Timothy J. Walton occupies some quantum state between a physicist and a mathematician, having obtained his PhD from the physics department at Lancaster University in 2008 but now masquerading as a lecturer in mathematics at the University of Bolton.
by Timothy J. Walton.
Applying techniques from classical electrodynamics to generate new gravitational wave perturbations
I must begin with a confession: I don’t view myself as a gravitational physicist. Despite my PhD at Lancaster University involving a formulation of relativistic elasticity and an awful lot of differential geometry, my research thus far has been within the realm of classical and quantum electrodynamics. But it was precisely within that domain, along one particular avenue of investigation, where the first seeds of an idea were sown. Following my earlier work on a class of exact finite energy, spatially compact solutions to the vacuum source-free Maxwell equations – pulsed electromagnetic waves – describing single cycle pulses of laser light , together with Shin Goto at Kyoto University in Japan and my former PhD supervisor Robin Tucker at Lancaster University, a new question arose: “do pulsed gravitational waves exist?’’
As I recall, this question was posed and began to take root during one of the regular meetings I have with Robin. Within my institution, I am fortunate enough Continue reading
Written by Geoffrey Compère, a Research Associate at the Université Libre de Bruxelles. He has contributed to the theory of asymptotic symmetries, the techniques of solution generation in supergravity, the Kerr/CFT correspondence and is generally interested in gravity, black hole physics and string theory.
Why mass and angular momentum might not be enough to characterize a stationary black hole
Geoffrey Compère having family time in the park Le
Cinquantenaire in Brussels.
“Black hole have no hair.” This famous quote originates from John Wheeler in the sixties. In other words, a stationary black hole in general relativity is only characterized by its mass and angular momentum. This is because multipole moments of the gravitational field are sources for gravitational waves which radiate the multipoles away and only the last two conserved quantities, mass and angular momentum, remain. That’s the standard story.
Now, besides gravitational waves, general relativity contains another physical phenomenon which does not exist in Newtonian theory: the memory effect. It was discovered beyond the Iron Curtain by Zeldovich and Polnarev in the seventies and rediscovered in the western world and further extended by Christodoulou in the nineties. While gravitational waves lead to spacetime oscillations, the memory effect leads to a finite permanent displacement of test observers in spacetime. The effect exists for any value of the cosmological constant but in asymptotically flat spacetimes, it can be understood in terms of an asymptotic diffeomorphism known as a BMS supertranslation.
In order to understand that, let’s go back to the sixties where the radiative properties of sources were explored in general relativity; it was found by Bondi, van der Burg, Mezner and Sachs that there is a fundamental ambiguity in the coordinate frame at null infinity. Most expected that Continue reading
Following from the seminal work of Dain, a great deal is now known concerning geometric inequalities relating the area, charge, and angular momentum of axisymmetric black hole horizons in (possibly dynamical) spacetimes. A key feature of these results is that they are quasi-local: they depend on spacetime only near the horizon itself and so are not sensitive to the asymptotic behaviour of the geometry.
For Einstein-Maxwell theory the celebrated uniqueness theorems tell us under certain conditions, that the Kerr-Newman (KN) family of solutions are the only stationary, axisymmetric and asymptotically flat black hole spacetimes. These are the model geometries that originally motivated the inequalities. However if we relax the condition of asymptotic flatness there are many other families of black hole solutions. While in general these will not contain event horizons (whose standard definitions require flat or AdS asymptotics) they still contain singularities and Killing horizons. In this paper we focussed Continue reading