Why does momentum depend on inertia?

Momentum is characterized in terms of inertial mass for particles moving at less than the speed of light, but entirely in terms of their energy for those lacking inertia. Does this difference suggest a physically distinct origin of momentum in the two cases and, if so, what is actually being conserved in interactions involving both types of particle? In this paper, we consider a recently proposed gravitational origin for rest-mass energy to demonstrate that a single definition of momentum applies to all particles, massless or otherwise. When introduced into this description, inertial mass is merely a surrogate for the particle's 'free' energy, but does not imply an origin of momentum different from that of particles without mass.

Automated summary

This paper discusses the origin and conservation of momentum, particularly for particles with mass and those without mass. The study shows that momentum can be uniformly defined for all particles, regardless of their mass, and there is no different origin based on mass.