129 research outputs found
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
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
Rotational superradiant scattering in a vortex flow
When an incident wave scatters off of an obstacle, it is partially reflected and partially transmitted. In theory, if the obstacle is rotating, waves can be amplified in the process, extracting energy from the scatterer. Here we describe in detail the first laboratory detection of this phenomenon, known as superradiance 1, 2, 3, 4. We observed that waves propagating on the surface of water can be amplified after being scattered by a draining vortex. The maximum amplification measured was 14% ± 8%, obtained for 3.70 Hz waves, in a 6.25-cm-deep fluid, consistent with the superradiant scattering caused by rapid rotation. We expect our experimental findings to be relevant to black-hole physics, since shallow water waves scattering on a draining fluid constitute an analogue of a black hole 5, 6, 7, 8, 9, 10, as well as to hydrodynamics, due to the close relation to over-reflection instabilities 11, 12, 13
Can quantum mechanics fool the cosmic censor?
We revisit the mechanism for violating the weak cosmic-censorship conjecture
(WCCC) by overspinning a nearly-extreme charged black hole. The mechanism
consists of an incoming massless neutral scalar particle, with low energy and
large angular momentum, tunneling into the hole. We investigate the effect of
the large angular momentum of the incoming particle on the background geometry
and address recent claims that such a back-reaction would invalidate the
mechanism. We show that the large angular momentum of the incident particle
does not constitute an obvious impediment to the success of the overspinning
quantum mechanism, although the induced back-reaction turns out to be essential
to restoring the validity of the WCCC in the classical regime. These results
seem to endorse the view that the "cosmic censor" may be oblivious to processes
involving quantum effects.Comment: 5 pages, to appear as a Rapid Communication in Phys. Rev.
Detecting rotational superradiance in fluid laboratories
Rotational superradiance was predicted theoretically decades ago, and is chiefly responsible for a number of important effects and phenomenology in black-hole physics. However, rotational superradiance has never been observed experimentally. Here, with the aim of probing superradiance in the lab, we investigate the behavior of sound and surface waves in fluids resting in a circular basin at the center of which a rotating cylinder is placed. We show that with a suitable choice for the material of the cylinder, surface and sound waves are amplified. Two types of instabilities are studied: one sets in whenever superradiant modes are confined near the rotating cylinder and the other, which does not rely on confinement, corresponds to a local excitation of the cylinder. Our findings are experimentally testable in existing fluid laboratories and, hence, offer experimental exploration and comparison of dynamical instabilities arising from rapidly rotating boundary layers in astrophysical as well as in fluid dynamical systems
Synchronous frequencies of extremal Kerr black holes: Resonances, scattering, and stability
The characteristic damping times of the natural oscillations of a Kerr black hole become arbitrarily large as the extremal limit is approached. This behavior is associated with the so-called zero damped modes (ZDMs), and suggests that extremal black holes are characterized by quasinormal modes whose frequencies are purely real. Since these frequencies correspond to oscillations whose angular phase velocity matches the horizon angular velocity of the black hole, they are sometimes called "synchronous frequencies." Several authors have studied the ZDMs for near-extremal black holes. Recently, their correspondence to branch points of the Green's function of the wave equation was linked to the Aretakis instability of extremal black holes. Here we investigate the existence of ZDMs for extremal black holes, showing that these real-axis resonances of the field are unphysical as natural black hole oscillations: the corresponding frequency is always associated with a scattering mode. By analyzing the behavior of these modes near the event horizon we obtain new insight into the transition to extremality, including a simple way to understand the Aretakis instability
Test of the weak cosmic censorship conjecture with a charged scalar field and dyonic Kerr-Newman black holes
A thought experiment considered recently in the literature, in which it is
investigated whether a dyonic Kerr-Newman black hole can be destroyed by
overcharging or overspinning it past extremality by a massive complex scalar
test field, is revisited. Another derivation of the result that this is not
possible, i.e. the weak cosmic censorship is not violated in this thought
experiment, is given. The derivation is based on conservation laws, on a null
energy condition, and on specific properties of the metric and the
electromagnetic field of dyonic Kerr-Newman black holes. The metric is kept
fixed, whereas the dynamics of the electromagnetic field is taken into account.
A detailed knowledge of the solutions of the equations of motion is not needed.
The approximation in which the electromagnetic field is fixed is also
considered, and a derivation for this case is also given. In addition, an older
version of the thought experiment, in which a pointlike test particle is used,
is revisited. The same result, namely the non-violation of the cosmic
censorship, is rederived in a way which is simpler than in earlier works.Comment: 18 pages, LaTe
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