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| Issue 12, 2024 |
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Extracts from the Internet | ||
Direction of time1 December 1998Relativistic quantum theory naturally leads to the requirement (also known as the CPT theorem) that physical processes be invariant under the combined action of charge conjugation C (particle-antiparticle interchange), parity P (mirror reflection), and time reversal T. Unlike classical physics, in quantum theory the C, P, and T operations may or may not preserve invariance when taken separately or in pairs. The violation of P invariance was discovered back in 1957 in the β decay of cobalt nuclei. In 1964, double CP invariance was found to be violated in the decay of neutral kaons. Together with the CPT theorem, CP violation also implies T violation meaning that the forward and backward movement of time are not equivalent. The first direct evidence for this has been seen in recent CERN experiments in which kaons K0 and antikaons produced by an antiproton beam colliding with hydrogen atoms were detected by their decay into electrons, positrons, pions, and neutrinos. By monitoring the transformations of K0 and anti-K0 into one another, it was found that anti-K0 turn into K0 more frequently than the other way round, directly indicating that the two directions of time are not equivalent. The CERN discovery was soon confirmed at Fermilab using a somewhat different technique. It is CP and T violations at the early stages of the Universe which are believed to have caused the matter-antimatter disproportionality. Source: http://www.cern.ch/cplear/Welcome.html Quantum wire1 December 1998The term quantum wire is used to cover one-atom-thick structures which are currently of considerable interest for electronic miniaturization. According to calculations by N’D’Lang and P Avouris of the IBM Research Center, a quantum wire of carbon atoms shows oscillating (rather than monotonically increasing) conductance as its length is increased, with maxima occurring at even numbers of atoms due to there being more allowable electronic states in this case. H Ohnishi and colleagues in Tokyo observed such oscillations on a quantum wire of gold atoms they obtained between the tip of a scanning tunneling microscope and the surface of a gold specimen. As the tip-surface separation was increased, the wire became longer and thinner and its conductance changed by quantum jumps of 2e2/h. A similar jumplike conductance was observed at the University of Leiden, where a (gold) quantum wire was produced as a bridge between the two ends of a broken gold wire. Source: http://publish.aps.org/FOCUS/, Nature Science Update Protoplanetary disk1 December 1998A protoplanetary disk around a young star 1300 light years away has been observed with the VLA radio telescope. The disk has an unprecedentedly small radius of the order of Neptune's orbit and is apparently very similar to that from which the solar system developed billions years ago. The rotation of the disk around the star was detected due to its two water masers, molecular clusters that amplify radio-frequency radiation. An important finding is a jet of matter ejected perpendicular to the disk and emitting at radio frequencies. The jet is believed to have developed in the accretion disk near a very young star under the action of stellar wind. The discovery is significant for understanding the origin of the solar system. Source: http://www.nrao.edu/ Gravitational lensing1 December 1998Light from one in every 500 quasars experiences gravitational lensing, i. e., is deviated by the gravitational field of a galaxy it encounters on its path, with the result that instead of one, a number of images of the quasar are observed. While the brightnesses of the quasar and its images change in time, these are not in-phase changes - indeed time lags of as much as a year may occur. Knowing the time lag and the angular distance between the images, the distance to the quasar and the rate of expansion of the Universe can be calculated. Owing to the Hubble telescope, further progress in this field became possible. In one particular case, four quasar images and a gravitational lens in the form of an elliptic galaxy were observed at infrared and optical wavelengths in the rare situation of a lens being located nearly exactly on the straight line between the quasar and the Earth. Owing to the high resolution of the Hubble telescope, the exact mass distribution in the gravitational lens was obtained. It was found that the Universe is expanding somewhat slower - and is therefore somewhat older - than was earlier believed. This eliminates the age discrepancy between the Universe as a whole and its oldest stars. Also, the presence of the cosmological Λ term is possible. Source: http://www.stsci.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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