2,318 research outputs found
Classical GR as a topological theory with linear constraints
We investigate a formulation of continuum 4d gravity in terms of a
constrained topological (BF) theory, in the spirit of the Plebanski
formulation, but involving only linear constraints, of the type used recently
in the spin foam approach to quantum gravity. We identify both the continuum
version of the linear simplicity constraints used in the quantum discrete
context and a linear version of the quadratic volume constraints that are
necessary to complete the reduction from the topological theory to gravity. We
illustrate and discuss also the discrete counterpart of the same continuum
linear constraints. Moreover, we show under which additional conditions the
discrete volume constraints follow from the simplicity constraints, thus
playing the role of secondary constraints. Our analysis clarifies how the
discrete constructions of spin foam models are related to a continuum theory
with an action principle that is equivalent to general relativity.Comment: 4 pages, based on a talk given at the Spanish Relativity Meeting 2010
(ERE2010, Granada, Spain
Classical GR as a topological theory with linear constraints
We investigate a formulation of continuum 4d gravity in terms of a
constrained topological (BF) theory, in the spirit of the Plebanski
formulation, but involving only linear constraints, of the type used recently
in the spin foam approach to quantum gravity. We identify both the continuum
version of the linear simplicity constraints used in the quantum discrete
context and a linear version of the quadratic volume constraints that are
necessary to complete the reduction from the topological theory to gravity. We
illustrate and discuss also the discrete counterpart of the same continuum
linear constraints. Moreover, we show under which additional conditions the
discrete volume constraints follow from the simplicity constraints, thus
playing the role of secondary constraints. Our analysis clarifies how the
discrete constructions of spin foam models are related to a continuum theory
with an action principle that is equivalent to general relativity.Comment: 4 pages, based on a talk given at the Spanish Relativity Meeting 2010
(ERE2010, Granada, Spain
Classical GR as a topological theory with linear constraints
We investigate a formulation of continuum 4d gravity in terms of a
constrained topological (BF) theory, in the spirit of the Plebanski
formulation, but involving only linear constraints, of the type used recently
in the spin foam approach to quantum gravity. We identify both the continuum
version of the linear simplicity constraints used in the quantum discrete
context and a linear version of the quadratic volume constraints that are
necessary to complete the reduction from the topological theory to gravity. We
illustrate and discuss also the discrete counterpart of the same continuum
linear constraints. Moreover, we show under which additional conditions the
discrete volume constraints follow from the simplicity constraints, thus
playing the role of secondary constraints. Our analysis clarifies how the
discrete constructions of spin foam models are related to a continuum theory
with an action principle that is equivalent to general relativity.Comment: 4 pages, based on a talk given at the Spanish Relativity Meeting 2010
(ERE2010, Granada, Spain
Chaotic quantum ratchets and filters with cold atoms in optical lattices: properties of Floquet states
Recently, cesium atoms in optical lattices subjected to cycles of
unequally-spaced pulses have been found to show interesting behavior: they
represent the first experimental demonstration of a Hamiltonian ratchet
mechanism, and they show strong variability of the Dynamical Localization
lengths as a function of initial momentum. The behavior differs qualitatively
from corresponding atomic systems pulsed with equal periods, which are a
textbook implementation of a well-studied quantum chaos paradigm, the quantum
delta-kicked particle (delta-QKP). We investigate here the properties of the
corresponding eigenstates (Floquet states) in the parameter regime of the new
experiments and compare them with those of the eigenstates of the delta-QKP at
similar kicking strengths. We show that, with the properties of the Floquet
states, we can shed light on the form of the observed ratchet current as well
as variations in the Dynamical Localization length.Comment: 9 pages, 9 figure
Lamm, Valluri, Jentschura and Weniger comment on "A Convergent Series for the QED Effective Action" by Cho and Pak [Phys. Rev. Lett. vol. 86, pp. 1947-1950 (2001)]
Complete results were obtained by us in [Can. J. Phys. 71, 389 (1993)] for
convergent series representations of both the real and the imaginary part of
the QED effective action; these derivations were based on correct intermediate
steps. In this comment, we argue that the physical significance of the
"logarithmic correction term" found by Cho and Pak in [Phys. Rev. Lett. 86,
1947 (2001)] in comparison to the usual expression for the QED effective action
remains to be demonstrated. Further information on related subjects can be
found in Appendix A of hep-ph/0308223 and in hep-th/0210240.Comment: 1 page, RevTeX; only "meta-data" update
QED effective action at finite temperature
The QED effective Lagrangian in the presence of an arbitrary constant
electromagnetic background field at finite temperature is derived in the
imaginary-time formalism to one-loop order. The boundary conditions in
imaginary time reduce the set of gauge transformations of the background field,
which allows for a further gauge invariant and puts restrictions on the choice
of gauge. The additional invariant enters the effective action by a topological
mechanism and can be identified with a chemical potential; it is furthermore
related to Debye screening. In concordance with the real-time formalism, we do
not find a thermal correction to Schwinger's pair-production formula. The
calculation is performed on a maximally Lorentz covariant and gauge invariant
stage.Comment: 9 pages, REVTeX, 1 figure, typos corrected, references added, final
version to appear in Phys. Rev.
