1,068 research outputs found

### Are there hyperentropic objects ?

By treating the Hawking radiation as a system in thermal equilibrium, Marolf
and R. Sorkin have argued that hyperentropic objects (those violating the
entropy bounds) would be emitted profusely with the radiation, thus opening a
loophole in black hole based arguments for such entropy bounds. We demonstrate,
on kinetic grounds, that hyperentropic objects could only be formed extremely
slowly, and so would be rare in the Hawking radiance, thus contributing
negligibly to its entropy. The arguments based on the generalized second law of
thermodynamics then rule out weakly self-gravitating hyperentropic objects and
a class of strongly self-gravitating ones.Comment: LaTeX, 4 page

### How does the entropy/information bound work ?

According to the universal entropy bound, the entropy (and hence information
capacity) of a complete weakly self-gravitating physical system can be bounded
exclusively in terms of its circumscribing radius and total gravitating energy.
The bound's correctness is supported by explicit statistical calculations of
entropy, gedanken experiments involving the generalized second law, and
Bousso's covariant holographic bound. On the other hand, it is not always
obvious in a particular example how the system avoids having too many states
for given energy, and hence violating the bound. We analyze in detail several
purported counterexamples of this type (involving systems made of massive
particles, systems at low temperature, systems with high degeneracy of the
lowest excited states, systems with degenerate ground states, or involving a
particle spectrum with proliferation of nearly massless species), and exhibit
in each case the mechanism behind the bound's efficacy.Comment: LaTeX, 10 pages. Contribution to the special issue of Foundation of
Physics in honor of Asher Peres; C. Fuchs and A. van der Merwe, ed

### Entropy Bounds and Black Hole Remnants

We rederive the universal bound on entropy with the help of black holes while
allowing for Unruh--Wald buoyancy. We consider a box full of entropy lowered
towards and then dropped into a Reissner--Nordstr\"om black hole in equilibrium
with thermal radiation. We avoid the approximation that the buoyant pressure
varies slowly across the box, and compute the buoyant force exactly. We find,
in agreement with independent investigations, that the neutral point
generically lies very near the horizon. A consequence is that in the generic
case, the Unruh--Wald entropy restriction is neither necessary nor sufficient
for enforcement of the generalized second law. Another consequence is that
generically the buoyancy makes only a negligible contribution to the energy
bookeeping, so that the original entropy bound is recovered if the generalized
second law is assumed to hold. The number of particle species does not figure
in the entropy bound, a point that has caused some perplexity. We demonstrate
by explicit calculation that, for arbitrarily large number of particle species,
the bound is indeed satisfied by cavity thermal radiation in the thermodynamic
regime, provided vacuum energies are included. We also show directly that
thermal radiation in a cavity in $D$ dimensional space also respects the bound
regardless of the value of $D$. As an application of the bound we show that it
strongly restricts the information capacity of the posited black hole remnants,
so that they cannot serve to resolve the information paradox.Comment: 12 pages, UCSBTH-93-2

### Tensor-vector-scalar-modified gravity: from small scale to cosmology

The impressive success of the standard cosmological model has suggested to
many that its ingredients are all one needs to explain galaxies and their
systems. I summarize a number of known problems with this program. They might
signal the failure of standard gravity theory on galaxy scales. The requisite
hints as to the alternative gravity theory may lie with the MOND paradigm which
has proved an effective summary of galaxy phenomenology. A simple nonlinear
modified gravity theory does justice to MOND at the nonrelativistic level, but
cannot be consistently promoted to relativistic status. The obstacles were
first sidestepped with the formulation of TeVeS, a covariant modified gravity
theory. I review its structure, its MOND and Newtonian limits, and its
performance in face of galaxy phenomenology. I also summarize features of TeVeS
cosmology and describe the confrontation with data from strong and weak
gravitational lensingComment: Invited talk at the Royal Society's Theo Murphy Meeting "Testing
general relativity with cosmology", Feb. 2011. LaTeX, 15 page

### Entropy bounds for charged and rotating systems

It was shown in a previous work 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. In this paper, we go further and derive
improved upper bounds to the entropy of {\it extensive} charged and rotating
systems. Furthermore, it is shown that for charged and rotating systems
(including non-extensive ones), the total energy that appear in both the
Bekenstein entropy bound (BEB) and the causal entropy bound (CEB) can be
replaced by the {\it internal} energy of the system. In addition, we propose
possible corrections to the BEB and the CEB.Comment: 12 pages, revte

### Does the generalized second law require entropy bounds for a charged system?

We calculate the net change in generalized entropy occurring when one carries
out the gedanken experiment in which a box initially containing energy $E$,
entropy $S$ and charge $Q$ is lowered adiabatically toward a
Reissner-Nordstr\"{o}m black hole and then dropped in. This is an extension of
the work of Unruh-Wald to a charged system (the contents of the box possesses a
charge $Q$). Their previous analysis showed that the effects of acceleration
radiation prevent violation of the generalized second law of thermodynamics. In
our more generic case, we show that the properties of the thermal atmosphere
are equally important when charge is present. Indeed, we prove here that an
equilibrium condition for the the thermal atmosphere and the physical
properties of ordinary matter are sufficient to enforce the generalized second
law. Thus, no additional assumptions concerning entropy bounds on the contents
of the box need to be made in this process. The relation between our work and
the recent works of Bekenstein and Mayo, and Hod (entropy bound for a charged
system) are also discussed.Comment: 18pages, RevTex, no figure

### Black hole polarization and new entropy bounds

Zaslavskii has suggested how to tighten Bekenstein's bound on entropy when
the object is electrically charged. Recently Hod has provided a second tighter
version of the bound applicable when the object is rotating. Here we derive
Zaslavskii's optimized bound by considering the accretion of an ordinary
charged object by a black hole. The force originating from the polarization of
the black hole by a nearby charge is central to the derivation of the bound
from the generalized second law. We also conjecture an entropy bound for
charged rotating objects, a synthesis of Zaslavskii's and Hod's. On the basis
of the no hair principle for black holes, we show that this last bound cannot
be tightened further in a generic way by knowledge of ``global'' conserved
charges, e.g., baryon number, which may be borne by the object.Comment: 21 pages, RevTex, Regularization of potential made clearer. Error in
energy of the particle corrected with no consequence for final conclusions.
New references adde

### Selection Rules for Black-Hole Quantum Transitions

We suggest that quantum transitions of black holes comply with selection
rules, analogous to those of atomic spectroscopy. In order to identify such
rules, we apply Bohr's correspondence principle to the quasinormal ringing
frequencies of black holes. In this context, classical ringing frequencies with
an asymptotically vanishing real part \omega_R correspond to virtual quanta,
and may thus be interpreted as forbidden quantum transitions. With this
motivation, we calculate the quasinormal spectrum of neutrino fields in
spherically symmetric black-hole spacetimes. It is shown that \omega_R->0 for
these resonances, suggesting that the corresponding fermionic transitions are
quantum mechanically forbidden.Comment: 4 pages, 2 figure

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