Comparison of two field theoretical forms of gravitational wave equations

TL;DR

This paper compares two field theoretical forms of gravitational wave equations, revealing differences in their energy-momentum tensors and showing their relation to Einstein's equations and Thirring's approximation.

Contribution

It demonstrates that the Papapetrou wave equation is not a new form of Einstein's equation and clarifies the relationship between different gravitational wave formulations.

Findings

Weinberg and Papapetrou equations differ in energy-momentum tensors for interacting fields.

Thirring's approximation makes Papapetrou and Weinberg equations equivalent.

Papapetrou's equation is not a distinct form of Einstein's equation.

Abstract

In the lowest nonlinear approximation I compare two gravitational wave equations,- those of Weinberg and Papapetrou. The first one is simply a form of Einstein equation and the second is claimed to be yet another field theoretical form in which the energy-momentum tensor is obtained by Belinfante or Rosenfeld method. I show that for interacting gravitational field these methods lead to different energy-momentum tensors. Both these tensors need to be complemented "by hand" with some interaction energy-momentum tensors in order that the conservation laws of the total energy-momentum tensor give equation of motion for particles in agreement with general relativity. In approximation considered by Thirring, the Papapetrou wave equation must coincide with that of Thirring. But they differ because Thirring inserted the necessary interaction term. I show that Thirring wave equation is equivalent to Weinberg's one. Hence the Papapetrou equation is not yet another form of Einstein equation.