Eric Poisson

Department of Physics

University of Guelph

Guelph, Ontario, N1G 2W1

(519) 824-4120 x53991
Eric Poisson
Department of Physics
University of Guelph

Description of research

The research activities of the Guelph Gravitation Group have recently been divided into two broad streams. The first stream is concerned with the physics of black holes in tidal environments. The second stream is concerned with the gravitational self-force.

Black holes in tidal environments

What happens to a black hole when it is not isolated, but placed in the presence of other bodies which exert tidal forces on it? To answer this question requires a description of the tidal environment, a computation of the gravitational perturbation created by the external bodies, and the extraction of physical, measurable consequences. Among the most exciting of those is the effect of the tidal coupling on the phasing of gravitational waves; a measurement of this effect will allow a black hole to be observationally distinguished from other types of compact bodies.

Graduate students Stephanne Taylor and Simon Comeau, and post-doctoral fellow Igor Vlasov have been working with me on this topic. In an application of a general formalism developed by myself, Stephanne Taylor calculated the tidal fields acting on a black hole when the tidal environment is provided by another black hole situated at a large distance; in this context the mutual gravity of the black holes is weak and can be calculated within the post-Newtonian approximation to general relativity. In another application, Simon Comeau calculated the tidal fields acting on a small black hole moving in the deep relativistic field of a much larger, and rapidly rotating, black hole. Meanwhile, Igor Vlasov and I are attempting to generalize the formalism so that the tidal deformation of a black hole can be described with more accuracy; this is a very challenging calculation that forces us to solve the equations of second-order black-hole perturbation theory.

As a variation on this theme, graduate student Taylor Binnington and I are currently studying the tidal deformation of compact bodies that are not black holes. We are seeking a meaningful description of this deformation, in terms of well-defined ``relativistic Love numbers'' that generalize the standard Newtonian description.

Gravitational self-force

The term ``gravitational self-force'' refers to the motion of a small-mass body around a large black hole, in a treatment that goes beyond the test-mass description. In this treatment, the small mass creates a (small but significant) perturbation in the gravitational field of the large black hole. The perturbation affects the motion of the small body --- the motion is no longer geodesic, but accelerated, and the body is said to move in response to its own gravitational self-force. The perturbation also propagates outward in the form of gravitational waves. What is the nature of the self-forced motion, and what information concerning the strong-field dynamics can be extracted from the gravitational waves? These are the questions that my research group and I have been exploring.

Graduate students Adam Pound and Roland Haas have been working with me on this topic. Most of the work on the self-force models the small body as a point particle, and this invariably gives rise to singularities associated with the infinite mass density of the particle. Adam Pound is currently rebuilding the foundations of the self-force by replacing the point particle with an extended body, a black hole. To carry this out he is formulating a novel strategy to integrate the Einstein field equations so as to determine the motion of the small black hole and the gravitational waves that will be emitted in the process. Roland Haas, on the other hand, has devised powerful analytical and numerical methods to deal with point particles and their singular fields, and has implemented a concrete evaluation of the self-force for particles moving in the Schwarzchild spacetime.