________________________________________________________________________ a Advanced Studies in Physics Centre of the Romanian Academy, Bucharest, Romania. b Academy of Romanian Scientists, Bucharest, Romania. *Corresponding author: E-mail: eliadestefanescu@yahoo.fr; Chapter 7 Print ISBN: 978-93-48119-75-9, eBook ISBN: 978-93-48119-32-2 A Unitary Approach to Relativistic Dynamics and Quantum Particle Interactions Eliade Stefanescu a,b* DOI: https://doi.org/10.9734/bpi/crpps/v4/2904 Peer-Review History: This chapter was reviewed by following the Advanced Open Peer Review policy. This chapter was thoroughly checked to prevent plagiarism. As per editorial policy, a minimum of two peer-reviewers reviewed the manuscript. After review and revision of the manuscript, the Book Editor approved the manuscript for final publication. Peer review comments, comments of the editor(s), etc. are available here: https://peerreviewarchive.com/review-history/2904 ABSTRACT In this paper, a Unitary Quantum Relativistic Theory was developed. The wavy nature of a quantum particle involves the relativistic dynamics of the wave packet of this particle, and a field of interaction described by Lorentz’s force and three of the four Maxwell equations: the electromagnetic induction law, and the flow laws of the electric and magnetic fields. These equations arise from the group velocity, which is of the form of the Lagrange equation, and from a relativistic principle for wave functions, which asserts that in any system of reference, a wave function has a bounded spectrum with a velocity limit c. When a magnetic circuit law is considered for a field interacting with a quantum particle, this is an electromagnetic field, propagating with the velocity c. This means that the limit velocity c of a quantum particle according to the theory of relativity is also obtained as a necessary condition of quantum dynamics. The relativistic particle wavefunctions considered in this framework, describing finite distributions of densities in the two conjugated spaces of the coordinates and momentum, provide a better description of a collisional phenomenon, in a finite volume occupied by these particles and the photon mediating this collision according to Quantum Electrodynamics. Keywords: Wave-function; wave-packet; group velocity; Hamiltonian; Lagrangian; Lorentz force; Hamilton equations; Lagrange equation; Lorentz transformation; Maxwell equations.