Relativistic Constraints on the Structure of Fundamental Forces
  It is proved that Special Relativity imposes constraints on the structure of fundamental forces. The orthogonality of the 4-force exerted on an elementary particle and its 4-velocity is discussed. The significance of the energy-momentum tensor associated with the field is analyzed. Relying on these issues, it is proved that the Lorentz force is consistent with all constraints whereas a force derived from a scalar potential does not satisfy all requirements. This analysis explains a general discussion of Goldstein, Poole and Safko. 
E. Comay