The relativistic dynamics for a system of non-interacting particles in Hamiltonian form is separated by a contact transformation into motion of their center of mass and internal motion. Interaction at a distance between them is then introduced into the expression for the rest-mass in terms of the internal variables. This gives a dynamics for which invariance over space displacements and rotation is trivial and which is rigorously invariant over Lorentz transformations. Earlier approximate treatments may be reduced by contact transformations to special cases of the general treatment.

• Received 19 June 1953

DOI:https://doi.org/10.1103/PhysRev.92.1300