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| Issue 12, 2024 |
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Extracts from the Internet | ||
Testing the isotropy of the the speed of light1 October 2009Ch. Eisele, A. Yu. Nevsky and S. Schiller (Institut fur Experimentalphysik, Heinrich-Heine-University at Dusseldorf, Germany) conducted a test of independence of the speed of light of the direction of propagation, with record accuracy so far. The setting of the experiment resembles that of the classical Michelson – Morley experiment. The setup includes two mutually perpendicular optical waveguides with slightly different resonance frequencies in which standing electromagnetic waves are excited by a laser. The difference between frequencies in the two waveguides is measured by observing beats of the sum signal as a function of spatial orientation of the apparatus. In the 13 months that the experiment was run the apparatus was turned by 90 degrees approximately 175,000 times. Considerable effort was made to exclude external factors of different types that produce systematic errors. No anisotropy of the speed of light was detected at accuracy of ≈ 10-17 which confirms the local Lorentz invariance at this level. The measurements conducted are important for testing the suggested approaches to constructing the Unified field theory: indeed, the Lorentz invariance holds only approximately in some models. Source: Phys. Rev. Lett. 103 090401 (2009) Ferromagnetism in Fermi gas1 October 2009W. Ketterle (Massachusetts Institute of Technology) and his colleagues succeeded for the first time in detecting ferromagnetic properties of ultracold gas consisting of 6Li atoms. Ferromagnetism was previously observed in gases only in the state of Bose – Einstein condensate. The possibility of transition of Fermi gases to ferromagnetic state when the interparticle interaction is repulsive has been discussed in a number of theoretical papers but no definitive conclusions have been made on the feasibility of such transitions. In the new experiment, a cloud of ultracold gas which is a mixture of atoms with oppositely oriented spins was held in an optical trap. The transition to ferromagnetic state at a temperature of less than 1 µK was identified by an indirect method of detecting a drop in the rate of inelastic three-particle collisions that result in the formation of molecules, by detecting the point when the minimal kinetic energy of particles is reached, and by recognizing certain characteristics of expansion of the gas cloud. These properties were exactly those predicted for the ferromagnetic gas. The gas field contained about 100 magnetic domains of volumes of about 5 µm3 with about 50 atoms of the gas in each. This experiment demonstrated that ferromagnetism may arise in a system of Fermi particles possessing no crystal lattice. Source: Science 325 1521 (2009) Buffer-gas cooling1 October 2009S.C. Doret (Massachusetts Institute of Technology) and his colleagues developed a new method of cooling a gas to the state of Bose – Einstein condensate — the gas was cooled via collisions of its atoms with atoms of auxiliary buffer gas. The Bose – Einstein condensate is typically produced by a preliminary laser cooling but this method works with only a few gases. In the experiment of S.C. Doret and his colleagues, 4He atoms were evaporated by light pulses from the inner walls of the vessel and a small fraction of them (≈ 10-5) were raised by microwave pulses to an excited state 4He*. Atoms of 4He in the ground state acted as buffer gas; collisions with them cooled 4He* atoms to a temperature of ≈ 500 µK. Then in a short time 4He atoms were again absorbed by vessel walls so what was left in the vessel was a cooled gas of 4He* atoms. Further cooling for the transition to the Bose – Einstein condensate used the conventional evaporation technique. The buffer-gas method can cool many atomic and molecular gases for which other methods of cooling are inapplicable. Source: Phys. Rev. Lett. 103 103005 (2009) Improved atomic force microscope1 October 2009Researchers at the IBM research laboratory in Zurich (Switzerland) and the Institute for Nanomaterials Science (Utrecht, the Netherlands) improved the resolution of the atomic force microscope (AFM) to a level at which individual atoms composing a molecule can be observed. This became feasible by using a single molecule of carbon monoxide CO as the tip of the needle of the AFM. The factor that limited the resolution of AFM with a metal tip was the distortion of the structure of the specimen by van der Waals forces when the tip approached the specimen surface, or atoms jumped to the tip off the specimen surface, adsorbed on the needle, and also destroyed the observed features. The advantages of a CO tip used as the needle tip lie in the considerable stability of this molecule against outside influences and van der Waals forces. As an illustration, L. Gross and his colleagues carried out observations of a carefully scrutinized molecule C22H14. The improved microscope made it possible to resolve all five carbon rings as well as all constituent carbon and hydrogen atoms in the molecule. It was possible to measure interatomic distances of only 0.14 nm, which is a record resolution of an atomic force microscope. Source: Science 325 1110 (2009) Search for gravitational waves1 October 2009It is assumed that cosmic gamma bursts of short duration (less than two seconds) occur because of a merger of two neutron stars, or a neutron star and a black hole in a binary system, while long-duration gamma bursts with a softer spectrum are generated in supernova explosions. In both cases gamma radiation must be accompanied with a powerful burst of gravitational waves. The data collected by the detectors LIGO (in the USA) and VIRGO (in Italy) were compared with the catalog of 137 gamma bursts most of which were observed by the Swift satellite. The study found no statistically significant correlation of signals which puts a bound on the total energy of gravitational radiation, or (if energy is given) on the distance to the sources of bursts. Typical gamma bursts occur in other galaxies, at cosmologically large distances. A gamma burst can be recorded by LIGO or VIRGO only if the burst source happens to be relatively near. Preliminary evaluations showed that such events cannot be dismissed as impossible but that their probability is very low. An earlier search by the LIGO detector of a gravitational signal from a relatively near and extremely bright gamma burst GRB 030329 also failed to find a signal. According to calculations, gravitational waves from gamma bursts should be confidently recorded by future improved detectors with sensitivity raised by approximately an order of magnitude compared to today's LIGO and VIRGO. Source: arXiv:0908.3824v1 [astro-ph.HE] |
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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