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Number 381, July 10, 1998 by Phillip F. Schewe and Ben Stein
NEW REASONS TO SEARCH FOR EXTRA DIMENSIONS. Grand Unified Theories (GUTs) are the framework wherein three of nature's forces---the strong, the weak, and the hypercharge forces (the latter being a better way of describing the electromagnetic force at energies above several hundred GeV---come together as facets of one underlying force. Unfortunately, these theories fully come into play only at the very high energies (10^16 GeV) which prevailed in the very early (and hot) universe. Only at such high energies do these three forces have equal (unified) strengths. However, a new proposal by three physicists at CERN (contact Keith Dienes, keith.dienes@cern.ch, 011-41-22-767-2459) makes the case that the unification energy can be considerably lower if extra spacetime dimensions exist. Such dimensions are a generic prediction of string theory. Normally we wouldn't detect such dimensions because they would be curled up (or "compactified") into balls about 10^-35 m across, far smaller than the best resolution of today's accelerator experiments, roughly 10^-18 m. However, the CERN physicists point out that if the extra dimensions are significantly larger, on the order of 10^-19 m, then their effect would be to lower the GUT unification energy to the TeV-scale (10^3 GeV). This in turn would allow GUT physics to be observed directly in the next round of accelerator experiments. Signatures of the large extra dimensions would be striking. Once an elementary particle, such as a Z boson (one of the particles carrying the weak force), is accelerated to TeV-scale energies, its "Compton wavelength" (the size of the Z as a wavelike object) will become smaller than the extra dimension and it can essentially "fall into" and rattle around inside the extra dimension. This (like the "harmonics," or higher-frequency overtones produced on a violin) would produce echoes which would appear as new particle resonances with TeV-scale masses. By studying the new particles (called "Kaluza-Klein excitations" after the scientists who pioneered the idea of extra dimensions), one can determine the size of the extra dimensions and the properties of GUT (and string) theories directly at energies far lower than was previously thought possible. Some physicists had despaired of putting GUT to a direct test, but this may change. (Dienes, Dudas, Gherghetta, online preprint at xxx.lanl.gov/abs/hep-ph/9803466; see figure at Physics News Graphics. Interestingly, this proposal coincides with a fictionalized account, by Gordon Kane, of a future discovery of extra dimensions, in Physics Today, May)
HEAVY-FERMION SUPERCONDUCTIVITY (in at least some materials) is mediated by the magnetic interactions among the charge carriers. In conventional low-temperature superconductors, electrons pair up via the mediation of vibrations of the underlying solid (set in motion by virtue of the electron's negative charge as it passes through a lattice of positive ions whose motion, in turn, attracts another electron); equivalently, one can imagine the vibrations to be organized into particles called phonons. In heavy-fermion materials, by contrast, the electrons in question are inner-shell electrons which, because they are tightly bound, move as if they were heavier (by a factor or 100 or more) than normal electrons. Now physicists at the University of Cambridge have shown that superconductivity in Ce-Pd-Si and Ce-In compounds arises from the magnetic interactions of the electron spins. Instead of ripples of lattice distortions (phonons) doing the binding, it is ripples in the density of spins that are the cause of the pairing. Thus in these materials magnetism and superconductivity (sometimes regarded as antithetical) may both result from spin density waves. This "magnetic glue" might also have a role in high-temperature superconductivity. (Nature, 2 July 1998.)
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