The world, as we know and feel it every day, consists of matter. Whereas the Big Bang is considered to have created both matter and antimatter in equal quantities, the present-day Universe clearly seems to display a huge asymmetry: antimatter is rarely observed, and if it is, it's only in highly exotic environments or in some radioactive reactions. Writing in Physical Review Letters, Ulmer et al.1 describe an experiment that paves the way for a high-precision test of the theoretically expected matter–antimatter symmetry.
Matter–antimatter symmetry is the most fundamental symmetry in the standard model of elementary particle physics. According to this symmetry, under a CPT transformation — which is a simultaneous inversion of the particle properties charge (C) and parity (P), and a reversal of time (T) — an antiparticle behaves exactly like its mirror-image particle. Ultra-high-precision tests of the CPT symmetry have been performed in different physical systems (for example, with mesons, leptons and baryons) by comparing the properties of particles and their antiparticles. Yet, until now a violation of this symmetry has never been observed.