In 1935 the Japanese physicist Hideki Yukawa predicted that there must be a particle, now known as the pion, which would transmit the strong interaction. The pion was duly discovered more than ten years later. However, we now know that although pion exchange is an important component of the static force, when the force acts between a pair of particles with high energy a very large number of particles collaborate in transmitting it. Regge theory provides a simple quantitative description of the combined effect of all these particle exchanges.

It was soon realised that the exchanges of the known particles, even though several hundred are listed in the data tables, are not sufficient to describe a striking feature of the strong force: that it retains its strength as the energy increases and even becomes yet stronger. To explain this, it must be that something else is exchanged. This new object was named after the Russian physicist Isaac Pomeranchuk. It was originally called the pomeranchukon, but this was later abbreviated to pomeron. Events in which a pomeron is exchanged are often called diffractive events. The reason for this is that pomeron exchange dominates in high-energy elastic scattering and, as we describe in chapter 3, when plotted against scattering angle the differential cross section has a striking dip, reminiscent of the intensity distribution in optical diffraction. However, we explain that actually the mechanism for dip generation in high-energy scattering is more complicated than in optical diffraction.