Number 300, December 20, 1996 by Phillip F. Schewe and Ben Stein CAN HYDROGEN BE A SUPERCONDUCTOR? That hydrogen can be metallic was demonstrated in an experiment earlier this year (Update 263), which also established that the H nuclei (protons) remained largely paired. Now two Cornell physicists, Neil Ashcroft (nwa1@cornell.edu) and C.F. Richardson, predict that diatomic hydrogen (H2) should have a superconducting phase, but only at megabar pressures. They believe that the nature of the superconductivity would be a modified version of the BCS process in which electrons, which normally repel each other, would form into (Cooper) pairs by exchanging vibrational modes (phonons) set up in the positive-ion lattice consisting of proton pairs. When the hydrogen remains diatomic, the basic electronic structure depends on both electrons and the holes electrons leave behind, and this mitigates somewhat the normal electron-electron repulsion which works against superconductivity. Theoretical predictions of superconducting transition temperatures have been notoriously difficult to make in the past, but nevertheless the authors suggest that hydrogen, of all the elements, might exhibit room-temperature superconductivity. Looking for the trademark zero-resistance in a tiny sample contained within a diamond-anvil cell cannot be accomplished directly so far (current probes are easily pinched off by the diamonds) but indirect methods (using inductive techniques) might succeed. (C.F. Richardson and N.W. Ashcroft, upcoming article in Physical Review Letters.) MOUNTAINS ON THE SUN. The SOHO spacecraft, dedicated to observing the sun and doppler-mapping the rise and fall of material and the passage of vibrations across the sun's face, has detected the presence of extended structures a third of a mile high on the solar surface. Jeffrey Kuhn of Michigan State, speaking at this week's American Geophysical Union meeting in San Francisco, said the bumps persisted in the same place on the surface for a month or more. (San Jose Mercury News, 18 December 1996.) THE CASIMIR FORCE , a 1948 theoretical prediction in which the seemingly desolate "vacuum" creates a tiny force between a pair of conductors, has been precisely measured for the first time. According to quantum mechanics, empty space (the "vacuum") is not truly empty but instead contains fleeting electromagnetic waves and particles that pop into and out of existence. However, when the vacuum is bounded by a pair of conducting surfaces, the only electromagnetic waves that can exist are those with wavelengths shorter than the distance between the surfaces. The exclusion of the longer wavelengths results in a tiny force between the conductors. To measure the Casimir force, Steve Lamoreaux, now at Los Alamos (505-667-5005), employs a torsion pendulum, a twisting horizontal bar suspended by a tungsten wire. The attraction between a gold-plated sphere and a second gold plate causes a small twisting force in the bar. By applying a voltage sufficient to keep the twisting angle of the bar fixed, Lamoreaux determined the force caused by the attraction of the plates. His results agree with theory to a 5% level. (Physical Review Letters, 6 January 1997.) Researchers previously measured the Casimir-Polder force (Update 122), a different but related effect in which the vacuum creates an attraction between a conducting plate and a neutral atom.