PHYSICS NEWS UPDATE A digest of physics news items prepared by Phillip F. Schewe, AIP Publ

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PHYSICS NEWS UPDATE A digest of physics news items prepared by Phillip F. Schewe, AIP Public Information Number 159 January 6, 1994 NEW LAMB SHIFT MEASUREMENTS DISAGREE WITH QED, the theory of quantum electrodynamics. Ironically, QED was devised in the 1950s to explain the Lamb shift (named after Willis Lamb), the slight shift in the energy of an electron bound to a nucleus due to energy fluctuations in the vacuum, which can create electron-positron pairs or virtual photons seemingly out of nothing. Researchers at the Max Planck Institut for Quantum Optics in Garching, Germany have determined experimentally that the Lamb effect should cause the energy of an electron in the lowest energy state, the 1S state, to be shifted upward by 8172.86 Mhz for the hydrogen atom and 8184.00 MHz in the deuterium atom, values which disagree with QED's predictions of 8173.12 MHz for hydrogen and 8184.13 MHz for deuterium, even when the uncertainties are taken into account. In what they have called "the most stringent test of QED for a bound atom to date," the researchers employed the latest advances in laser and optical spectroscopy to make the measurements of the 1S Lamb shift, which can be measured to greater precision than the 2S and 2P Lamb shifts traditionally studied. However, the researchers do not particularly suspect that any "new physics" is at work here; they believe that a new, unpublished theoretical calculation taking into account previously ignored effects in QED should bring the results into agreement with theory. (M. Weitz et al., Physical Review Letters, 17 January 1994; contact Martin Weitz at Stanford University, 415-723-4666). SELF-ORGANIZED CRITICALITY IN FRAGMENTING is what happens when you shatter a frozen potato. Danish scientists have fractured several objects such as gypsum, soap, paraffin, and potatoes, and inventoried all the fragments with masses larger than a milligram (Lene Oddershede et al, Phys. Rev. Lett. 8 November 1993). The probability of finding a fragment of a certain mass was found to scale inversely with a power of the mass. The particular power measured (the scaling exponent) depends on the object's shape -- ball, cube, plate, bar, etc. -- but not on the material itself. The researchers take this as evidence for self organized criticality. This is the name for a type of behavior in which the evolution of a system over a wide range of size scales is correlated. Some scientists suspect that self organized criticality is at work in such systems as sandpiles, forest fires, and even earthquakes. (Dallas Morning News, 20 December 1993.) THE NUMBER OF FOREIGN STUDENTS receiving physics PhD's in the U.S. increased from 420 to 650 per year from 1987 to 1992. More than half of the increase is due to students from China. A new AIP survey shows 14,534 physics graduate students enrolled in 1992, the latest year in the sample. Of these 15% were female and 41% were of non-U.S. citizenship. In 1992, 1346 physics PhD's were awarded; of these 11% were to females and 48% were to foreign students, the highest foreign percentage yet recorded. Only 22% of the new foreign PhD's took up employment outside the U.S. The leading physics subfields for new PhD's were condensed matter (35%), particles (12.8%), and nuclear (8.3%). In astronomy in 1992, 93 PhD's were awarded, 18% to females and 16% to foreign students. (For more information contact Patrick Mulvey, 301-209- 3076.)


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