152 research outputs found
No time machines in classical general relativity
Irrespective of local conditions imposed on the metric, any extendible
spacetime U has a maximal extension containing no closed causal curves outside
the chronological past of U. We prove this fact and interpret it as
impossibility (in classical general relativity) of the time machines, insofar
as the latter are defined to be causality-violating regions created by human
beings (as opposed to those appearing spontaneously).Comment: A corrigendum (to be published in CQG) has been added to correct an
important mistake in the definition of localit
Group Chase and Escape
We describe here a new concept of one group chasing another, called "group
chase and escape", by presenting a simple model. We will show that even a
simple model can demonstrate rather rich and complex behavior. In particular,
there are cases in which an optimal number of chasers exists for a given number
of escapees (or targets) to minimize the cost of catching all targets. We have
also found an indication of self-organized spatial structures formed by both
groups.Comment: 13 pages, 12 figures, accepted and to appear in New Journal of
Physic
The Bond-Algebraic Approach to Dualities
An algebraic theory of dualities is developed based on the notion of bond
algebras. It deals with classical and quantum dualities in a unified fashion
explaining the precise connection between quantum dualities and the low
temperature (strong-coupling)/high temperature (weak-coupling) dualities of
classical statistical mechanics (or (Euclidean) path integrals). Its range of
applications includes discrete lattice, continuum field, and gauge theories.
Dualities are revealed to be local, structure-preserving mappings between
model-specific bond algebras that can be implemented as unitary
transformations, or partial isometries if gauge symmetries are involved. This
characterization permits to search systematically for dualities and
self-dualities in quantum models of arbitrary system size, dimensionality and
complexity, and any classical model admitting a transfer matrix representation.
Dualities like exact dimensional reduction, emergent, and gauge-reducing
dualities that solve gauge constraints can be easily understood in terms of
mappings of bond algebras. As a new example, we show that the (\mathbb{Z}_2)
Higgs model is dual to the extended toric code model {\it in any number of
dimensions}. Non-local dual variables and Jordan-Wigner dictionaries are
derived from the local mappings of bond algebras. Our bond-algebraic approach
goes beyond the standard approach to classical dualities, and could help
resolve the long standing problem of obtaining duality transformations for
lattice non-Abelian models. As an illustration, we present new dualities in any
spatial dimension for the quantum Heisenberg model. Finally, we discuss various
applications including location of phase boundaries, spectral behavior and,
notably, we show how bond-algebraic dualities help constrain and realize
fermionization in an arbitrary number of spatial dimensions.Comment: 131 pages, 22 figures. Submitted to Advances in Physics. Second
version including a new section on the eight-vertex model and the correction
of several typo
Godel-type Universes in String-inspired Charged Gravity
We consider a string-inspired, gravitational theory of scalar and
electromagnetic fields and we investigate the existence of axially-symmetric,
G\"{o}del-type cosmological solutions. The neutral case is studied first and an
"extreme" G\"{o}del-type rotating solution, that respects the causality, is
determined. The charged case is considered next and two new configurations for
the, minimally-coupled to gravity, electromagnetic field are presented. Another
configuration motivated by the expected distribution of currents and charges in
a rotating universe is studied and shown to lead to a G\"{o}del-type solution
for a space-dependent coupling function. Finally, we investigate the existence
of G\"{o}del-type cosmological solutions in the framework of the one-loop
corrected superstring effective action and we determine the sole configuration
of the electromagnetic field that leads to such a solution. It turns out that,
in all the charged cases considered, Closed Timelike Curves do appear and the
causality is always violated.Comment: 26 pages, LaTex file, a few comments and references added, version to
appear in Physical Review
Association between hemodynamic activity and motor performance in six-month-old full-term and preterm infants: a functional near-infrared spectroscopy study
FAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOThis study aimed to assess task-induced activation in motor cortex and its association with motor performance in full-term and preterm born infants at six months old. A cross-sectional study of 73 sixmonth- old infants was conducted (35 full-term and 38 preterm infants). Motor performance was assessed using the Bayley Scales of Infant Development third edition-Bayley-III. Brain hemodynamic activity during motor task was measured by functional near-infrared spectroscopy (fNIRS). Motor performance was similar in full-term and preterm infants. However, differences in hemodynamic response were identified. Full terms showed a more homogeneous unilateral and contralateral activated area, whereas in preterm-born the activation response was predominantly bilateral. The full-term group also exhibited a shorter latency for the hemodynamic response than the preterm group. Hemodynamic activity in the left sensorimotor region was positively associated with motor performance measured by Bayley-III. The results highlight the adequacy of fNIRS to assess differences in task-induced activation in sensorimotor cortex between groups. The association between motor performance and the hemodynamic activity require further investigation and suggest that fNIRS can become a suitable auxiliary tool to investigate aspects of neural basis on early development of motor abilities.5118FAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOFAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO215502012/02500-82013/07559-
Time as an operator/observable in nonrelativistic quantum mechanics
The nonrelativistic Schroedinger equation for motion of a structureless
particle in four-dimensional space-time entails a well-known expression for the
conserved four-vector field of local probability density and current that are
associated with a quantum state solution to the equation. Under the physical
assumption that each spatial, as well as the temporal, component of this
current is observable, the position in time becomes an operator and an
observable in that the weighted average value of the time of the particle's
crossing of a complete hyperplane can be simply defined: ... When the
space-time coordinates are (t,x,y,z), the paper analyzes in detail the case
that the hyperplane is of the type z=constant. Particles can cross such a
hyperplane in either direction, so it proves convenient to introduce an
indefinite metric, and correspondingly a sesquilinear inner product with
non-Hilbert space structure, for the space of quantum states on such a surface.
