Using Clocks in Space to Search for New Physics

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 the "second."

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 example, NIST website).

According to Kostelecky (kostelec@indiana.edu, 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.)