Physics Tipsheet #86, November 25, 2008
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Highlights in this Issue:
Portable Precision: A New Type of Atomic ClockV.V. Flambaum, V.A. Dzuba, and A. Derevianko The most accurate atomic clocks in the world are based on the output of cesium atoms. These ultra-precise fountain clocks measure the frequency and time interval of seconds by using a fountain-like movement of cesium atoms. Unfortunately, fountain clocks aren’t easily transportable- they tend to be huge, stationary apparatuses stuck in laboratories. Physicists from the University of New South Wales, Australia and the University of Nevada, Reno propose a method to reduce the size of atomic clocks to handy, compact devices using specially engineered optical lattices. Optical lattices are created by trapping atoms in a standing wave light field formed by laser beams. But the lasers can hamper the time keeping ability of the atoms. By applying an external magnetic field to the lattice in a specific direction, the atomic clock is rendered insensitive to the laser field strength. This property allows the atomic clock to function properly at a smaller size. While a portable cesium clock could benefit numerous scientific and general applications, the expected accuracy of the optical lattice clocks has yet to be explored. Calling for further theoretical and experimental investigation, the authors assert that even if the precision of such clocks turns out to be less competitive than the fountains, the optical lattice clocks have a clear advantage of a smaller apparatus size, making them useful in applications like navigation systems and precision tests of fundamental symmetries in space. –NR Voiding the Cosmic Void: We're not at Center of the Universe After AllJ. P. Zibin, Adam Moss, and Douglas Scott Models of the universe that place us near the center of a large, sparse region don't jibe with astronomical observations. Cosmologists at the University of British Columbia reached the conclusion through a new analysis that reaffirms the presence of a perplexing dark energy. In recent years many studies have indicated that the expansion of the universe is accelerating, which may be due to a mysterious form of dark energy that makes up most of the contents of the universe. Alternative interpretations have suggested that the accelerated expansion may be merely an illusion, if we happened to live near the center of an enormous cosmic void, empty of most matter. The researchers examined the latest data, in particular subtle features in the cosmic microwave background radiation (the afterglow of the Big Bang) and ripples in the large-scale distribution of matter. They found that void models, unlike standard dark energy models, do a very poor job of explaining all of the latest data, taken together. The new study helps to solidify our place in the Universe as a completely typical and unremarkable one. It also reaffirms that most of the stuff in the universe is far from ordinary: the dark energy remains as enigmatic as ever. 50 Years of PRLMartin Blume Physical Review Letters turns 50 this year. Martin Blume is celebrating the green journal's birthday by summarizing the most intriguing papers to appear in PRL each year since 1958. To see past editions of visit Marty's Milestone PRL project. This week, Marty is taking a look at a milestone, Bose-Einstein Condensate papers from 1995 and 1996, that led to the Nobel Prize for Physics in (2001). Bose-Einstein Condensation in a Gas of Sodium Atoms Collective Excitations of a Bose-Einstein Condensate in a Dilute Gas The 2001 Nobel Prize in Physics was awarded to Eric Cornell, Wolfgang Ketterle, and Carl Wieman "for the achievement of Bose-Einstein condensation in dilute gases of alkali atoms, and for early fundamental studies of the properties of the condensates." Their work followed many important contributions by others over a period of years in which the achievement of a Bose-Einstein condensate was a goal. The first observation of the condensate was by the group of Wieman and Cornell, published in Science in 1995. Their 1996 Letter is included here as it is an initial study of the "properties of the condensates" as mentioned in the Nobel citation. The observation of the condensates required trapping the atoms in a small volume and cooling them to the required temperature for condensation. Ketterle was able to do this for a collection of sodium atoms using a novel trap that produced a very high density condensate – higher than the density of rubidium atoms in the condensate of Wieman and Cornell. Together their research and the many following developments have produced a greater understanding of quantum mechanics and of statistical physics. Nadia Ramlagan and James Riordon contributed to this Tip Sheet. About APSThe American Physical Society is the leading professional organization of physicists, representing over 46,000 physicists in academia and industry in the United States and internationally. APS has offices in College Park, MD (Headquarters), Ridge, NY, and Washington, DC.
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