From gravity to fluid dynamics
Black holes are one of the most remarkable predictions of the theory of general relativity. In the FLUIDGRAVITY (Fluid dynamics and quantum gravity) project, recent advances in brane cosmology, string theory and fluid dynamics motivated studies of black holes in more than four dimensions with surprising results. In a spacetime with more than four dimensions, black holes exist with exotic shapes and unusual dynamics. The solutions of Einstein's equations include multiple asymptotic boundaries and past event horizons that are not observed in real black holes formed by the gravitational collapse of a massive star. FLUIDGRAVITY scientists focused on one such feature: the inner horizon of a black hole that is inaccessible to an external observer. They studied solutions of the scalar wave equation for a black hole and how the information obtained is translated into physically meaningful observables. In addition, families of exact solutions of the highly non-linear equations of force-free electromagnetism were found, exhibiting complex phenomena observed in the near-horizon region. In particular, much of the physics of electrodynamic energy extraction was captured for rotating black holes. On the other hand, field equations cannot be solved analytically. Therefore, scientists developed techniques to extract properties of the scattering of particles off a black hole without an analytic solution. The new approach allows estimating the dependence of observables based on analytic properties of the solutions. With these studies, FLUIDGRAVITY scientists found themselves at the start of a difficult but promising journey. Their solutions are valid for a curved space time – but the Universe is much more complex. Subsequently, they used the relationship between general relativity and hydrodynamics to study how black holes behave when perturbed. Ongoing research may tell physicists more about the effects of turbulence on the black hole space time and whether rotating gravitational vortices are observable.
Keywords
Gravity, fluid dynamics, quantum gravity, black holes, four dimensions