12,993 research outputs found
Twofold Hidden Conformal Symmetries of the Kerr-Newman Black Hole
In this paper, we suggest that there are two different individual 2D CFTs
holographically dual to the Kerr-Newman black hole, coming from the
corresponding two possible limits --- the Kerr/CFT and Reissner-Nordstr\"om/CFT
correspondences, namely there exist the Kerr-Newman/CFTs dualities. A probe
scalar field at low frequencies turns out can exhibit two different 2D
conformal symmetries (named by - and -pictures, respectively) in its
equation of motion when the associated parameters are suitably specified. These
twofold dualities are supported by the matchings of entropies, absorption cross
sections and real time correlators computed from both the gravity and the CFT
sides. Our results lead to a fascinating "microscopic no hair conjecture" ---
for each macroscopic hair parameter, in additional to the mass of a black hole
in the Einstein-Maxwell theory, there should exist an associated holographic
CFT description.Comment: 21 pages, 1 figure, typos correcte
4D Scattering Amplitudes and Asymptotic Symmetries from 2D CFT
We reformulate the scattering amplitudes of 4D flat space gauge theory and
gravity in the language of a 2D CFT on the celestial sphere. The resulting CFT
structure exhibits an OPE constructed from 4D collinear singularities, as well
as infinite-dimensional Kac-Moody and Virasoro algebras encoding the asymptotic
symmetries of 4D flat space. We derive these results by recasting 4D dynamics
in terms of a convenient foliation of flat space into 3D Euclidean AdS and
Lorentzian dS geometries. Tree-level scattering amplitudes take the form of
Witten diagrams for a continuum of (A)dS modes, which are in turn equivalent to
CFT correlators via the (A)dS/CFT dictionary. The Ward identities for the 2D
conserved currents are dual to 4D soft theorems, while the bulk-boundary
propagators of massless (A)dS modes are superpositions of the leading and
subleading Weinberg soft factors of gauge theory and gravity. In general, the
massless (A)dS modes are 3D Chern-Simons gauge fields describing the soft,
single helicity sectors of 4D gauge theory and gravity. Consistent with the
topological nature of Chern-Simons theory, Aharonov-Bohm effects record the
"tracks" of hard particles in the soft radiation, leading to a simple
characterization of gauge and gravitational memories. Soft particle exchanges
between hard processes define the Kac-Moody level and Virasoro central charge,
which are thereby related to the 4D gauge coupling and gravitational strength
in units of an infrared cutoff. Finally, we discuss a toy model for black hole
horizons via a restriction to the Rindler region.Comment: 66 pages, 8 figures; v2: version to appear in JHE
Quantum Hair on Black Holes
A black hole may carry quantum numbers that are {\it not} associated with
massless gauge fields, contrary to the spirit of the ``no-hair'' theorems. We
describe in detail two different types of black hole hair that decay
exponentially at long range. The first type is associated with discrete gauge
charge and the screening is due to the Higgs mechanism. The second type is
associated with color magnetic charge, and the screening is due to color
confinement. In both cases, we perform semi-classical calculations of the
effect of the hair on local observables outside the horizon, and on black hole
thermodynamics. These effects are generated by virtual cosmic strings, or
virtual electric flux tubes, that sweep around the event horizon. The effects
of discrete gauge charge are non-perturbative in , but the effects of
color magnetic charge become -independent in a suitable limit. We
present an alternative treatment of discrete gauge charge using dual variables,
and examine the possibility of black hole hair associated with discrete {\it
global} symmetry. We draw the distinction between {\it primary} hair, which
endows a black hole with new quantum numbers, and {\it secondary} hair, which
does not, and we point out some varieties of secondary hair that occur in the
standard model of particle physics.Comment: (100 pages
Noninvasive depth estimation using tissue optical properties and a dual-wavelength fluorescent molecular probe in vivo
Translation of fluorescence imaging using molecularly targeted imaging agents for real-time assessment of surgical margins in the operating room requires a fast and reliable method to predict tumor depth from planar optical imaging. Here, we developed a dual-wavelength fluorescent molecular probe with distinct visible and near-infrared excitation and emission spectra for depth estimation in mice and a method to predict the optical properties of the imaging medium such that the technique is applicable to a range of medium types. Imaging was conducted at two wavelengths in a simulated blood vessel and an in vivo tumor model. Although the depth estimation method was insensitive to changes in the molecular probe concentration, it was responsive to the optical parameters of the medium. Results of the intra-tumor fluorescent probe injection showed that the average measured tumor sub-surface depths were 1.31 ± 0.442 mm, 1.07 ± 0.187 mm, and 1.42 ± 0.182 mm, and the average estimated sub-surface depths were 0.97 ± 0.308 mm, 1.11 ± 0.428 mm, 1.21 ± 0.492 mm, respectively. Intravenous injection of the molecular probe allowed for selective tumor accumulation, with measured tumor sub-surface depths of 1.28 ± 0.168 mm, and 1.50 ± 0.394 mm, and the estimated depths were 1.46 ± 0.314 mm, and 1.60 ± 0.409 mm, respectively. Expansion of our technique by using material optical properties and mouse skin optical parameters to estimate the sub-surface depth of a tumor demonstrated an agreement between measured and estimated depth within 0.38 mm and 0.63 mm for intra-tumor and intravenous dye injections, respectively. Our results demonstrate the feasibility of dual-wavelength imaging for determining the depth of blood vessels and characterizing the sub-surface depth of tumors in vivo
