Number 571, January 2, 2002
by Phil Schewe, James Riordon, and Ben Stein
A New Limit on the Overall Validity of Special
Relativity
A new limit on the overall validity of special relativity theory has
been established by a group of physicists the University of Konstanz
(Germany) quantum optics lab in collaboration with the University of
Düsseldorf. In a sense this is the highest accuracy overall test
of special relativity, a pillar of modern physics. One of the principles
of relativity theory is that the velocity of light, c, will be the same
as measured by all observers. Thus, for example, an observer on a train
moving very quickly toward a signal lamp will record the same light
speed as an observer at rest next to the train tracks; the velocity
of the train does not in any make the apparent light speed any greater.
In a Michelson-Morley-type experiment (MM), the universality of observed
light speed is demonstrated by comparing light beams moving in different
directions.
In another class of experiments, called Kennedy-Thorndike (KT) measurements,
one tests that c does not depend on the velocity of the laboratory.
Since present MM precision is higher than the best KT precision, the
Konstanz researchers aimed for a better KT test as a way of confirming,
to a new level of accuracy, that c is independent of both the speed
and direction of the lab. Basically they keep watch over a set of standing
light waves in a chilled cavity over a 190-day period, during which
the Earth traces out more than one-half of its orbit around the sun,
altering the velocity of the "lab" by an amount equal to 60
km/sec. If c were to vary with lab speed, then the standing waves (constantly
compared to a highly stable atomic clock) would fall out of tune with
the cavity; the cavity itself, made of sapphire, has very little thermal
expansion at a temperature of 4 K, and could be counted upon to keep
its shape. In this way the stability of the resonance frequency translated
into a three-fold improvement in accuracy over past KT experiments.
A 100-fold improvement in the near future is anticipated. (Achim Peters,
49-7531-88-3823, achim.peters@uni-konstanz.de; Holger Mueller, holger.mueller@uni-konstanz.de)
(Braxmaier et
al., Physical Review Letters, 7 January 2002; also see
researchers' webpage
on the experiment.)
Slowing and Storing Light in a Solid
When light encounters a medium in which the index of refraction changes
dramatically with wavelength, the group velocity of light, the speed
at which the wave pulse propagates, can be considerably lowered, even
to zero. The energy and information in the original light beam can be
stored, without any heating, in the form of a wave of excitations in
the spins of the atoms in the medium. Earlier this year two different
experiments at Harvard stopped and stored light in a vapor sample (Update
521).
Now the feat has been carried out in a solid material in an experiment
carried out at MIT and at the Air Force Research Laboratory in Hanscom,
Massachusetts. This is a nice advance since in general information processing
is carried out in solid-state integrated devices. The medium used, a
yttrium-silicate crystal doped with atoms of the rare earth praseodymium,
is already commonly used as a medium for high-density optical data storage.
The researchers (contact Philip Hemmer, 781-377-5170, philip.hemmer@hanscom.af.mil)
foresee many applications for slow or stopped light in a solid, in areas
such as quantum computing, ultra-sensitive magnetometry, and acousto-optics
(if light is slowed to subsonic speeds, strong coupling between light
and sound waves becomes possible). (Turukhin
et al., Physical Review Letters, 14 January 2001.)
A Sonic Crystal
A sonic crystal is to sound waves in air what a photonic crystal is
to light waves or a semiconductor is to electrons-it permits the passage
of waves at some energies but not others. Scientists in Spain (contact
Francisco Meseguer Rico, fmese@fis.upv.es, 349-6387-9841; Jose Sanchez
Dehesa, jsdehesa@upvnet.upv.es) are the first to use a sonic crystal,
an arrangement made of aluminum rods (see figure),
as an acoustic lens for focusing sound waves at audible frequencies.
They also create thereby an interferometer which, like its lightwave
counterpart, causes two wavetrains of soundwaves to interfere with each
other in a characteristic pattern. (Cervera
et al., Physical Review Letters, 14 January 2002.)
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