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| Issue 12, 2024 |
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Extracts from the Internet | ||
Testing mass-energy equivalence directly1 February 2006Einstein's formula E=mc2 that relates the mass of a body to the energy its contains has been given a new direct test at the Laue Langevin Institute in Grenoble, France. The test involved the study of reactions in which Si and S nuclei capture a neutron to form the respective 29Si and 33S isotopes. The masses of the isotopes before and after the reaction were measured from the cyclotron frequencies of their ions in a Penning trap. The isotope mass ratio was measured with a precision two to three orders of magnitude better than previous experiments. After capturing a neutron the nuclei deexcited to their ground states with emission of photons, whose frequency and energy were determined by measuring the photon diffraction angles from the crystal. From the series of experiments performed, 10-7, which is 55 times the precision of similar results obtained five years ago from the annihilation of electron-positron pairs. Source: Nature 438 1096 (2005) Electron bubbles in superfluid helium1 February 2006An electron placed in a superfluid helium environment repels the neighboring helium atoms, making a spherical cavity around itself. The size of this `electron bubble' is such as to minimize the sum of the quantized kinetic energy of the electron, the surface tension energy, and the work of the pressure forces acting in the liquid. In a number of previous experiments, bubble radii of 19A have been observed. Now A Ghost and H Maris of Brown University in the US have discovered a new, larger type of an electron bubble in liquid helium. In their experiments, electron emission from a beta- radioactive source or a metal tip was directed into a container of helium in which, in addition to that, a controlled-amplitude ultrasound wave was generated and focused to the center. The ultrasound caused the electron bubbles to rapidly grow in size and then collapse, leading to cavitation effects which were detected by laser light scattering. The threshold ultrasound amplitude for the onset of cavitation determines the pressure in the wave and hence the size of the bubbles. In the phase diagram of temperature vs threshold pressure, three regions corresponding to different types of electron bubbles were observed. The first type are the ordinary electron bubbles - those that have been observed before. The second, newly-discovered region is interpreted in terms of electron bubbles attached to vortex filaments in the superfluid helium. The properties of bubbles near vortices have been the subject of theoretical studies, and it is known that they can grow to large sizes due to the fact that the pressure near vortices is less than the average because of the Bernoulli effect. The third type of bubbles form at low temperature and have the largest threshold pressure. Their nature is not yet clear. It is hypothesized that these bubbles interact simultaneously with two vertices or, alternatively, with vortex filaments each having an angular momentum 2h. Source: Phys. Rev. Lett. 95 265301 (2005) Direct observation of non-Poissonian atom statistics in adegenerate gas1 February 2006Until recently, the quantum statistics of degenerate Fermi and Bose gases have only been examined by studying the macroscopic properties of the gases, but now an experiment by C.S.Chuu and his colleagues at the University of Texas has for the first time revealed the deviation from the classical statistics by counting atoms directly. The team measured fluctuations in the number of Bose condensed 87Rb atoms held in a trap. The system under study was in the Thomas-Fermi limit, meaning that the number of atoms and the trap potential depth are related. The counting techniques used relied on either laser light absorption by atoms or the fluorescence of excited atoms, depending on whether the number of atoms exceeded or was less than a thousand, respectively. The marked instrument noise reduction the team achieved compared with previous experiments provided a sufficient counting accuracy to directly observe deviations from the Poisson fluctuation law for the number of atoms, N<500. As expected theoretically, the magnitude of the fluctuations exceeds N1/2 in correspondence with Bose-Einstein statistics. Source: Phys. Rev. Lett. 95 260403 (2005) A new source of coherent radiation1 February 2006A new type of coherent radiation source, involving a mechanism fundamentally different from that of the laser, can be created according to theoretical calculations and numerical simulations performed at Livermore and MIT. It is established that as a mechanical shock wave propagates through a dielectric crystal, its front makes the atoms at the crystal sites to vibrate in phase, leading to the radiation of coherent light waves in the frequency range from one to a hundred terahertz (1terahertz=1012Hz). Radiation generated in this way may have properties which cannot be obtained using conventional lasers. The experimental realization of this theoretical prediction is an important and interesting problem to be solved in the near future. Source: Phys. Rev. Lett. 96 013904 (2005) A giant gas cloud above the Galaxy's disk plane1 February 2006A giant cloud of gas, 10,000 light-years above the Galaxy's plane and 23,000 light years from Earth, has been discovered by Y.Pidopryhora (National Radio Astronomy Observatory (NRAO) and Ohio University), J.Lockman (NRAO), and J.Shields (Ohio University) using the GBT telescope. The cloud is a million times as massive as the Sun, and its outflow from the disk is about a hundred times as energetic as a supernova explosion. Whereas the upper layers of the cloud are primarily comprised of neutral hydrogen, which is seen at a wavelength of 21 cm, it interior is likely to be filled with ionized hydrogen. The hydrogen is strongly disturbed in that there are many smaller outflows closer to the plane of the disk. The astronomers believe that the gas was initially in a cluster of young stars in one of the Galaxy's arms and that it was blown from there either by supernova explosions or by stellar winds. In the latter case, the age of the cloud should be between 10 and 30 million years. Such gas outflows may be crucial for the evolution of galaxies because they transfer heavy chemical elements and control the star formation rate, among other things. Source: http://www.nrao.edu/pr/2006/plume/ |
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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