43,168 research outputs found
String Theory : Where are we now?
This is a brief overview on the current status of string theory for
non-specialists. The purpose is to give an aspect on the nature of string
theory as a unified theory of all interactions including quantum gravity and to
discuss future perspectives. Particular emphases are put on the mysteries why
string theory contains gravity and why it resolves the ultraviolet problems.Comment: 19 pages, 1 figure, written version of a general talk in the workshop
"Frontier of Theoretical Physics", Beijing, Nov, 1999, minor typos correcte
Alchemical normal modes unify chemical space
In silico design of new molecules and materials with desirable quantum
properties by high-throughput screening is a major challenge due to the high
dimensionality of chemical space. To facilitate its navigation, we present a
unification of coordinate and composition space in terms of alchemical normal
modes (ANMs) which result from second order perturbation theory. ANMs assume a
predominantly smooth nature of chemical space and form a basis in which new
compounds can be expanded and identified. We showcase the use of ANMs for the
energetics of the iso-electronic series of diatomics with 14 electrons, BN
doped benzene derivatives (C(BN)H with ),
predictions for over 1.8 million BN doped coronene derivatives, and genetic
energy optimizations in the entire BN doped coronene space. Using Ge lattice
scans as reference, the applicability ANMs across the periodic table is
demonstrated for III-V and IV-IV-semiconductors Si, Sn, SiGe, SnGe, SiSn, as
well as AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, and InSb. Analysis of our
results indicates simple qualitative structure property rules for estimating
energetic rankings among isomers. Useful quantitative estimates can also be
obtained when few atoms are changed to neighboring or lower lying elements in
the periodic table. The quality of the predictions often increases with the
symmetry of system chosen as reference due to cancellation of odd order terms.
Rooted in perturbation theory the ANM approach promises to generally enable
unbiased compound exploration campaigns at reduced computational cost
Steepest Entropy Ascent Model for Far-Non-Equilibrium Thermodynamics. Unified Implementation of the Maximum Entropy Production Principle
By suitable reformulations, we cast the mathematical frameworks of several
well-known different approaches to the description of non-equilibrium dynamics
into a unified formulation, which extends to such frameworks the concept of
Steepest Entropy Ascent (SEA) dynamics introduced by the present author in
previous works on quantum thermodynamics. The present formulation constitutes a
generalization also for the quantum thermodynamics framework. In the SEA
modeling principle a key role is played by the geometrical metric with respect
to which to measure the length of a trajectory in state space. In the near
equilibrium limit, the metric tensor is related to the Onsager's generalized
resistivity tensor. Therefore, through the identification of a suitable metric
field which generalizes the Onsager generalized resistance to the arbitrarily
far non-equilibrium domain, most of the existing theories of non-equilibrium
thermodynamics can be cast in such a way that the state exhibits a spontaneous
tendency to evolve in state space along the path of SEA compatible with the
conservation constraints and the boundary conditions. The resulting unified
family of SEA dynamical models is intrinsically and strongly consistent with
the second law of thermodynamics. Non-negativity of the entropy production is a
readily proved general feature of SEA dynamics. In several of the different
approaches to non-equilibrium description we consider here, the SEA concept has
not been investigated before. We believe it defines the precise meaning and the
domain of general validity of the so-called Maximum Entropy Production
Principle. It is hoped that the present unifying approach may prove useful in
providing a fresh basis for effective, thermodynamically consistent, numerical
models and theoretical treatments of irreversible conservative relaxation
towards equilibrium from far non-equilibrium states.Comment: 15 pages, 4 figures, to appear in Physical Review
Reducing "Structure From Motion": a General Framework for Dynamic Vision - Part 1: Modeling
The literature on recursive estimation of structure and motion from monocular image sequences comprises a large number of different models and estimation techniques. We propose a framework that allows us to derive and compare all models by following the idea of dynamical system reduction.
The "natural" dynamic model, derived by the rigidity constraint and the perspective projection, is first reduced by explicitly decoupling structure (depth) from motion. Then implicit decoupling techniques are explored, which consist of imposing that some function of the unknown parameters is held constant. By appropriately choosing such a function, not only can we account for all models seen so far in the literature, but we can also derive novel ones
Differential geometric regularization for supervised learning of classifiers
We study the problem of supervised learning for both binary and multiclass classification from a unified geometric perspective. In particular, we propose a geometric regularization technique to find the submanifold corresponding to an estimator of the class probability P(y|\vec x). The regularization term measures the volume of this submanifold, based on the intuition that overfitting produces rapid local oscillations and hence large volume of the estimator. This technique can be applied to regularize any classification function that satisfies two requirements: firstly, an estimator of the class probability can be obtained; secondly, first and second derivatives of the class probability estimator can be calculated. In experiments, we apply our regularization technique to standard loss functions for classification, our RBF-based implementation compares favorably to widely used regularization methods for both binary and multiclass classification.http://proceedings.mlr.press/v48/baia16.pdfPublished versio
A general theory of linear cosmological perturbations: stability conditions, the quasistatic limit and dynamics
We analyse cosmological perturbations around a homogeneous and isotropic
background for scalar-tensor, vector-tensor and bimetric theories of gravity.
Building on previous results, we propose a unified view of the effective
parameters of all these theories. Based on this structure, we explore the
viable space of parameters for each family of models by imposing the absence of
ghosts and gradient instabilities. We then focus on the quasistatic regime and
confirm that all these theories can be approximated by the phenomenological
two-parameter model described by an effective Newton's constant and the
gravitational slip. Within the quasistatic regime we pinpoint signatures which
can distinguish between the broad classes of models (scalar-tensor,
vector-tensor or bimetric). Finally, we present the equations of motion for our
unified approach in such a way that they can be implemented in
Einstein-Boltzmann solvers
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