Is spacetime symmetric?

The synthesis of antihydrogen (a lone positron orbiting a nuclear antiproton) at CERN in 1995 showed that antimatter is not merely a theoretical dream. Later this year, experiments at CERN’s new Antiproton Decelerator (AD) will begin investigating the properties of antihydrogen, their objective being to search for tiny differences in behaviour between matter and antimatter. Any such disparity would have deep implications for our understanding of space and time, as was highlighted at a recent meeting on spacetime symmetries held at Indiana University, Bloomington.

At the microscopic level the universe seems invariant both under CPT (the combination of charge conjugation, C, parity inversion, P and time reversal, T) and relativistic Lorentz transformations (rotations and boosts). However, these symmetries could be violated by effects at the Planck scale, at distances so small (10^­33 cm) and energies so high (10¹⁹ GeV) that the gravitational force between two particles becomes comparable to the other forces of physics. Although such effects would be very small, they might be detected in sensitive experiments.

If nature is CPT invariant, the masses of a particle and its antiparticle should be exactly equal. Recent experiments at Fermilab and CERN have established mass equality for the neutral kaon and antikaon to about one part in 1019. This astonishing precision can be compared to measuring the distance between the Earth and the nearest stars (a few light years) to an accuracy of about 1 cm.

Opening the meeting, Bruce Winstein, spokesman for Fermilab’s KTeV experiment, summarized the status of these experiments and the KLOE experiment at Frascati’s DAPHNE collider. An ambitious proposal to improve the current bound by more than an order of magnitude in a dedicated CPT kaon experiment was presented by Gordon Thomson of Rutgers. Measurements constraining CPT violation in the B-meson system to about one part in 1016, recently performed by the OPAL and DELPHI collaborations at CERN using data from LEP, were reviewed by Martin Jimack of CERN.

A general extension of the standard model and quantum electrodynamics that includes CPT and Lorentz violation was presented by meeting organizer Alan Kostelecky of Indiana. This can be employed to identify promising observable signals that arise from a broad class of theories with CPT and Lorentz violation, including those in which Lorentz symmetry is spontaneously broken in an underlying unified theory at the Planck scale. Malcolm Perry of the University of Cambridge reviewed the status of string and M (membrane) theory and described a new mechanism for CPT violation that involves the dilaton field.

One crucial test of spacetime symmetries is to compare the properties of stable particles with those of their antiparticles. This is possible with high-precision measurements made in electromagnetic traps. New results were presented by experimentalist Richard Mittleman from Hans Dehmelt’s group at Washington. An analysis of several months of data from an experiment with single trapped electrons placed a bound of six parts in 10²¹ on a combination of Lorentz- and CPT-violating quantities. Another new bound was reported by Gerald Gabrielse of Harvard, who constrained certain Lorentz-violating quantities to four parts in 10²⁶ by comparing the cyclotron frequencies of an antiproton and a hydrogen ion in an electromagnetic trap. A bold plan for testing spacetime symmetries is to perform spectroscopic measurements on antihydrogen and compare them with those of hydrogen. This requires the production of trapped antihydrogen, soon to begin, employing CERN’s Antiproton Decelerator (AD). Talks at the meeting outlined the goals of the AD’s two key trapped antihydrogen collaborations, ATRAP and ATHENA.

Comparisons between specialized atomic clocks can provide sharp tests of spacetime symmetries. These experiments are, in principle, capable of discerning Lorentz violation at the remarkable level of about one part in 10³¹. Astrophysical observations are interesting too, because small effects could be amplified as light travels over astronomical distances. One possibility is to look for radiowave birefringence on cosmological scales. Roman Jackiw of MIT presented a theoretical study of such effects, while other talks described possible experiments along these lines.

Organized by particle theorist Alan Kostelecky and attended by about 70 physicists from about half a dozen countries, the meeting was the first conference specifically focusing on this topic.