In physics, classical Yang–Mills theory is a generalization of the Maxwell theory of electromagnetism where the * chromo* -electromagnetic field itself carries charges. As a classical field theory it has solutions which travel at the speed of light so that its quantum version should describe massless particles ( gluons ). However, the postulated phenomenon of color confinement permits only bound states of gluons, forming massive particles. This is the mass gap . Another aspect of confinement is asymptotic freedom which makes it conceivable that quantum Yang-Mills theory exists without restriction to low energy scales. The problem is to establish rigorously the existence of the quantum Yang–Mills theory and a mass gap.