267 research outputs found
Acceleration Radiation for Orbiting Electrons
This paper presents an analysis of the radiation seen by an observer in
circular acceleration, for a magnetic spin. This is applied to an electron in a
storage ring, and the subtilty of the interaction of the spin with the spatial
motion of the electron is explicated. This interaction is shown to be time
dependent (in the radiating frame), which explains the strange results found
for the electron's residual polarisation in the literature. Finally, some brief
comments about the radiation emitted by an accelerating detector are made where
it is shown that the spectrum is correlated in that particles are emitted in
pairs.Comment: 21pp 7fi
Classical aspects of Hawking radiation verified in analogue gravity experiment
There is an analogy between the propagation of fields on a curved spacetime
and shallow water waves in an open channel flow. By placing a streamlined
obstacle into an open channel flow we create a region of high velocity over the
obstacle that can include wave horizons. Long (shallow water) waves propagating
upstream towards this region are blocked and converted into short (deep water)
waves. This is the analogue of the stimulated Hawking emission by a white hole
(the time inverse of a black hole). The measurements of amplitudes of the
converted waves demonstrate that they appear in pairs and are classically
correlated; the spectra of the conversion process is described by a
Boltzmann-distribution; and the Boltzmann-distribution is determined by the
determined by the change in flow across the white hole horizon.Comment: 17 pages, 10 figures; draft of a chapter submitted to the proceedings
of the IX'th SIGRAV graduate school: Analogue Gravity, Lake Como, Italy, May
201
Quantum radiation by electrons in lasers and the Unruh effect
In addition to the Larmor radiation known from classical electrodynamics,
electrons in a laser field may emit pairs of entangled photons -- which is a
pure quantum effect. We investigate this quantum effect and discuss why it is
suppressed in comparison with the classical Larmor radiation (which is just
Thomson backscattering of the laser photons). Further, we provide an intuitive
explanation of this process (in a simplified setting) in terms of the Unruh
effect.Comment: 4 pages, 3 figure
Coordinates with Non-Singular Curvature for a Time Dependent Black Hole Horizon
A naive introduction of a dependency of the mass of a black hole on the
Schwarzschild time coordinate results in singular behavior of curvature
invariants at the horizon, violating expectations from complementarity. If
instead a temporal dependence is introduced in terms of a coordinate akin to
the river time representation, the Ricci scalar is nowhere singular away from
the origin. It is found that for a shrinking mass scale due to evaporation, the
null radial geodesics that generate the horizon are slightly displaced from the
coordinate singularity. In addition, a changing horizon scale significantly
alters the form of the coordinate singularity in diagonal (orthogonal) metric
coordinates representing the space-time. A Penrose diagram describing the
growth and evaporation of an example black hole is constructed to examine the
evolution of the coordinate singularity.Comment: 15 pages, 1 figure, additional citation
Black Hole Thermodynamics and Lorentz Symmetry
Recent developments point to a breakdown in the generalized second law of
thermodynamics for theories with Lorentz symmetry violation. It appears
possible to construct a perpetual motion machine of the second kind in such
theories, using a black hole to catalyze the conversion of heat to work. Here
we describe and extend the arguments leading to that conclusion. We suggest the
inference that local Lorentz symmetry may be an emergent property of the
macroscopic world with origins in a microscopic second law of causal horizon
thermodynamics.Comment: 4 pages; v2: Version to appear in Foundations of Physics. Potential
counterexamples addressed, argument given applying to LV theories where all
speeds (or horizons) coincide, and editing for clarit
Hawking Spectrum and High Frequency Dispersion
We study the spectrum of created particles in two-dimensional black hole
geometries for a linear, hermitian scalar field satisfying a Lorentz
non-invariant field equation with higher spatial derivative terms that are
suppressed by powers of a fundamental momentum scale . The preferred frame
is the ``free-fall frame" of the black hole. This model is a variation of
Unruh's sonic black hole analogy. We find that there are two qualitatively
different types of particle production in this model: a thermal Hawking flux
generated by ``mode conversion" at the black hole horizon, and a non-thermal
spectrum generated via scattering off the background into negative free-fall
frequency modes. This second process has nothing to do with black holes and
does not occur for the ordinary wave equation because such modes do not
propagate outside the horizon with positive Killing frequency. The horizon
component of the radiation is astonishingly close to a perfect thermal
spectrum: for the smoothest metric studied, with Hawking temperature
, agreement is of order at frequency
, and agreement to order persists out to
where the thermal number flux is ). The flux
from scattering dominates at large and becomes many orders of
magnitude larger than the horizon component for metrics with a ``kink", i.e. a
region of high curvature localized on a static worldline outside the horizon.
