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| Issue 12, 2024 |
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Extracts from the Internet | ||
Low Speed Atom Scattering1 February 1999The scattering of atoms at low relative speed is of interest in that it highlights the quantum properties of the atoms. Because of the typically large velocity spread of cooled atoms, until recently no experiment with ultracold atoms has allowed an accurate knowledge of atomic pre-collision speeds. This problem was overcome in a recent Yale University experiment in which the interaction of two clouds of cesium atoms, first cooled down to 1 K and then tossed up to a height of several centimeters, was studied near the top of their trajectories. The relative atomic speeds were found from the time delay of the clouds in reaching the top, a laser technique being employed to measure the speeds before and after a collision. When at the top of the trajectory, atoms interact for a fairly long time and, importantly, without outer influences unavoidable in magnetic or laser traps. The interference of s- and p- waves was observed and a relative energy was found at which s-waves cancel and the atomic clouds pass through one another unscattered. Application to the construction of superhigh precision atomic clocks is suggested. Source: http://publish.aps.org/FOCUS/ Relativistic nonlinear optics1 February 1999An experiment carried out at Michigan University has for the first time allowed to see that the magnetic field of light does affect the motion of the electron the light is scattered upon. In the classical theory of Thomson scattering, only light's electric field affects the motion of a charge. The magnetic field may be neglected if the velocity obtained by the charge is much less than the speed of light - a usual situation experimentally. In the experiment, however, the laser light used was strong enough to make the charge - electron - to vibrate with relativistic velocities in the field of light. A superhigh-power laser beam acting on a beam of helium atoms ionized the atoms and was scattered by free electrons and ions. It is found that the real cross section differs from its Thomson value and that the scattered frequency depends on the angle. The implication is that electrons move along a very complicated path due to the combined forces of the electric and magnetic fields. With this experiment, a new field of research, "relativistic nonlinear optics,' seems to have been opened. Source: http://www.nature.com Background radiation fluctuations1 February 1999New data on the angular fluctuations of the background radiation temperature were obtained using a radio telescope at the Amundsen-Scott Antarctic South Pole Station. The background radiation, whose discovery in 1965 confirmed the so-called hot universe model, is highly anisotropic and possesses a Planck type spectrum at about 3K. Small spatial fluctuations in the radiation temperature are believed to have formed near the hydrogen recombination time point after the Big Bang and are now carrying information on processes that took place at that stage. Theoretically, the average fluctuation should depend periodically on the angle: an effect, which was predicted by A D Sakharov and subsequently studied in detail by Silk, Zel'dovich, and Syunyaev. The periodic dependence results from the interaction of adiabatic density-of-matter fluctuations with sound wave radiation. Owing to the South Pole location of the telescope, one and the same portion of the sky was observed and statistically significant results obtained in spite of the Earth's rotation. That radiation intensity decreases with decreasing angular scale amounts, in the researchers' view, to the observation of the first `acoustic peak' of the predicted oscillation. The particular position of the peak depends on the parameters of the cosmological model used. Observations suggest that the total density of matter in the universe (including the hidden mass, baryon matter, and possibly the Λ-term) is close to the critical density and that the spatial geometry of the universe in this case is very nearly Euclidean, in consistency with the inflation model of the early universe. Source: http://unisci.com NGC 5907 Galaxy1 February 1999In 1994, the observation of a faint stellar halo around the spiral galaxy NGC 5907 39 million light years away revealed a halo brightness profile very nearly coincident with the density profile of the dark matter halo as obtained from the galaxy rotation curve. This is unusual because for most galaxies brightness falls off much faster away from the center. Also, spectral analysis showed that the halo stars are metal-rich compared with other spiral galaxies. Recent Hubble observations revealed yet another unexpected result, namely, that the galaxy halo has much fewer massive bright stars than expected for normal galaxies of the same brightness. This implies that the glow of the halo is overwhelmingly dominated by dwarf stars invisible in the telescope. Thus far, no good explanation is available to account for such a strong disproportion between bright and faint stars. Source: http://www.berkley.edu |
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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