A framework for real-time physically-based hair rendering
Hair rendering has been a major challenge in computer graphics for several years due to the complex light interactions involved. Complexity mainly stems from two aspects: the number of hair strands, and the resulting complexity of their interaction with light. In general, theoretical approaches towards a realistic hair visualization aim to develop a proper scattering model on a per-strand level, which can be extended in practice to the whole hair volume with ray tracing even though it is usually expensive in computational terms. Aiming at achieving real-time hair rendering, I analyze each component contributing to it from both theoretical and practical points of view in this work. Most approaches, both real- and non-real-time build on top of the Marschner scattering model, such as recent efficient state-of-the-art techniques introduced in Unreal Engine and Frostbite, among others. Interactive applications cannot afford the complexity of ray tracing, and they target efficiency by explicitly dealing with each component involved in both single-strand and inter-strand light interactions, applying the necessary simplifications to match the time budget. I have further implemented a framework, separating the different components, which combines aspects of these approaches towards the best possible quality and performance. The implementation achieves real-time good-looking hair, and its flexibility has allowed to perform experiments on performance, scalability, and contribution to quality of the different components
Superradiance -- the 2020 Edition
Superradiance is a radiation enhancement process that involves dissipative
systems. With a 60 year-old history, superradiance has played a prominent role
in optics, quantum mechanics and especially in relativity and astrophysics. In
General Relativity, black-hole superradiance is permitted by the ergoregion,
that allows for energy, charge and angular momentum extraction from the vacuum,
even at the classical level. Stability of the spacetime is enforced by the
event horizon, where negative energy-states are dumped. Black-hole
superradiance is intimately connected to the black-hole area theorem, Penrose
process, tidal forces, and even Hawking radiation, which can be interpreted as
a quantum version of black-hole superradiance. Various mechanisms (as diverse
as massive fields, magnetic fields, anti-de Sitter boundaries, nonlinear
interactions, etc...) can confine the amplified radiation and give rise to
strong instabilities. These "black-hole bombs" have applications in searches of
dark matter and of physics beyond the Standard Model, are associated to the
threshold of formation of new black hole solutions that evade the no-hair
theorems, can be studied in the laboratory by devising analog models of
gravity, and might even provide a holographic description of spontaneous
symmetry breaking and superfluidity through the gauge-gravity duality.
This work is meant to provide a unified picture of this multifaceted subject.
We focus on the recent developments in the field, and work out a number of
novel examples and applications, ranging from fundamental physics to
astrophysics.Comment: 279 pages. Second Edition of the "Lecture Notes in Physics" book by
Springer-Verlag. Overall improvement, typos and incorrect statements of
Edition 1 are now corrected; new sections were added, reflecting activity in
the field. Bounds on ultralight fields are summarized in Table 4, and updated
online regularly at https://centra.tecnico.ulisboa.pt/network/grit/ and
https://web.uniroma1.it/gmunu
Virtual Black Holes
One would expect spacetime to have a foam-like structure on the Planck scale
with a very high topology. If spacetime is simply connected (which is assumed
in this paper), the non-trivial homology occurs in dimension two, and spacetime
can be regarded as being essentially the topological sum of and
bubbles. Comparison with the instantons for pair creation of black holes
shows that the bubbles can be interpreted as closed loops of
virtual black holes. It is shown that scattering in such topological
fluctuations leads to loss of quantum coherence, or in other words, to a
superscattering matrix that does not factorise into an matrix and its
adjoint. This loss of quantum coherence is very small at low energies for
everything except scalar fields, leading to the prediction that we may never
observe the Higgs particle. Another possible observational consequence may be
that the angle of QCD is zero without having to invoke the
problematical existence of a light axion. The picture of virtual black holes
given here also suggests that macroscopic black holes will evaporate down to
the Planck size and then disappear in the sea of virtual black holes.Comment: 24p, LaTeX, 3 postscript figures included with epsf sent in a
seperate uuencoded fil
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