Physics News Update, Number 438

Number 438, July 9, 1999 by Phillip F. Schewe and Ben Stein

CLUSTERING IN GRANULAR GASES Granular materials (e.g., salt, sand, sugar) share properties with solids (they support a load) and liquids (they pour) but have unique properties of their own owing to the complex ways in which thousands to millions of grains collide with each other. To understand better the ways in which grains move and organize themselves, it would be nice if gravitational interactions could be minimized so that only inter-grain and grain-wall interactions were important. For this reason, French researchers (Eric Falcon, Ecole Normale Superieure, 011-33-1-44-323501, eric.falcon@ips.ens.fr) resorted to outer space. They have performed the first experiment with vibrated granular media in a low-gravity environment. On board a sounding rocket, inelastic frictional collisions among the grains themselves and with the container walls were the only interaction mechanisms at work. Once fluidized (agitated) the grains formed a uniform gas. At higher densities, though, the grains formed dense, motionless, 3-dimensional clusters surrounded by low-density regions. (Falcon et al., Physical Review Letters, 12 July 1999.)

QUANTUM COMPUTERS PERFORM THEIR FIRST SIMULATION Until now, quantum computers have done simple arithmetic (Update 310) and searched small databases (Update 367). But one of the first applications envisioned for them, proposed in 1982 by Richard Feynman, was that they could simulate quantum-mechanical processes better and more efficiently than classical computers. Demonstrating Feynman's idea for the first time, researchers (David Cory, MIT, 617-253-3806, dcory@mit.edu) have used a quantum computer to solve a senior-year undergraduate physics problem. Namely, they simulated a "truncated harmonic oscillator", the series of energy levels--assumed to be finite for simplicity--experienced by a quantum particle such as an electron which is bound to another object such as a proton. To simulate this system, they used an NMR quantum computer, a device in which an external magnetic field aligns a group of atomic nuclei in a liquid, solid, or gas, so that the tiny magnet associated with each atom's nucleus is either along the field (a state known as "spin-down", which can represent a 0 in binary code) or opposed to it ("spin-up", which can represent a 1). Like previous designs, the NMR computer consisted of molecules in the liquid state; in this case the researchers manipulated the spins of two atomic nuclei within each molecule. The manipulation results in the possible energy states for this two-spin system exactly simulating the possible energy states for the quantum particle. Future steps could include modeling the somewhat more sophisticated real-world system of an electron in a hydrogen atom. (Somaroo et al., Physical Review Letters, 28 June 1999.)

AIRLINER CONTRAILS, the thin line-shaped ice clouds formed from water vapor in exhaust gases, account for about 0.1% of the worldwide cloud cover, and as much as 0.5-2% over parts of Europe and the Eastern Northern Atlantic. A group of atmospheric scientists (Patrick Minnis, NASA Langley Research Center, p.minnis@larc.nasa.gov) have made of a study of these clouds in order to forecast their possible future radiative forcing effect, that is, the amount by which contrails would enhance (through greenhouse action) the solar and earth-emitted infrared radiation retained on Earth. The conclusion: between 1992 and 2050 contrail cloud cover will increase by a factor of 6. During this period the contrail fraction of anthropogenic radiative forcing may increase from about 1% (1992) to 2 or 3%. These are global estimates; regional averages (such as for the northern temperate zone) will be greater still. ( Minnis et al., Geophysical Research Letters, 1 July 1999.)