Protons and neutrons (“nucleons”) are gripped in the atomic nucleus by a strong force (Figure 4.1). The electromagnetic force is transmitted by photons, and, in 1935, Hideki Yukawa proposed that the strong force also has an agent — the pi-meson, or pion, labelled π . In his theory, it is the exchange of a pion between pairs of protons or neutrons that attracts them (Figure 5.1). However, unlike the electromagnetic force which has an infinite reach, the influence of the strong force extends hardly beyond the breadth of two nucleons, a mere 10−15 m. To explain this, Yukawa proposed that the pion had a mass. Had the pion been massless like the photon, the strong force would have been infinite range like the electromagnetic force. However, the range of 10−15 m required the pion to have a mass of about 1/7 that of a proton.