Number 402, November 13, 1998 by Phillip F. Schewe and Ben Stein
IMMISCIBLE QUANTUM LIQUIDS. The wavelike overlap of cooled alkali atoms known as Bose Einstein condensation (BEC) represents a new form of condensed matter in which physicists can pursue studies of fluid dynamics, sound propagation, persistent currents, and many of the coherence phenomena occurring in other "super" states such as superfluids and superconductors. One notable BEC innovation introduced in the past year by Wolfgang Ketterle and his colleagues at MIT was the development of an all-optical trap (Update 362) which can hold condensate atoms in a number of distinct (hyperfine) internal states. And just as helium-3 (which has a magnetic substructure) is a more complex superfluid than nonmagnetic helium-4, so the multi-component MIT condensate ought to exhibit behavior not seen in single-component BEC. Indeed, at the New Horizons in Science meeting in Boston last week Ketterle reported that when he immersed his BEC in a uniform magnetic field and a stream of radio waves, those portions of the condensate in different hyperfine states (m=0 and m=1) quickly segregated themselves into alternating domains (differing in energies equivalent to only a few nanokelvins) as if they were oil and vinegar. Furthermore, these layers unexpectedly persist; in effect this arrangement of the condensate constitutes a metastable macroscopically occupied excited state.
THE ARROW OF TIME has been directly measured by two groups of physicists, one at CERN in Geneva (http://www.cern.ch/cplear/Welcome.html) and one at Fermilab (http://fnphyx-www.fnal.gov/experiments/ktev/ktev.html) near Chicago. Time reversal (T) is one of those symmetries, along with charge conjugation (or C, the operation which turns particles into antiparticles) and parity (or P, the reversal of a particle's coordinates from x,y,z to -x,-y,-z) that were once thought to be preserved in interactions at the atomic level. But then experiments showed that P, C, and the combination CP were not sacred. And since the triple symmetry of CPT is still thought to be valid, T by itself was thought to be vulnerable. That is, it is now thought that physics does differentiate between the forward or backward movement of time. The two groups have now seen evidence for this T violation in the observed decay rates for neutral K mesons. (Science, 2 Oct.; Science News, 31 Oct.)
SONIC BANDGAPS, frequency ranges in which sound waves are excluded from a material because of the material's geometrical structure, have been created by researchers in Spain, opening the possibility for a fundamentally new way of soundproofing highways and other sources of noise. Acoustic bandgap materials are analogous to optical bandgap materials (also known as photonic crystals), in which arrangements of thin bars can cause light waves to interfere in carefully controlled ways. Such interference prevents the crystal from transmitting light waves within a certain range of colors (Update 55). Taking inspiration from such photonic crystals, and from a beautiful outdoor sculpture in Madrid, Francisco Meseguer of the Institute of Material Science in Madrid (firstname.lastname@example.org) has designed a metallic structure that produces bandgaps in the audible frequency range for sound waves entering the material from all directions. Described at the recent Acoustical Society of America meeting in Norfolk, this "sound sculpture" consists of one-meter-long metal bars arranged in a hybrid honeycomb-triangular pattern.(See http://www.acoustics.org/meseg2.htm; also see Sanchez-Perez et al., Physical Review Letters, 15 June; Physical Review Focus, 15 June)
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