20,294 research outputs found
Dynamical Formation of Horizons in Recoiling D Branes
A toy calculation of string/D-particle interactions within a world-sheet
approach indicates that quantum recoil effects - reflecting the gravitational
back-reaction on space-time foam due to the propagation of energetic particles
- induces the appearance of a microscopic event horizon, or `bubble', inside
which stable matter can exist. The scattering event causes this horizon to
expand, but we expect quantum effects to cause it to contract again, in a
`bounce' solution. Within such `bubbles', massless matter propagates with an
effective velocity that is less than the velocity of light in vacuo, which may
lead to observable violations of Lorentz symmetry that may be tested
experimentally. The conformal invariance conditions in the interior geometry of
the bubbles select preferentially three for the number of the spatial
dimensions, corresponding to a consistent formulation of the interaction of D3
branes with recoiling D particles, which are allowed to fluctuate independently
only on the D3-brane hypersurface.Comment: 25 pages LaTeX, 4 eps figures include
The status of traditional Scottish animal breeds and plant varieties and the implications for biodiversity
The aim of this scoping study was to evaluate the effects on Scottish biodiversity of
changes in the use of traditional breeds and varieties. The overall objectives were:
a) The evaluation of the importance of genetic loss from the reduction in use of these
breeds and varieties, for example, the loss of unusual characteristics that might have
been of particular local use.
b) An assessment of the impacts of reduction in the ability to conduct further breeding or
research on rare and traditional varieties and breeds.
c) Identification of the loss of certain farming techniques associated with particular
varieties and breeds.
d) An assessment of possible losses of biodiversity associated with reduction in the use of
these breeds and varieties and the farming systems associated with them
Test of the Additivity Principle for Current Fluctuations in a Model of Heat Conduction
The additivity principle allows to compute the current distribution in many
one-dimensional (1D) nonequilibrium systems. Using simulations, we confirm this
conjecture in the 1D Kipnis-Marchioro-Presutti model of heat conduction for a
wide current interval. The current distribution shows both Gaussian and
non-Gaussian regimes, and obeys the Gallavotti-Cohen fluctuation theorem. We
verify the existence of a well-defined temperature profile associated to a
given current fluctuation. This profile is independent of the sign of the
current, and this symmetry extends to higher-order profiles and spatial
correlations. We also show that finite-time joint fluctuations of the current
and the profile are described by the additivity functional. These results
suggest the additivity hypothesis as a general and powerful tool to compute
current distributions in many nonequilibrium systems.Comment: 4 pages, 4 figure
Space-Time Foam may Violate the Principle of Equivalence
The interactions of different particle species with the foamy space-time
fluctuations expected in quantum gravity theories may not be universal, in
which case different types of energetic particles may violate Lorentz
invariance by varying amounts, violating the equivalence principle. We
illustrate this possibility in two different models of space-time foam based on
D-particle fluctuations in either flat Minkowski space or a stack of
intersecting D-branes. Both models suggest that Lorentz invariance could be
violated for energetic particles that do not carry conserved charges, such as
photons, whereas charged particles such electrons would propagate in a
Lorentz-inavariant way. The D-brane model further suggests that gluon
propagation might violate Lorentz invariance, but not neutrinos. We argue that
these conclusions hold at both the tree (lowest-genus) and loop (higher-genus)
levels, and discuss their implications for the phenomenology of quantum
gravity.Comment: 20 pages, 4 figures, the version accepted for publication in the
International Journal of Modern Physics
The Theory of the Nucleon Spin
I discuss two topics of current interest in the study of the spin structure
of the nucleon. First, I discuss whether there is a sum rule for the components
of the nucleon's angular moments. Second, I discuss the measurement of the
nucleon's transversity distribution in light of recent results reported by the
HERMES collaboration at DESY.Comment: 15 pages, 8 figures, LaTeX using rspublic.cls and BoxedEPS macros; as
submitted to Phil Trans A of the Royal Society for forthcoming volume: The
Quark Structure of Matter; email correspondence to [email protected]
Quantum-Gravitational Diffusion and Stochastic Fluctuations in the Velocity of Light
We argue that quantum-gravitational fluctuations in the space-time background
give the vacuum non-trivial optical properties that include diffusion and
consequent uncertainties in the arrival times of photons, causing stochastic
fluctuations in the velocity of light ``in vacuo''. Our proposal is motivated
within a Liouville string formulation of quantum gravity that also suggests a
frequency-dependent refractive index of the particle vacuum. We construct an
explicit realization by treating photon propagation through quantum excitations
of -brane fluctuations in the space-time foam. These are described by
higher-genus string effects, that lead to stochastic fluctuations in couplings,
and hence in the velocity of light. We discuss the possibilities of
constraining or measuring photon diffusion ``in vacuo'' via -ray
observations of distant astrophysical sources.Comment: 17 pages LATEX, uses axodraw style fil
Generalized canonical ensembles and ensemble equivalence
This paper is a companion article to our previous paper (J. Stat. Phys. 119,
1283 (2005), cond-mat/0408681), which introduced a generalized canonical
ensemble obtained by multiplying the usual Boltzmann weight factor of the canonical ensemble with an exponential factor involving a continuous
function of the Hamiltonian . We provide here a simplified introduction
to our previous work, focusing now on a number of physical rather than
mathematical aspects of the generalized canonical ensemble. The main result
discussed is that, for suitable choices of , the generalized canonical
ensemble reproduces, in the thermodynamic limit, all the microcanonical
equilibrium properties of the many-body system represented by even if this
system has a nonconcave microcanonical entropy function. This is something that
in general the standard () canonical ensemble cannot achieve. Thus a
virtue of the generalized canonical ensemble is that it can be made equivalent
to the microcanonical ensemble in cases where the canonical ensemble cannot.
The case of quadratic -functions is discussed in detail; it leads to the
so-called Gaussian ensemble.Comment: 8 pages, 4 figures (best viewed in ps), revtex4. Changes in v2: Title
changed, references updated, new paragraph added, minor differences with
published versio
Prelude to Compressed Baryonic Matter
This is intended to appear as the introduction to "The CBM Physics Book:
compressed baryonic matter in laboratory experiments" (ed. B. Friman, C.
H\"ohne, S. Leupold, J. Knoll, J. Randrup, R. Rapp, P. Senger), to be published
by Springer. At the end there is a new proposal for numerically tractable
models of interacting many-body systems.Comment: 12 pages, to appear in "The CBM Book: compressed baryonic matter in
laboratory experiments
Local freedom in the gravitational field
In a cosmological context, the electric and magnetic parts of the Weyl
tensor, E_{ab} and H_{ab}, represent the locally free curvature - i.e. they are
not pointwise determined by the matter fields. By performing a complete
covariant decomposition of the derivatives of E_{ab} and H_{ab}, we show that
the parts of the derivative of the curvature which are locally free (i.e. not
pointwise determined by the matter via the Bianchi identities) are exactly the
symmetrised trace-free spatial derivatives of E_{ab} and H_{ab} together with
their spatial curls. These parts of the derivatives are shown to be crucial for
the existence of gravitational waves.Comment: New results on gravitational waves included; new references added;
revised version (IOP style) to appear Class. Quantum Gra
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