3,332 research outputs found
Non-equilibrium Thermodynamics of Spacetime: the Role of Gravitational Dissipation
In arXiv:gr-qc/9504004 it was shown that the Einstein equation can be derived
as a local constitutive equation for an equilibrium spacetime thermodynamics.
More recently, in the attempt to extend the same approach to the case of
theories of gravity, it was found that a non-equilibrium setting is indeed
required in order to fully describe both this theory as well as classical GR
(arXiv:gr-qc/0602001). Here, elaborating on this point, we show that the
dissipative character leading to a non-equilibrium spacetime thermodynamics is
actually related -- both in GR as well as in gravity -- to non-local
heat fluxes associated with the purely gravitational/internal degrees of
freedom of the theory. In particular, in the case of GR we show that the
internal entropy production term is identical to the so called tidal heating
term of Hartle-Hawking. Similarly, for the case of gravity, we show that
dissipative effects can be associated with the generalization of this term plus
a scalar contribution whose presence is clearly justified within the
scalar-tensor representation of the theory. Finally, we show that the allowed
gravitational degrees of freedom can be fixed by the kinematics of the local
spacetime causal structure, through the specific Equivalence Principle
formulation. In this sense, the thermodynamical description seems to go beyond
Einstein's theory as an intrinsic property of gravitation.Comment: 13 pages, 1 figur
Analogue model for quantum gravity phenomenology
So called "analogue models" use condensed matter systems (typically
hydrodynamic) to set up an "effective metric" and to model curved-space quantum
field theory in a physical system where all the microscopic degrees of freedom
are well understood. Known analogue models typically lead to massless minimally
coupled scalar fields. We present an extended "analogue space-time" programme
by investigating a condensed-matter system - in and beyond the hydrodynamic
limit - that is in principle capable of simulating the massive Klein-Gordon
equation in curved spacetime. Since many elementary particles have mass, this
is an essential step in building realistic analogue models, and an essential
first step towards simulating quantum gravity phenomenology. Specifically, we
consider the class of two-component BECs subject to laser-induced transitions
between the components, and we show that this model is an example for Lorentz
invariance violation due to ultraviolet physics. Furthermore our model suggests
constraints on quantum gravity phenomenology in terms of the "naturalness
problem" and "universality issue".Comment: Talk given at 7th Workshop on Quantum Field Theory Under the
Influence of External Conditions (QFEXT 05), Barcelona, Catalonia, Spain, 5-9
Sep 200
Threshold effects and Planck scale Lorentz violation: combined constraints from high energy astrophysics
Recent work has shown that dispersion relations with Planck scale Lorentz
violation can produce observable effects at energies many orders of magnitude
below the Planck energy M. This opens a window on physics that may reveal
quantum gravity phenomena. It has already constrained the possibility of Planck
scale Lorentz violation, which is suggested by some approaches to quantum
gravity. In this work we carry out a systematic analysis of reaction
thresholds, allowing unequal deformation parameters for different particle
dispersion relations. The thresholds are found to have some unusual properties
compared with standard ones, such as asymmetric momenta for pair creation and
upper thresholds. The results are used together with high energy observational
data to determine combined constraints. We focus on the case of photons and
electrons, using vacuum Cerenkov, photon decay, and photon annihilation
processes to determine order unity constraints on the parameters controlling
O(E/M) Lorentz violation. Interesting constraints for protons (with photons or
pions) are obtained even at O((E/M)^2), using the absence of vacuum Cerenkov
and the observed GZK cutoff for ultra high energy cosmic rays. A strong
Cerenkov limit using atmospheric PeV neutrinos is possible for O(E/M)
deformations provided the rate is high enough. If detected, ultra high energy
cosmological neutrinos might yield limits at or even beyond O((E/M)^2).Comment: 35 pages, 13 Figures, RevTex4. Version published in PRD. Expanded
introduction, updated discussion of possible constraint if GZK cutoff is
confirmed. Corrected typos. Added and updated reference
High energy constraints on Lorentz symmetry violations
Lorentz violation at high energies might lead to non linear dispersion
relations for the fundamental particles. We analyze observational constraints
on these without assuming any a priori equality between the coefficients
determining the amount of Lorentz violation for different particle species. We
focus on constraints from three high energy processes involving photons and
electrons: photon decay, photo-production of electron-positron pairs, and
vacuum Cerenkov radiation. We find that cubic momentum terms in the dispersion
relations are strongly constrained.Comment: 7 pages, 1 figure, Talk presented at CPT01; the Second Meeting on CPT
and Lorentz Symmetry, Bloomington, Indiana, 15-18 Aug. 2001. Minor numerical
error corrected, gamma-decay constraint update
Sonoluminescence as a QED vacuum effect: Probing Schwinger's proposal