From the discrete to the continuous - towards a cylindrically consistent dynamics
Discrete models usually represent approximations to continuum physics.
Cylindrical consistency provides a framework in which discretizations mirror
exactly the continuum limit. Being a standard tool for the kinematics of loop
quantum gravity we propose a coarse graining procedure that aims at
constructing a cylindrically consistent dynamics in the form of transition
amplitudes and Hamilton's principal functions. The coarse graining procedure,
which is motivated by tensor network renormalization methods, provides a
systematic approximation scheme towards this end. A crucial role in this coarse
graining scheme is played by embedding maps that allow the interpretation of
discrete boundary data as continuum configurations. These embedding maps should
be selected according to the dynamics of the system, as a choice of embedding
maps will determine a truncation of the renormalization flow.Comment: 22 page
Scattering through a straight quantum waveguide with combined boundary conditions
Scattering through a straight two-dimensional quantum waveguide Rx(0,d) with
Dirichlet boundary conditions on (-\infty,0)x{y=0} \cup (0,\infty)x{y=d} and
Neumann boundary condition on (-infty,0)x{y=d} \cup (0,\infty)x{y=0} is
considered using stationary scattering theory. The existence of a matching
conditions solution at x=0 is proved. The use of stationary scattering theory
is justified showing its relation to the wave packets motion. As an
illustration, the matching conditions are also solved numerically and the
transition probabilities are shown.Comment: 26 pages, 3 figure
Monitoring induced gene expression of single cells in a multilayer microchip
We present a microfluidic system that facilitates long-term measurements of single cell response to external stimuli. The difficulty of addressing cells individually was overcome by using a two-layer microfluidic device. The top layer is designed for trapping and culturing of cells while the bottom layer is employed for supplying chemical compounds that can be transported towards the cells in defined concentrations and temporal sequences. A porous polyester membrane that supports transport and diffusion of compounds from below separates the microchannels of both layers. The performance and potential of the device are demonstrated using human embryonic kidney cells (HEK293) transfected with an inducible gene expression system. Expression of a fluorescent protein (ZsGreen1-DR) is observed while varying the concentration and exposure time of the inducer tetracycline. The study reveals the heterogeneous response of the cells as well as average responses of tens of cells that are analyzed in parallel. The microfluidic platform enables systematic studies under defined conditions and is a valuable tool for general single cell studies to obtain insights into mechanisms and kinetics that are not accessible by conventional macroscopic methods. Figure A two-layer microfluidic device is presented that facilitates measurements of single cell response to external stimul
Energy-momentum tensor in thermal strong-field QED with unstable vacuum
The mean value of the one-loop energy-momentum tensor in thermal QED with
electric-like background that creates particles from vacuum is calculated. The
problem differes essentially from calculations of effective actions (similar to
that of Heisenberg--Euler) in backgrounds that do not violate the stability of
vacuum. The role of a constant electric background in the violation of both the
stability of vacuum and the thermal character of particle distribution is
investigated. Restrictions on the electric field and its duration under which
one can neglect the back-reaction of created particles are established.Comment: 7 pages, Talk presented at Workshop "Quantum Field Theory under the
Influence of External Conditions", Leipzig, September 17-21, 2007;
introduction extended, version accepted for publication in J.Phys.
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