Recent presentations

Riding the tide on black holes

General relativity, Einstein's greatest scientific achievement and today's most accurate description of gravitation, is turning 100 this year. Black holes, the most striking predictions of general relativity, are currently in the process of being firmly established as real astrophysical bodies. In this talk I describe black holes, their properties as dictated by general relativity, and how they fit in the astrophysical universe. In particular, I will describe how a companion body can raise a tide on a black hole, much as the Moon raises a tide on Earth, and what consequences this can have on the motion of the two-body system.

This public lecture was presented during a joint meeting of the Royal Canadian Institute for the Advancement of Science and the Royal Astronomical Society of Canada (Mississauga Centre), on 5 November 2015 at the Mississauga Central Library. The lecture is intended for a general audience.

Advanced Newtonian gravity

Introduction to post-Newtonian gravity

These lectures, delivered at CITA in April, 2015, cover some of the perhaps lesser known results of Newtonian gravity and provide an introduction to post-Newtonian gravity in the context of a weak-field, slow-motion approximation to general relativity. The lectures were essentially an infomercial for the recent book Gravity: Newtonian, post-Newtonian, Relativistic, by Eric Poisson and Clifford Will.

The lectures are intended for a general physics audience.

Of black holes and tides

The Newtonian theory of tidal deformations and dynamics is by now well developed. Tides are responsible for diverse phenomena, such as the existence of a dark face of the Moon, the spectacular volcanic activity on Io, and the dislodging of comets from the Oort Cloud. Compact bodies such as neutron stars and black holes can also be subjected to tidal forces, and the description of the resulting deformation and dynamics must be derived from general relativity. This talk offers an accessible overview of the current effort to formulate a comprehensive relativistic theory of tidal deformations and dynamics. We will see that like in Newtonian theory, the tidal deformation of compact bodies can be characterized in terms of "Love numbers", dimensionless quantities that depend on the body's internal structure. We will also see that like in Newtonian theory, the tidal deformation can lead to a transfer of angular momentum from the body to the orbital motion, and dissipation of energy within the body.

This talk is intended for a general physics audience.

Tidal deformation and dynamics of compact bodies

The Newtonian theory of tidal deformations and dynamics can be extended to general relativity and applied to compact bodies such as neutron stars and black holes. Like in Newtonian theory the tidal deformation of compact bodies can be characterized in terms of "Love numbers", dimensionless quantities that depend on the body's internal structure. Like in Newtonian theory the tidal deformation can lead to a transfer of angular momentum from the body to the orbital motion and dissipation of energy within the body. This talk provides an introduction to these topics.

This talk is slightly more technical than the previous one.

Intrinsic geometry of a tidally deformed Kerr horizon

The intrinsic metric of a tidally deformed black-hole horizon can be presented in a coordinate system adapted to the horizon's null generators, with one coordinate acting as a running parameter along each generator, and two coordinates acting as constant generator labels. The metric is invariant under reparametrizations of the generators, and as such the horizon's intrinsic geometry is known to be gauge invariant. We consider a Kerr black hole deformed by a slowly-evolving external tidal field, and describe the intrinsic geometry of its event horizon in terms of the electric and magnetic tidal moments that characterize the tidal environment. When the black hole is slowly rotating, the horizon's geometry can be described in terms of a deviation from an otherwise spherical surface, and the deformation can be characterized by gauge-invariant Love numbers. Some aspects of this tidal deformation have direct analogues in Newtonian physics. Some do not, and I will describe the similarities and differences between the tidal deformation of rotating black holes in general relativity and rotating fluid bodies in Newtonian physics.

This talk is fairly technical and is intended for an expert audience.

Self-force on a charge outside a five-dimensional black hole

To hold a particle in place in the gravitational field of a black hole requires an external agent to exert a force that compensates for the black hole's gravity. When the particle is electrically charged, the force required of the external agent is decreased. The difference is accounted by the particle's self-force, which originates from the particle's interaction with its own electric field, which is deformed by the curvature of the spacetime. The self-force acting on an electric charge outside a four-dimensional (three spatial dimensions plus time) was calculated in 1980 by Smith and Will, using Copson's exact solution to Maxwell's equations in the Schwarzschild spacetime. In this talk I describe a recent calculation of the self-force for a five-dimensional black hole. This calculation requires very different methods, since the five-dimensional version of Copson's solution is not known.

This talk is fairly technical and is intended for an expert audience.

Cosmic censorship and self-force

In the first part of the talk I review the scenarios proposed by Hubeny and Jacobson-Sotiriou that suggest that a near-extremal black hole can be turned into a naked singularity by absorbing a particle. In the Hubeny case, the black hole is charged, and it absorbs a particle with sufficient charge to produce an overcharged final state. In the Jacobson-Sotiriou case, the black hole is spinning, and it absorbs a particle with sufficient angular momentum to produce an overspinning state. I explain why such scenarios violate the third law of black-hole mechanics (although there may be a loophole), and seek a mechanism that prevents such overcharged or overspinning final states. I argue that self-force effects on the particle could provide this mechanism, and describe earlier partial attempts to incorporate the self-force into the scenarios. In the second part of the talk I describe ongoing work with Peter Zimmerman on the formulation of the self-force when the background metric is not a solution to the vacuum field equations (which is the case when the background spacetime describes a charged black hole).

This talk is fairly technical and is intended for an expert audience.