Several years ago Schwinger proposed a physical mechanism for
sonoluminescence in terms of photon production due to changes in the properties
of the quantum-electrodynamic (QED) vacuum arising from a collapsing dielectric
bubble. This mechanism can be re-phrased in terms of the Casimir effect and has
recently been the subject of considerable controversy. The present paper probes
Schwinger's suggestion in detail: Using the sudden approximation we calculate
Bogolubov coefficients relating the QED vacuum in the presence of the expanded
bubble to that in the presence of the collapsed bubble. In this way we derive
an estimate for the spectrum and total energy emitted. We verify that in the
sudden approximation there is an efficient production of photons, and further
that the main contribution to this dynamic Casimir effect comes from a volume
term, as per Schwinger's original calculation. However, we also demonstrate
that the timescales required to implement Schwinger's original suggestion are
not physically relevant to sonoluminescence. Although Schwinger was correct in
his assertion that changes in the zero-point energy lead to photon production,
nevertheless his original model is not appropriate for sonoluminescence. In
other works (see quant-ph/9805023, quant-ph/9904013, quant-ph/9904018,
quant-ph/9905034) we have developed a variant of Schwinger's model that is
compatible with the physically required timescales.Comment: 18 pages, ReV_TeX 3.2, 9 figures. Major revisions: This document is
now limited to providing a probe of Schwinger's original suggestion for
sonoluminescence. For details on our own variant of Schwinger's ideas see
quant-ph/9805023, quant-ph/9904013, quant-ph/9904018, quant-ph/990503
Lorentz symmetry breaking: phenomenology and constraints
In this talk I shall review several motivations for considering departures from exact Lorentz invariance and the different theoretical frameworks adopted to describe these departures. Among these, I shall focus on an effective field theory approach and discuss the phenomenology and constraints of Lorentz symmetry breaking in the Standard Model as well as in Gravity. In particular I will focus on current constraints on UV breaking inspired by quantum gravity scenarios and briefly discuss the open issues and future perspectives for this field of research
Sonoluminescence: Two-photon correlations as a test of thermality
In this Letter we propose a fundamental test for probing the thermal nature
of the spectrum emitted by sonoluminescence. We show that two-photon
correlations can in principle discriminate between real thermal light and the
quasi-thermal squeezed-state photons typical of models based on the dynamic
Casimir effect. Two-photon correlations provide a powerful experimental test
for various classes of sonoluminescence models.Comment: 6 pages, revtex 3; revised to include more discussion of finite
volume effects; physics conclusions unchanged; to appear in Physics Letters
Vacuum Effects in Gravitational Fields: Theory and Detectability
In this thesis, we investigate quantum vacuum effects in the presence of
gravitational fields. After discussing the general theory of vacuum effects in
strong fields we apply it to the relevant issue of the interaction of the
quantum vacuum with black hole geometries. In particular we consider the
long-standing problem of the interpretation of gravitational entropy. After
these investigations, we discuss the possible experimental tests of particle
creation from the quantum vacuum. This leads us to study acoustic geometries
and their way of ``simulating'' gravitational structures, such as horizons and
black holes. We study the stability of these structures and the problems
related to setting up experimental detection of ``phonon Hawking flux'' from
acoustic horizons. This line of research then leads us to propose a new model
for explaining the emission of light in the phenomenon of Sonoluminescence,
based on the dynamical Casimir effect. This is possibly amenable to
experimental investigation. Finally we consider high energy phenomena in the
early universe. In particular we discuss inflation and possible alternative
frameworks for solving the cosmological puzzles.Comment: Latex2e, 237 pages, 36 figures. Ph.D. Thesi
Sonoluminescence and the QED vacuum
In this talk I shall describe an extension of the quantum-vacuum approach to
sonoluminescence proposed several years ago by J.Schwinger. We shall first
consider a model calculation based on Bogolubov coefficients relating the QED
vacuum in the presence of an expanded bubble to that in the presence of a
collapsed bubble. In this way we shall derive an estimate for the spectrum and
total energy emitted. This latter will be shown to be proportional to the
volume of space over which the refractive index changes, as Schwinger
predicted. After this preliminary check we shall deal with the physical
constraints that any viable dynamical model for SL has to satisfy in order to
fit the experimental data. We shall emphasize the importance of the timescale
of the change in refractive index. This discussion will led us to propose a
somewhat different version of dynamical Casimir effect in which the change in
volume of the bubble is no longer the only source for the change in the
refractive index.Comment: 15 pages, 1 figure, uses sprocl.sty. Talk at the 4th Workshop on
Quantum Field Theory Under the Influence of External Conditions, Leipzig,
14-18 September, 199
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