A weighty mass difference

摘要

Nuclear physics, and many major aspects of the physical world as we know it, hinges on the 0.14% difference in mass between neutrons and protons. Theoretically, that mass difference ought to be a calculable consequence of the quantum theory of the strong nuclear force (quantum chromodynamics; QCD) and the electromagnetic force (quantum electrodynamics; QED). But the required calculations are technically difficult and have long hovered out of reach. In a paper published in Science, Borsanyi et al. report breakthrough progress on this problem.