2,053 research outputs found
The gravity of magnetic stresses and energy
In the framework of designing laboratory tests of relativistic gravity, we
investigate the gravitational field produced by the magnetic field of a
solenoid. Observing this field might provide a mean of testing whether stresses
gravitate as predicted by Einstein's theory. A previous study of this problem
by Braginsky, Caves and Thorne predicted that the contribution to the
gravitational field resulting from the stresses of the magnetic field and of
the solenoid walls would cancel the gravitational field produced by the
mass-energy of the magnetic field, resulting in a null magnetically-generated
gravitational force outside the solenoid. They claim that this null result,
once proved experimentally, would demonstrate the stress contribution to
gravity. We show that this result is incorrect, as it arises from an incomplete
analysis of the stresses, which neglects the axial stresses in the walls. Once
the stresses are properly evaluated, we find that the gravitational field
outside a long solenoid is in fact independent of Maxwell and material
stresses, and it coincides with the newtonian field produced by the linear mass
distribution equivalent to the density of magnetic energy stored in a unit
length of the solenoid. We argue that the gravity of Maxwell stress can be
directly measured in the vacuum region inside the solenoid, where the newtonian
noise is absent in principle, and the gravity generated by Maxwell stresses is
not screened by the negative gravity of magnetic-induced stresses in the
solenoid walls.Comment: 10 pages, final version accepted for publication in PR
The Tolman-Bondi--Vaidya Spacetime: matching timelike dust to null dust
The Tolman-Bondi and Vaidya solutions are two solutions to Einstein equations
which describe dust particles and null fluid, respectively. We show that it is
possible to match the two solutions in one single spacetime, the
Tolman-Bondi--Vaidya spacetime. The new spacetime is divided by a null surface
with Tolman-Bondi dust on one side and Vaidya fluid on the other side. The
differentiability of the spacetime is discussed. By constructing a specific
solution, we show that the metric across the null surface can be at least
and the stress-energy tensor is continuous.Comment: 5 pages, no figur
Planck Fluctuations, Measurement Uncertainties and the Holographic Principle
Starting from a critical analysis of recently reported surprisingly large
uncertainties in length and position measurements deduced within the framework
of quantum gravity, we embark on an investigation both of the correlation
structure of Planck scale fluctuations and the role the holographic hypothesis
is possibly playing in this context. While we prove the logical independence of
the fluctuation results and the holographic hypothesis (in contrast to some
recent statements in that direction) we show that by combining these two topics
one can draw quite strong and interesting conclusions about the fluctuation
structure and the microscopic dynamics on the Planck scale. We further argue
that these findings point to a possibly new and generalized form of quantum
statistical mechanics of strongly (anti)correlated systems of degrees of
freedom in this fundamental regime.Comment: 19 pages, Latex, no figures, some new references, to appear
ModPhysLett
Failure of the work-Hamiltonian connection for free energy calculations
Extensions of statistical mechanics are routinely being used to infer free
energies from the work performed over single-molecule nonequilibrium
trajectories. A key element of this approach is the ubiquitous expression
dW/dt=\partial H(x,t)/ \partial t which connects the microscopic work W
performed by a time-dependent force on the coordinate x with the corresponding
Hamiltonian H(x,t) at time t. Here we show that this connection, as pivotal as
it is, cannot be used to estimate free energy changes. We discuss the
implications of this result for single-molecule experiments and atomistic
molecular simulations and point out possible avenues to overcome these
limitations
Energy dependence on fractional charge for strongly interacting subsystems
The energies of a pair of strongly-interacting subsystems with arbitrary
noninteger charges are examined from closed and open system perspectives. An
ensemble representation of the charge dependence is derived, valid at all
interaction strengths. Transforming from resonance-state ionicity to ensemble
charge dependence imposes physical constraints on the occupation numbers in the
strong-interaction limit. For open systems, the chemical potential is evaluated
using microscopic and thermodynamic models, leading to a novel correlation
between ground-state charge and an electronic temperature.Comment: 4 pages, 3 figs.; as accepted (Phys. Rev. Lett.
