When it comes to the most tested and precise scientific theories, quantum electrodynamics (QED) ranks at or near the top of the list. The theory of light–matter interactions has predicted, for example, the value of the electron’s magnetic dipole moment to 12 significant figures, and observations published last year are in agreement.1 That’s equivalent to measuring the distance from New York City to Los Angeles to a precision better than the width of a human hair.2 Yet despite QED’s superlative predictions, the theory is more readily validated in light atoms than in heavy atoms.
For low-mass atoms, perturbation theory can precisely predict QED effects, such as a slight change in the transition energy of an electron that’s decaying from an excited atomic orbital to a low-energy orbital. But in a high-mass atom, relativistic and QED effects cannot be well approximated as small disturbances to the system. That’s because...
