The Energy-Momentum Tensor in General Relativity and in Alternative Theories of Gravitation, and the Gravitational vs. Inertial Mass

Abstract

We establish a general relation between the canonical energy-momentum tensor of Lagrangian dynamics and the tensor that acts as the source of the gravitational field in Einstein's equations, and we show that there is a discrepancy between these tensors when there are direct nonminimal couplings between matter and the Riemann tensor. Despite this discrepancy, we give a general proof of the exact equality of the gravitational and inertial masses for any arbitrary system of matter and gravitational fields, even in the presence of nonminimal second-derivative couplings and-or linear or nonlinear second-derivative terms of any kind in the Lagrangian. The gravitational mass is defined by the asymptotic Newtonian potential at large distance from the system, and the inertial mass is defined by the volume integral of the energy density determined from the canonical energy-momentum tensor. In the Brans-Dicke scalar field theory, we establish that the nonminimal coupling and long range of the scalar field leads to an inequality between the gravitational and inertial masses, and we derive an exact formula for this inequality and confirm that it is approximately proportional to the gravitational self-energy (the Nordvedt effect), but with a constant of proportionality different from what is claimed in the published literature in calculations based on the PPN scheme. Similar inequalities of gravitational and inertial masses are expected to occur in other scalar and vector theories.