(a) The CPT symmetry can be likened to a mirror that reflects spatial coordinates, flips charge and other additive quantum numbers, and reverses time. To test for cracks in this CPT mirror, physicists check whether the magnetic moment of the proton (left) has the same magnitude as that of the antiproton (right). (Technically, the moments have opposite signs due to the way magnetic moment is defined relative to the spin.) (b) To measure the antiproton’s magnetic moment, the ATRAP Collaboration measures the cyclotron and spin-flip frequencies, fc and fs, respectively. The ratio of these frequencies gives the antiproton’s magnetic moment, μp̅ =-fs/fc μN, in terms of the nuclear magneton μN.