Einstein's theory of relativity holds several things sacred. One is
the idea that if you rotate a particle or object, or boost it up to
a high velocity, the laws of physics affecting the object should stay
the same. This is called Lorentz invariance.
But in some "extensions" of the standard model of particle
physics, interactions of particles with certain hypothetical universal
fields (very roughly analogous to the way in which Higgs bosons are
supposed to make some particles massive) might lead to subtle violations
of Lorentz invariance.
In a new paper Alan Kostelecky of Indiana University and his colleagues
show how this can happen, and how such a violation could be detected
in clock-comparison experiments now being readied for the International
Space Station (ISS).
In general an atomic clock works by shooting microwaves into a sample
of cooled cesium atoms and reading out the microwave-absorption frequency
which corresponds to a specific quantum transition for electrons in
the cesium atoms. The microwave frequency setting is used to define
If one can cool the atoms to lower temperatures (thus reducing the
blurring caused by their movement) or observe them for longer periods,
the precision of the whole readout process (and the standardization
of the second) would improve.
The world's best clock, NIST F-1, currently has an uncertainty of one
part in 1015. It achieves this by chilling Cs atoms in a
trap and then gently boosting them upwards. Where they reach the top
of their trajectory (subject always to the attraction of gravity) and
are at their slowest is where they are subjected to the microwave bath.
A related apparatus mounted on the ISS could gain in precision because
the atoms would never fall (at least not relative to the atom trap setup)
and could be sampled for longer periods. The goal is to have several
such "space clocks" in orbit within a few years (see, for
According to Kostelecky (email@example.com, 812-855-1485) certain
Lorentz-violation effects, expected to show up as a tiny shifting of
an atom's energy level, would be more readily accessible in space thanks
to the speeds, rotation rates, and clock orientations available on space
platforms (see animations).
With sensitivities in space comparable to those in Earth-based experiments,
the expected tests of Lorentz-violating effects would be measured with
uncertainties at the level of parts in 1027. (Bluhm
et al., Physical Review Letters, 4 March 2002.)