>... A detailed formalism for computing average crossing times on a z=constant
hyperplane, and average dwell times and delay times for a zone of interaction
between a pair of z=constant hyperplanes, is presented.Comment: 31 pages, no figures. Differs from published version by minor
corrections and additions, and two citation
Schroedinger equation for joint bidirectional motion in time
The conventional, time-dependent Schroedinger equation describes only
unidirectional time evolution of the state of a physical system, i.e., forward
or, less commonly, backward. This paper proposes a generalized quantum dynamics
for the description of joint, and interactive, forward and backward time
evolution within a physical system. [...] Three applications are studied: (1) a
formal theory of collisions in terms of perturbation theory; (2) a
relativistically invariant quantum field theory for a system that kinematically
comprises the direct sum of two quantized real scalar fields, such that one
field evolves forward and the other backward in time, and such that there is
dynamical coupling between the subfields; (3) an argument that in the latter
field theory, the dynamics predicts that in a range of values of the coupling
constants, the expectation value of the vacuum energy of the universe is forced
to be zero to high accuracy. [...]Comment: 30 pages, no figures. Related material is in quant-ph/0404012.
Differs from published version by a few added remarks on the possibility of a
large-scale-average negative energy density in spac
The Nonlinear Stability of the Trivial Solution to the Maxwell-Born-Infeld System
In this article, we use an electromagnetic gauge-free framework to establish
the existence of small-data global solutions to the Maxwell-Born-Infeld (MBI)
system on the Minkowski space background in 1 + 3 dimensions. Because the
nonlinearities in the system satisfy a version of the null condition, we are
also able to show that these solutions decay at exactly the same rates as
solutions to the linear Maxwell-Maxwell system. In addition, we show that on
any Lorentzian manifold, the MBI system is hyperbolic in the interior of the
field-strength regime in which its Lagrangian is real-valued.Comment: A few additional comments and some references were added. Some typos
were corrected. 73 page
Dynamics and stability of the Godel universe
We use covariant techniques to describe the properties of the Godel universe
and then consider its linear response to a variety of perturbations. Against
matter aggregations, we find that the stability of the Godel model depends
primarily upon the presence of gradients in the centrifugal energy, and
secondarily on the equation of state of the fluid. The latter dictates the
behaviour of the model when dealing with homogeneous perturbations. The
vorticity of the perturbed Godel model is found to evolve as in almost-FRW
spacetimes, with some additional directional effects due to shape distortions.
We also consider gravitational-wave perturbations by investigating the
evolution of the magnetic Weyl component. This tensor obeys a simple plane-wave
equation, which argues for the neutral stability of the Godel model against
linear gravity-wave distortions. The implications of the background rotation
for scalar-field Godel cosmologies are also discussed.Comment: Revised version, to match paper published in Class. Quantum Gra
Unsuitability of the moving light clock system for the Lorentz factor derivation
The moving light clock system was analyzed with respect to the orientation of
the wavefront of the light pulse observed in the moving and stationary frames
of reference. The plane wavefront of the light pulse was oriented horizontally
in both the frames. The wavefront observed in the stationary frame was not
perpendicular to the direction of the light pulse propagation. This showed
different characteristics of the light pulse than that assumed in the Lorentz
factor derivation. According to the horizontal orientation of the wavefront,
velocity c was determined as the vertical component of the light pulse motion
observed in the stationary frame. Application of this velocity distribution in
the Lorentz factor derivation showed the same travel time for the light pulse
observed in the moving and stationary frames of reference. The moving light
clock system was therefore found to be unsuitable for the Lorentz factor
derivation and illustration of time dilation, and shown to illustrate the
relativity of the observation of light rather than the relativity of time.Comment: 4 pages, 5 figure
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