Before matrix mechanics, the old quantum theory described the motion of a particle by a classical orbit, with well defined position and momentum X (t), P (t), with the restriction that the time integral over one period T of the momentum times the velocity must be a positive integer multiple of the Planck constant ∫ ∫ 0 T P d X d t d t = ∫ ∫ 0 T P d X = n h . While this restriction correctly selects orbits with more or less the right energy values E n, the old quantum mechanical formalism did not describe time dependent processes, such as the emission or absorption of radiation.