20 research outputs found
Superradiant scattering of dispersive fields
Motivated by analogue models of classical and quantum field theory in curved
spacetimes and their recent experimental realizations, we consider wave
scattering processes of dispersive fields exhibiting two extra degrees of
freedom. In particular, we investigate how standard superradiant scattering
processes are affected by subluminal or superluminal modifications of the
dispersion relation. We analyze simple 1-dimensional toy-models based on
fourth-order corrections to the standard second order wave equation and show
that low-frequency waves impinging on generic scattering potentials can be
amplified during the process. In specific cases, by assuming a simple step
potential, we determine quantitatively the deviations in the amplification
spectrum that arise due to dispersion, and demonstrate that the amplification
can be further enhanced due to the presence of extra degrees of freedom. We
also consider dispersive scattering processes in which the medium where the
scattering takes place is moving with respect to the observer and show that
superradiance can also be manifest in such situations.Comment: 31 pages, 11 figures; published in CQ
Analogue model for anti-de Sitter as a description of point sources in fluids
We introduce an analogue model for a nonglobally hyperbolic spacetime in
terms of a two-dimensional fluid. This is done by considering the propagation
of sound waves in a radial flow with constant velocity. We show that the
equation of motion satisfied by sound waves is the wave equation on
. Since this spacetime is not globally hyperbolic, the
dynamics of the Klein-Gordon field is not well defined until boundary
conditions at the spatial boundary of are prescribed. On the analogue
model end, those extra boundary conditions provide an effective description of
the point source at . For waves with circular symmetry, we relate the
different physical evolutions to the phase difference between ingoing and
outgoing scattered waves. We also show that the fluid configuration can be
stable or unstable depending on the chosen boundary condition.Comment: 6 pages, 1 figure. To appear in Phys Rev
Synchronized stationary clouds in a static fluid
The existence of stationary bound states for the hydrodynamic velocity field
between two concentric cylinders is established. We argue that rotational
motion, together with a trapping mechanism for the associated field, is
sufficient to mitigate energy dissipation between the cylinders, thus allowing
the existence of infinitely long lived modes, which we dub stationary clouds.
We demonstrate the existence of such stationary clouds for sound and surface
waves when the fluid is static and the internal cylinder rotates with constant
angular velocity . These setups provide a unique opportunity for the
first experimental observation of synchronized stationary clouds. As in the
case of bosonic fields around rotating black holes and black hole analogues,
the existence of these clouds relies on a synchronization condition between
and the angular phase velocity of the cloud.Comment: v2: 7 pages, 4 figures. Accepted for publication in Physics Letters
Quasinormal Mode Oscillations in an Analogue Black Hole Experiment
The late stages of the relaxation process of a black hole are expected to depend only on its mass and angular momentum and not on the details of its formation process. Inspired by recent analogue gravity experiments, which demonstrate that certain black hole processes take place in gravitational and hydrodynamical systems alike, we conduct an experiment to search for quasinormal mode oscillations of the free surface of a hydrodynamical vortex flow. Our results demonstrate the occurrence and hint at the ubiquity of quasinormal ringing in nonequilibrium analog black hole experiments
Challenging the weak cosmic censorship conjecture with charged quantum particles
Motivated by the recent attempts to violate the weak cosmic censorship
conjecture for near-extreme black-holes, we consider the possibility of
overcharging a near-extreme Reissner-Nordstr\"om black hole by the quantum
tunneling of charged particles. We consider the scattering of spin-0 and
spin-1/2 particles by the black hole in a unified framework and obtain
analytically, for the first time, the pertinent reflection and transmission
coefficients without any small charge approximation. Based on these results, we
propose some gedanken experiments that could lead to the violation of the weak
cosmic censorship conjecture due to the (classically forbidden) absorption of
small energy charged particles by the black hole. As for the case of scattering
in Kerr spacetimes, our results demonstrate explicitly that scalar fields are
subject to (electrical) superradiance phenomenon, while spin-1/2 fields are
not. Superradiance impose some limitations on the gedanken experiments
involving spin-0 fields, favoring, in this way, the mechanisms for creation of
a naked singularity by the quantum tunneling of spin-1/2 charged fermions. We
also discuss the implications that vacuum polarization effects and quantum
statistics might have on these gedanken experiments. In particular, we show
that they are not enough to prevent the absorption of incident small energy
particles and, consequently, the formation of a naked singularity.Comment: 9 pages; Final version to appear in PR