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| Issue 12, 2024 |
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Extracts from the Internet | ||
Sterile neutrino1 May 2007In 1995 an LSND experiment looking for antineutrino oscillations yielded the mass scale of oscillating states ~1eV2. This result contradicted the data of other experiments with solar, reactor and atmospheric neutrinos which pointed to mass square being smaller by 3-4 orders of magnitude. A hypothesis was proposed that an additional flavor of neutrino may have influenced the oscillations observed by the LSND – a sterile neutrino which is its own antiparticle. To check the LSND data, a new more accurate experiment MiniBooNE was conducted at Fermilab to search for oscillations. Neutrinos with energies 475-3000MeV muon were generated in the accelerator in pion and kaon decays created in proton collisions with the target. The search for neutrino was conducted by using Cerenkov detectors placed at a distance of 500 m from the accelerator. The MiniBooNE experimental data show that the level of oscillations does not exceed the number of background events, in agreement with the predictions of the Standard model of elementary particles; this rules out any involvement of a hypothetical sterile neutrino. Hence, the result of the LSND experiment has not been confirmed. To be precise, the MiniBooNE studied oscillations of neutrinos while the LSND – those of antineutrinos, but the anticipated agreement of the two results was based on the CPT-theorem. The MiniBooNE experiment also discovered a new interesting result: the observed number electron neutrino at low energies E<475MeV was substantially higher than the predicted quantity. The excess could hardly originate in neutrino oscillations but so far has no explanation and requires further investigation. Sources: http://arxiv.org/abs/0704.1500 Testing general relativity1 May 2007New preliminary results were published on measurements of general relativity effects by the space observatory Gravity Probe B that circumnavigates the Earth on a polar orbit. This satellite was designed for studying two effects: the geodetic precession (the geodetic effect) and the dragging of the reference frame (frame-dragging). The Gravity Probe B is one of the most complicated and precise space instruments ever. The idea of this experiment emerged as early as 1959; its specific features took many years to finalize. The experiment was run for 17 months and was completed in October 2005 but processing the huge volume of collected data still continues. The instrument monitored the precession of gyroscopes: nearly ideal quartz spheres with smooth (to within several atomic layers) surface coated with a thin layer of superconducting niobium. The permanent spatial orientation of the satellite was maintained using a telescope trained on one of the stars. Microscopic shifting of gyroscope axes was measured by superconducting sensors (squids). A number of other complicated technical problems were also solved. The measured effect of geodetic precession of gyroscopes in the orbital plane was found to be 6.6'' per year, which coincides with general relativity predictions to within 1%. The research team hopes that additional filtering out of noise and distortions would improve the accuracy to 0.01%. The geodetic precession is caused by the warping of spacetime due to the mass of the Earth. This precession manifests itself in changing the orientation of the vector, and in this particular case - of the gyroscope axis when it travels along a closed loop (the Earth’s orbit). The second effect - dragging of reference frame due to Earth’s rotation - is weaker by a factor of 170, so the corresponding result will hopefully be extracted from the data by the end of this year. Sources: http://einstein.stanford.edu/ Newton's Second Law for very small accelerations1 May 2007J. Gundlach and coworkers at the Washington University tested Newton's Second Law F=ma for very small accelerations. They studied oscillations of a torsion pendulum of mass 70g suspended on an elastic fiber and swiveling with a period of 13 minutes. Complete agreement with Newton's law was obtained down to acceleration as low as a=5x10-12sm/s2; the accuracy of this experiment improves that of preceding experiments by a factor of 1000. Results of such experiments are important e.g. in astrophysics. The visible (baryonic) mass of galaxies and clusters of galaxies is not sufficient for explaining the observed high stellar velocities and high gas temperatures. In view of these observations, modifications of Newtonian dynamics were suggested for low acceleration range. The high accuracy with which Newton's law holds rejects this scenario and favors the presence of dark matter (hidden mass) that creates additional attractive field (on direct proof of the existence of dark mass in colliding clusters of galaxies see Phys. Uspekhi 49 999 (2006)). There is also a problem of anomalous acceleration of space probes Pioneer-10 and 11 at a level of 10-7-10-8sm/s2. The new laboratory measurements exclude the explanation of this anomaly in terms of deviation from Newton's law. To further test the validity of Newton's law for small accelerations experiments are needed in which F is the force of gravitational attraction. Sources: Phys. Rev. Lett. 98 150801 (2007) Electronically driven melting of crystals1 May 2007Electronically driven melting of semiconductor crystals exposed to high-power ultrashort x-ray pulses generated by a free electron laser was studied at the Stanford Synchrotron Radiation Laboratory (SSRL). The electronically driven melting begins with heating of electrons as x-ray photons are scattered by them. Photons knock out electrons from outer atomic shells thus breaking chemical bonds, after which the crystal rapidly decomposes into atoms without imparting thermal motion energy to atomic cores. In contrast to the above mechanism, ordinary melting of crystals is caused by gradual increase in the amplitude of atomic vibrations at the nodes of the crystal lattice. This experiment also tested a promising new technique for studying dynamic behavior of atoms in crystals by analyzing the scattering of ultrashort x-ray pulses. Sources: http://www.physorg.com/news96220225.html Eclipse of a black hole1 May 2007The Chandra X-ray telescope observed a rare event: the eclipse of a black hole and its accretion disk by a gas cloud. The telescope studied x-ray emission from the core of the galaxy NGC1365 lying at a distance of 60 million light years from Earth. Turbulent friction in the gas in the accretion disk around the black hole causes loss of angular momentum by the gas so that it get heated and gradually spirals into the black hole. Strong heating in the central part of the disk leads to generation of x-ray, UV and optical radiation. The x-ray luminosity of the galaxy NGC1365 was found to greatly decrease within just 48 hours. Astronomers interpret this event as resulting from a gas cloud moving across the line of sight. The cloud rotates around the black hole at a distance of about 0.01 light year. The parameters of the eclipse show that the size of the emitting part of the accretion disk is about seven astronomical units – only about 10 times the size of horizon of this black hole. Sources: http://chandra.harvard.edu/press/07_releases/press_041207.html |
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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