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Number 453, October 19, 1999 by Phillip F. Schewe and Ben Stein
EXTRA INVISIBLE DIMENSIONS are for particle physicists what they are for Star Trek captains: a device for covering a lot of ground quickly and explaining anomalous behavior. In physics the importation of extra dimensions into the standard theory helps to make peace between quantum mechanics and general relativity, but it doesn't explain the great disparity (the "hierarchy problem") between the temperature at which the weak and electromagnetic forces fuse together (102 GeV, expressed in energy units) and the temperature at which gravity joins up with the other forces (1018 GeV), a temperature so hot, or an energy so high, that such conditions have not prevailed since a tiny moment after the big bang. Some theories contend that we are not aware of the extra dimensions because they extend only a very short distance, far smaller than the size of an atom. Yet another way of playing with spacetime is to introduce a new dimension essentially infinite in extent but one in which gravitons, the carriers of gravity, would largely be locked up in localized regions, at least in the extra dimension. This exciting new idea, advanced by Lisa Randall of Princeton (609-258-4322, randall@feynman.princeton.edu; on leave from MIT, randall@baxter.mit.edu, 617-253-4818) and Raman Sundrum, now at Stanford, has the effect of fusing gravity with the other known forces at the more reasonable energy of 103 GeV (rather than at 1018 GeV), thus solving the hierarchy problem. One testable implication of the new hypothesis would be the existence of exotic new particles which could be detectable at energies to be available in a few years at the Large Hadron Collider (LHC) under construction in Geneva. (Two articles by Randall and Sundrum in Physical Review Letters.)
WAVE PROPERTIES OF BUCKYBALLS have been observed in an experiment at the University of Vienna. Physical objects from quarks to planets have wavelike attributes. The quantum nature of a bowling ball, unfortunately, is not manifest since its equivalent quantum (or de Broglie) wavelength is so tiny that interference effects (for example, the left part of the ball negating the right part of the ball) cannot be detected in a practical experiment. However, the wave properties of some composite entities, such as atoms and even small molecules, have previously been demonstrated. Now Anton Zeilinger at the University of Vienna (zeilinger-office@exp.uniwire.ac.at) has been able to perform the same feat for fullerenes, the largest objects (by a factor of ten) for which wavelike behavior has been seen. The researchers send a beam of the soccerball-shaped C-60 molecules (with velocities of around 200 m/sec) through a system of baffles and a grating (with slits 50 nm wide,100 nm apart) which yields a striking interference pattern characteristic of quantum behavior. Ironically the pattern indicating wave behavior is built up from an ensemble of individual sightings, each of which depends upon a buckyball's particle-like ability to make itself felt in an electrode. The interference is not negated thereby since it is not known by which path the C-60 came to be at the electrode.(Arndt et al., Nature, 14 October 1999.)
STRIPED SUPERCONDUCTIVITY. In high-temperature ceramic superconductors, currents flow mostly in the plane. But if special dopants (such as neodymium) are added to La-Sr-Cu-O materials, the supercurrents seem to be further restricted to narrow lanes or stripes. In these materials rows of charges are separated by insulating antiferromagnetic regions (in which neighboring atomic spins oppose each other), so they are referred to as charge-ordered or spin-ordered materials. Since the stripes occur preferentially at lower temperatures, physicists are not sure whether the stripes help or hurt superconductivity. Two new experiments (in which the superconductivity is turned off, the better to study underlying electronic properties) add some fresh perspective. A University of Tokyo team (Noda et al.) uses a strong magnetic field to produce a Hall effect, in which electrons should be pushed sideways by the field. A resistance to this effect is taken as evidence for a "self-organized" one-dimensional charge flow. Meanwhile a Stanford-LBL-Tokyo team (Zhou et al.) shoots UV photons into their samples and observe the ejected electrons that come flying out. The telltale photo-electron pattern maps back to charge flows in the sample that must have been organized into stripes. (Both articles appear in Science, 8 Oct.)
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