This assumption is justified because we are ultimately interested in the radiation field at some given finite post-Newtonian precision like 3PN, and because only a finite number of multipole moments can contribute at any finite order of approximation. With a finite number of multipoles in the linearized metric (26View Equation, 27View Equation, 28View Equation), there is a maximal multipolarity lmax(n) at any post-Minkowskian order n, which grows linearly with n.