FIGURE 1. The nucleon–nucleon potential.

At distances of a few fermi, the force between two nucleons is weakly attractive, indicated by a negative potential. According to Hideki Yukawa's model², this force is mediated by the exchange of particles known as mesons. The -meson, or pion, the lightest of the mesons, accounts for the attractive force at the largest distances where it is felt, whereas heavier mesons (, , ) take over closer in. The picture changes abruptly, however, below a separation of just under 1 fermi. Here the force becomes strongly repulsive, preventing nucleons merging. Ishii et al.¹ provide the first theoretical calculations from quantum chromodynamics, the theory of the strong force, that reproduce the empirical form of this potential.