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Skriven 2004-12-13 20:10:13 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 712
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 712 December 13, 2004
by Phillip F. Schewe, Ben Stein

IS SPECIAL RELATIVITY WRONG?  The centennial of Albert Einstein's miracle year
of 1905 has arrived and so it is pertinent to ask how one of his most famous
theories is doing.  Physicists don't necessarily believe that Einstein's rules
about the nature of spacetime are mistaken, but as part of the continual
scientific effort to extend what is known about the universe physicists search
for subtle hints of a departure from expected behavior.  Special relativity
predicts that clocks traveling in various directions and with various fixed
speeds relative to each other will tell time differently, but in such a way
that spacetime has no preferred or distinguishable direction, a proposition
known as Lorentz invariance.  Physicists, always on the lookout for departures
from received opinion, and also motivated by theoretical suggestions that such
effects might be expected, take this as an invitation precisely to search for
such a special direction or to find that the variation of clock rates does not
adhere to Einstein's equations.  Such effects are described by the
"Standard-Model Extension" (SME) and they can come in several forms.  One
disproof of special relativity would be the finding that matter and antimatter
behaved differently.  Another would be a birefringence violation: observing
that light with different polarizations travels at different velocities through
vacuum.  Still another disruption of the Einsteinian view would occur if the
universe were pervaded by an underlying oriented energy field, one that
interacted weakly with known particles so as to favor one direction over
another. A new experiment puts this latter violation to its most stringent test
yet.  As so often happens when searching for extremely subtle effects, no
departure from known physics was found but a new upper bound could be
established.  Ronald Walsworth and his Harvard-Smithsonian colleagues, in
conjunction with theorist Alan Kostelecky at Indiana University, look at how
atoms prepared in special magnetic states (the precision of their light
emissions allow them to serve as "clocks") vary in their timekeeping when
moving at certain velocities (or "boosts") relative to the hypothetical
Lorentz-symmetry-violating fields that may permeate the universe.  In this case
the two clocks consist of a sample of helium-3 atoms and a sample of xenon-129
atoms held in a container within a fixed magnetic field.  The clock rate in
each case is the rate at which the atomic nuclei precess in the magnetic field.
The emissions from one atomic species were fed into a feedback mechanism for
controlling the magnetic field, so in effect the one set of atoms (or, to be
more precise, their nuclear spins) acted as a reference clock while the other
species served as the test clock. The whole apparatus, and the absolute
orientation of the applied magnetic field in spacetime (and along with it the
orientation of the atoms and their emissions) change as the Earth rotates daily
and as the Earth takes its annual course around the sun.  Furthermore, to
achieve the
necessary level of precision (based on the light let loose by the atoms), the
Harvard researchers achieved the difficult experimental feat of having the two
atom samples operate in a maser mode (that is, they performed like a laser)
within the same container.  The existence of a Lorentz-violating field, one
that like a magnetic field favors a particular orientation in an otherwise
isotropic spacetime, could cause the two clocks to become more out of synch as
they move relative to the Lorentz-violating field.  The main result of the
experiment was to put a stringent new limit on a coupling of material particles
(primarily the neutron) to such fields. The upshot: no Lorentz "boost"
violations are seen at a level of one part in 10^-27.  (Cane et al., Physical
Review Letters, 3 December 2004; previous relativity test summarized at
http://www.aip.org/pnu/2003/split/623-2.html; contact Ron Walsworth at
617-495-7274, rwalsworth@cfa.harvard.edu; background articles in Physics Today,
July 2004, Scientific American, Sept 04; Harvard website at
www.cfa-www.harvard.eduWalsworth/Activities/DNGM/DNGM2.html;
Kosetlecky site, http://www.physics.indiana.edu/~kostelec/faq.html#30 )

LASER LIGHTNING ROD.  Lightning on demand, drawing down a bolt of lightning for
performing scientific studies, is usually done by firing a rocket into an
overhead cloud.  The rocket spools out a long wire, providing a conducting path
between the charged-up cloud and the earth below.  Soon this might be done
using laser pulses.  A team of French and German scientists has performed
experiments in the lab in which a laser beam ionizes air molecules between an
artificial thunderhead (a high voltage electrode) with another electrode, the
equivalent of "earth" (a grounded electrode), several meters away.  The
experiment is unique in that it can trigger megavolt discharges across
self-guided plasma filaments in air generated by laser pulses.  (Here are the
potent characteristics of natural lightning: peak power of ten megawatts, peak
voltage of 100 MV, peak currents of tens of kilo-amps.)  One of the lab results
is the surprising discovery that rain does not much perturb the triggering or
guiding of the discharge process.  Next the team will perform open-air
lightning experiments.  The aim of this work will be to obtain the ability to
trigger lightning before it occurs naturally at sensitive sites such as
airports or electrical substations.  (Ackermann et al., Applied Physics
Letters, 6 December 2004; contact Jerome Kasparian, Universite Lyon,
jkaspari@lasim.univ-lyon1.fr)

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 * Origin: Big Bang (1:106/2000.7)