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{at}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{at}lasim.univ-lyon1.fr)

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