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)