Photon rockets and gravitational radiation

TL;DR

This paper clarifies why Kinnersley's photon rocket solution lacks gravitational radiation by explicitly calculating linearized gravitational waves from point-like photon rockets, showing dipolar anisotropy cancels radiation, while more general anisotropies produce genuine gravitational waves.

Contribution

It provides explicit linearized gravitational wave calculations for photon rockets, demonstrating the conditions under which gravitational radiation is absent or present.

Findings

Dipolar photon emission cancels gravitational radiation in Kinnersley's solution.

More general anisotropies in photon emission generate genuine gravitational waves.

The results reconcile non-radiative solutions with post-Minkowskian formalisms.

Abstract

The absence of gravitational radiation in Kinnersley's ``photon rocket'' solution of Einstein's equations is clarified by studying the mathematically well-defined problem of point-like photon rockets in Minkowski space (i.e. massive particles emitting null fluid anisotro\-pically and accelerating because of the recoil). We explicitly compute the (uniquely defined) {\it linearized} retarded gravitational waves emitted by such objects, which are the coherent superposition of the gravitational waves generated by the motion of the massive point-like rocket and of those generated by the energy-momentum distribution of the photon fluid. In the special case (corresponding to Kinnersley's solution) where the anisotropy of the photon emission is purely dipolar we find that the gravitational wave amplitude generated by the energy-momentum of the photons exactly cancels the usual $1/r$ gravitational wave amplitude generated by the accelerated motion of the rocket. More general photon anisotropies would, however, generate genuine gravitational radiation at infinity. Our explicit calculations show the compatibility between the non-radiative character of Kinnersley's solution and the currently used gravitational wave generation formalisms based on post-Minkowskian perturbation theory.