Of strings, shells, and bags

We review several approaches to explain the extended structure of hadrons and their charge content in a relativistic way. The first model is purely geometric and proposes to describe the hadron as a string moving in space‐time. Unfortunately, the introduction of a geometry gives rise to quantum fluctuations which spoil the relativistic invariance of the theory (except in an unphysical number of dimensions). Yet it give rise to a very nice picture of strong interactions in terms of the splitting of strings. Further, this model can only accommodate Abelian charges. The second approach is a hybrid one which associates local fields with the quark quantum numbers, but traps them relativistically in a region of space, called the bag. This bag model has very nice static solutions which roughly give the experimentally observed values for the static parameters of the hadrons. It remains to be seen if the model can accommodate its quantum fluctuations. Finally, the last approach is purely field theoretical and relies on the peculiarities of the quark binding potential. There the quarks end up on a shell.