GRAVITATIONAL FRAME-DRAGGING IS OBSERVED . Just as an electric charge emits an electric field and (if the charge is in motion) a magnetic field, so a massive body should, according to general relativity, emit a static gravitational field and (if the body is moving or rotating) an extra "gravitomagnetic" field (see Update 295 and the accompanying figure). Two groups of astronomers, one based in Rome (Luigi Stella, stella@coma.mporzio.astro.it) and one at MIT (Wei Cui, cui@space.mit.edu), now claim to have detected experimental evidence for this force in the form of subtle modulations in the x-ray signals coming from neutron stars and monitored by the Rossi X-Ray Timing Explorer (RXTE) satellite. The phenomenon is sometimes called "frame dragging" since its source is the distortions of space-time caused by the rotating mass. The results were reported yesterday at the High Energy Astrophysics Division meeting of the American Astronomical Society in Colorado.

BOILING TOPOLOGY . If Einstein hadn't introduced the so-called cosmological constant (a fudge factor, denoted by the Greek letter Lambda, meant to preserve the then-apparent static density of the universe) into his gravitational equations, nature might have invented one anyway. For one thing, according to modern field theories, the vacuum is not empty but filled with virtual particles which (through a process called quantum fluctuations) zip into and out of existence, and Lambda has come to be seen as the energy density of the vacuum. Furthermore, the "inflationary" version of the big bang model calls for something like a nonzero Lambda in order to expand the early universe at a rate much higher than we observe today. The universe having evolved to its present state, however, it would now be convenient if Lambda were very close to zero. Although not presenting a complete theory of quantum gravity, Steven Carlip (UC Davis, 916-752-8786,carlip@dirac.ucdavis.edu) has at least sought to explain the Lambda mystery in a model which spans the distance scales from near the Planck realm (10^-35 m), where space becomes granular (and where the fluctuations arise), all the way up to the cosmological scale (10 billion light years). Carlip compares the universe to a pot of water being heated. The addition of energy makes the water hotter, but only up to the boiling point. Thereafter, more energy serves only to boil the water faster. Analogously, larger Lambdas only make the universe "boil" more. That is, the topology of space-time is merely driven into an ever more complicated tangle of subtle distortions and wormholes. Carlip's framework, which calls upon recent developments in group theory and non-Euclidean geometry, makes it very unlikely that Lambda would have a negative value, and may well explain why Lambda is so close to zero.(Physical Review Letters, tent. 24 Nov.; as usual, journalists can obtain copies of PRL articles by contacting AIP Public Information at physnews@aip.org)

THE PHYSICS OF COFFEE STAINS . When a droplet of coffee or ink dries, why is the residue ring shaped rather uniform across the footprint of the original drop? Scientists at the University of Chicago suggest that as the droplet dries the outer boundary of the droplet remains pinned to the surface and that evaporating liquid at the frontier is replaced by liquid from the interior. Along with this flow comes most of the particles (e.g., coffee grounds) suspended in the solution. This new model may not be of much help in cleaning up after parties, but it might come in handy for various industrial processes. (Nature, 23 Oct.)