observation, taken with the 10-meter Keck telescope in Hawaii, of an optical object at the position of the anomalous x-ray pulsar 0142 + 61, whose emission might originate from the magnetosphere of a magnetar. The accretion scenario is excluded because the accretion disk would have to be much brighter than actually observed. The magnetar model was originally proposed as an explanation for the sporadic gamma-ray bursts, presumed to be created by the fracture the neutron star surface undergoes when charged particles are accelerated in a strong magnetic field. Source: http://xxx.lanl.gov/ |
The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) — the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics. Permanent editor is Yu.N. Eroshenko. It is compiled from a multitude of Internet sources. | |
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