Instantons and the real world

It now seems likely that the key to understanding quantum chromodynamics (QCD) relies on a detailed knowledge of the vacuum state - that is, a state of zero energy. Previously believed empty, it appears to contain 'pseudo-particles' otherwise called instantons, which can affect the interactions between the quarks. Instantons are not particles and have no direct physical interpretation. They are solutions to equations describing the gauge fields of QCD and represent large vacuum fluctuations of these fields. Recent studies of the theory of instantons and their role in quark interactions and confinement are considered. It is argued that since instanton effects can spoil the invariance of strong interactions under symmetry transformations a new type of fundamental particle, the 'axion', must exist. This particle is a neutral, spinless boson with mass of 0.01 to 0.1 % of the proton mass. Failure to establish the existence of the axion will create difficulties for theories trying to avoid the unfortunate consequences of instanton effects, while its detection may signal the credibility of their physical reality. The reasons why instantons have not been heard of previously if their effects are so important, and why their presence seems to be important only in theories like QCD and not in QED, are considered. (U.K.)