Comment on "Failure of the work-Hamiltonian connection for free-energy calculations" by Jose M. G. Vilar and J. Miguel Rubi
I point out that the arguments raised by Vilar and Rubi against the
work-Hamiltonian connection in free-energy calculations imply, if correct, the
failure of the statistical mechanical expression of the thermodynamical
free-energy via the logarithm of the partition function.Comment: To appear in Physical Review Letter
Quantum Macrostates, Equivalence of Ensembles and an H-Theorem
Before the thermodynamic limit, macroscopic averages need not commute for a
quantum system. As a consequence, aspects of macroscopic fluctuations or of
constrained equilibrium require a careful analysis, when dealing with several
observables. We propose an implementation of ideas that go back to John von
Neumann's writing about the macroscopic measurement. We apply our scheme to the
relation between macroscopic autonomy and an H-theorem, and to the problem of
equivalence of ensembles. In particular, we show how the latter is related to
the asymptotic equipartition theorem. The main point of departure is an
expression of a law of large numbers for a sequence of states that start to
concentrate, as the size of the system gets larger, on the macroscopic values
for the different macroscopic observables. Deviations from that law are
governed by the entropy.Comment: 16 pages; v1 -> v2: Sec. 3 slightly rewritten, 2 references adde
More examples of structure formation in the Lemaitre-Tolman model
In continuing our earlier research, we find the formulae needed to determine
the arbitrary functions in the Lemaitre-Tolman model when the evolution
proceeds from a given initial velocity distribution to a final state that is
determined either by a density distribution or by a velocity distribution. In
each case the initial and final distributions uniquely determine the L-T model
that evolves between them, and the sign of the energy-function is determined by
a simple inequality. We also show how the final density profile can be more
accurately fitted to observational data than was done in our previous paper. We
work out new numerical examples of the evolution: the creation of a galaxy
cluster out of different velocity distributions, reflecting the current data on
temperature anisotropies of CMB, the creation of the same out of different
density distributions, and the creation of a void. The void in its present
state is surrounded by a nonsingular wall of high density.Comment: LaTeX 2e with eps figures. 30 pages, 11 figures, 30 figure files.
Revision matches published versio
Black Hole Evaporation in an Expanding Universe
We calculate the quantum radiation power of black holes which are asymptotic
to the Einstein-de Sitter universe at spatial and null infinities. We consider
two limiting mass accretion scenarios, no accretion and significant accretion.
We find that the radiation power strongly depends on not only the asymptotic
condition but also the mass accretion scenario. For the no accretion case, we
consider the Einstein-Straus solution, where a black hole of constant mass
resides in the dust Friedmann universe. We find negative cosmological
correction besides the expected redshift factor. This is given in terms of the
cubic root of ratio in size of the black hole to the cosmological horizon, so
that it is currently of order but could have been significant at the formation epoch of
primordial black holes. Due to the cosmological effects, this black hole has
not settled down to an equilibrium state. This cosmological correction may be
interpreted in an analogy with the radiation from a moving mirror in a flat
spacetime. For the significant accretion case, we consider the Sultana-Dyer
solution, where a black hole tends to increase its mass in proportion to the
cosmological scale factor. In this model, we find that the radiation power is
apparently the same as the Hawking radiation from the Schwarzschild black hole
of which mass is that of the growing mass at each moment. Hence, the energy
loss rate decreases and tends to vanish as time proceeds. Consequently, the
energy loss due to evaporation is insignificant compared to huge mass accretion
onto the black hole. Based on this model, we propose a definition of
quasi-equilibrium temperature for general conformal stationary black holes.Comment: Accepted for publication in Class.Quant.Grav., 18 pages and 3 figure
Gyratons on Melvin spacetime
We present and analyze new exact gyraton solutions of algebraic type II on a
background which is static, cylindrically symmetric Melvin universe of type D.
For a vanishing electromagnetic field it reduces to previously studied gyratons
on Minkowski background. We demonstrate that the solutions are member of a more
general family of the Kundt spacetimes. We show that the Einstein equations
reduce to a set of mostly linear equations on a transverse 2-space and we
discuss the properties of polynomial scalar curvature invariants which are
generally non-constant but unaffected by the presence of gyratons.Comment: 15 pages, no figures, journal version extended by appendices B and
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