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