This non-thermal flux amounts to roughly 10\% of the total luminosity for the
kinkier metrics considered. The flux exhibits oscillations as a function of
frequency which can be explained by interference between the various
contributions to the flux.Comment: 32 pages, plain latex, 16 figures included using psfi
Black Hole Evaporation in the Presence of a Short Distance Cutoff
A derivation of the Hawking effect is given which avoids reference to field
modes above some cutoff frequency in the free-fall frame
of the black hole. To avoid reference to arbitrarily high frequencies, it is
necessary to impose a boundary condition on the quantum field in a timelike
region near the horizon, rather than on a (spacelike) Cauchy surface either
outside the horizon or at early times before the horizon forms. Due to the
nature of the horizon as an infinite redshift surface, the correct boundary
condition at late times outside the horizon cannot be deduced, within the
confines of a theory that applies only below the cutoff, from initial
conditions prior to the formation of the hole. A boundary condition is
formulated which leads to the Hawking effect in a cutoff theory. It is argued
that it is possible the boundary condition is {\it not} satisfied, so that the
spectrum of black hole radiation may be significantly different from that
predicted by Hawking, even without the back-reaction near the horizon becoming
of order unity relative to the curvature.Comment: 35 pages, plain LaTeX, UMDGR93-32, NSF-ITP-93-2
Pseudo-Schwarzschild Spherical Accretion as a Classical Black Hole Analogue
We demonstrate that a spherical accretion onto astrophysical black holes,
under the influence of Newtonian or various post-Newtonian pseudo-Schwarzschild
gravitational potentials, may constitute a concrete example of classical
analogue gravity naturally found in the Universe. We analytically calculate the
corresponding analogue Hawking temperature as a function of the minimum number
of physical parameters governing the accretion flow. We study both the
polytropic and the isothermal accretion. We show that unlike in a general
relativistic spherical accretion, analogue white hole solutions can never be
obtained in such post-Newtonian systems. We also show that an isothermal
spherical accretion is a remarkably simple example in which the only one
information--the temperature of the fluid, is sufficient to completely describe
an analogue gravity system. For both types of accretion, the analogue Hawking
temperature may become higher than the usual Hawking temperature. However, the
analogue Hawking temperature for accreting astrophysical black holes is
considerably lower compared with the temperature of the accreting fluid.Comment: Final Version to appear in the journal General Relativity &
Gravitation, Volume 27, Issue 11, 2005. 17 pages, Two colour and one black
and white figures. Typos corrected. Recent reference on analogue effect in
relativistic accretion disc adde
Extensive Entropy Bounds
It is shown that, for systems in which the entropy is an extensive function
of the energy and volume, the Bekenstein and the holographic entropy bounds
predict new results. More explicitly, the Bekenstein entropy bound leads to the
entropy of thermal radiation (the Unruh-Wald bound) and the spherical entropy
bound implies the "causal entropy bound". Surprisingly, the first bound shows a
close relationship between black hole physics and the Stephan-Boltzmann law
(for the energy and entropy flux densities of the radiation emitted by a hot
blackbody). Furthermore, we find that the number of different species of
massless fields is bounded by .Comment: 8 pages, revtex, To appear in Phys. Rev.
Analog model for an expanding universe
Over the last few years numerous papers concerning analog models for gravity
have been published. It was shown that the dynamical equation of several
systems (e.g. Bose-Einstein condensates with a sink or a vortex) have the same
wave equation as light in a curved-space (e.g. black holes). In the last few
months several papers were released which deal with simulations of the
universe.
In this article the de-Sitter universe will be compared with a freely
expanding three-dimensional spherical Bose-Einstein condensate. Initially the
condensate is in a harmonic trap, which suddenly will be switched off. At the
same time a small perturbation will be injected in the center of the condensate
cloud.
The motion of the perturbation in the expanding condensate will be discussed,
and after some transformations the similarity to an expanding universe will be
shown.Comment: Presented at the 4th Australasian conference on General Relativity
and Cosmology, Monash U, Melbourne, 7-9 January 200
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