Fifty years ago, Norman Ramsey and collaborators endeavored to make the first measurement of the electric dipole moment of the neutron. What they measured, to very high precision, was less than 5 × 10−20 e·cm and consistent with zero. In the intervening half-century, the neutron electric dipole moment has become one of the most remarkable precision measurements in modern physics (see the article by Norval Fortson, Patrick Sandars, and Stephen Barr in Physics Today, June 2003, page 33). Currently the limit has been improved to a vanishingly small 10−26 e·cm, and upcoming experiments may reach 10−28 e·cm.
This result is puzzling because within the standard model of particle physics the strong interaction should violate time-reversal symmetry (T), and thus CP symmetry—symmetry under the combined operators of charge conjugation C and parity P. But such a symmetry violation would result...