1,647 research outputs found
Casimir effect in a weak gravitational field and the spacetime index of refraction
In a recent paper [arXiv:0904.2904] using a conjecture it is shown how one
can calculate the effect of a weak stationary gravitational field on vacuum
energy in the context of Casimir effect in an external gravitational field
treated in 1+3 formulation of spacetime decomposition.. In this article,
employing quntum field theory in curved spacetime, we explicitly calculate the
effect of a weak static gravitational field on virtual massless scalar
particles in a Casimir apparatus. It is shown that, as expected from the
proposed conjecture, both the frequency and renormalized energy of the virtual
scalar field are affected by the gravitational field through its index of
refraction. This could be taken as a strong evidence in favour of the proposed
conjecture. Generalizations to weak {\it stationary} spacetimes and virtual
photons are also discussed.Comment: 11 pages, RevTex, typos corrected (combined with arXiv:0904.2904
published in PRD
Recommended from our members
Simulation, Learning and Control Methods to Improve Robotic Vegetable Harvesting
Agricultural robots are subject to a much harsher envi- ronment than those in the factory or lab and control strategies need to take this into account while maintaining a low cycle time. Three control strategies were tested on Vegebot, a lettuce-picking robot, in both simulation and on the real robot. Between a fast open loop that was vulnerable to environmental noise and a slow but robust visual servoing technique, a Learned Open Loop strategy was tested where the robot learned from successful picks to pick at an intermediate speed. This reduced the projected cycle time from 31s to 17.2s, a 45% reduction.This project was possible thanks to EPSRC Grant EP/L015889/1, the Royal Society ERA Foundation Translation Award (TA160113), EPSRC Doctoral Training Program ICASE AwardRG84492 (cofunded by G’s Growers), EPSRC Small Partnership AwardRG86264 (in collaboration with G’s Growers), and the BBSRC Small Partnership GrantRG81275. Special thanks to G Growers, George Walker and Josie Hughes for their invaluable assistance
Propagating, evanescent, and localized states in carbon nanotube-graphene junctions
We study the electronic structure of the junctions between a single graphene
layer and carbon nanotubes, using a tight-binding model and the continuum
theory based on Dirac fermion fields. The latter provides a unified description
of different lattice structures with curvature, which is always localized at
six heptagonal carbon rings around each junction. When these are evenly spaced,
we find that it is possible to curve the planar lattice into armchair (6n,6n)
as well as zig-zag (6n,0) nanotubes. We show that the junctions fall into two
different classes, regarding the low-energy electronic behavior. One of them,
constituted by the junctions made of the armchair nanotubes and the zig-zag
(6n,0) geometries when n is a multiple of 3, is characterized by the presence
of two quasi-bound states at the Fermi level, which are absent for the rest of
the zig-zag nanotubes. These states, localized at the junction, are shown to
arise from the effective gauge flux induced by the heptagonal carbon rings,
which has a direct reflection in the local density of states around the
junction. Furthermore, we also analyze the band structure of the arrays of
junctions, finding out that they can also be classified into two different
groups according to the low-energy behavior. In this regard, the arrays made of
armchair and (6n,0) nanotubes with n equal to a multiple of 3 are characterized
by the presence of a series of flat bands, whose number grows with the length
of the nanotubes. We show that such flat bands have their origin in the
formation of states confined to the nanotubes in the array. This is explained
in the continuum theory from the possibility of forming standing waves in the
mentioned nanotube geometries, as a superposition of modes with opposite
momenta and the same quantum numbers under the C_6v symmetry of the junction.Comment: 13 pages, 8 figure
Vacuum entanglement enhancement by a weak gravitational field
Separate regions in space are generally entangled, even in the vacuum state.
It is known that this entanglement can be swapped to separated Unruh-DeWitt
detectors, i.e., that the vacuum can serve as a source of entanglement. Here,
we demonstrate that, in the presence of curvature, the amount of entanglement
that Unruh-DeWitt detectors can extract from the vacuum can be increased.Comment: 6 pages, 1 figur
Anisotropic higher derivative gravity and inflationary universe
Stability analysis of the Kantowski-Sachs type universe in pure higher
derivative gravity theory is studied in details. The non-redundant generalized
Friedmann equation of the system is derived by introducing a reduced one
dimensional generalized KS type action. This method greatly reduces the labor
in deriving field equations of any complicate models. Existence and stability
of inflationary solution in the presence of higher derivative terms are also
studied in details. Implications to the choice of physical theories are
discussed in details in this paper.Comment: 9 page
The Gauge Fields and Ghosts in Rindler Space
We consider 2d Maxwell system defined on the Rindler space with metric
ds^2=\exp(2a\xi)\cdot(d\eta^2-d\xi^2) with the goal to study the dynamics of
the ghosts. We find an extra contribution to the vacuum energy in comparison
with Minkowski space time with metric ds^2= dt^2-dx^2. This extra contribution
can be traced to the unphysical degrees of freedom (in Minkowski space). The
technical reason for this effect to occur is the property of Bogolubov's
coefficients which mix the positive and negative frequencies modes. The
corresponding mixture can not be avoided because the projections to positive
-frequency modes with respect to Minkowski time t and positive -frequency modes
with respect to the Rindler observer's proper time \eta are not equivalent. The
exact cancellation of unphysical degrees of freedom which is maintained in
Minkowski space can not hold in the Rindler space. In BRST approach this effect
manifests itself as the presence of BRST charge density in L and R parts. An
inertial observer in Minkowski vacuum |0> observes a universe with no net BRST
charge only as a result of cancellation between the two. However, the Rindler
observers who do not ever have access to the entire space time would see a net
BRST charge. In this respect the effect resembles the Unruh effect. The effect
is infrared (IR) in nature, and sensitive to the horizon and/or boundaries. We
interpret the extra energy as the formation of the "ghost condensate" when the
ghost degrees of freedom can not propagate, but nevertheless do contribute to
the vacuum energy. Exact computations in this simple 2d model support the claim
made in [1] that the ghost contribution might be responsible for the observed
dark energy in 4d FLRW universe.Comment: Final version to appear in Phys. Rev. D. Comments on relation with
energy momentum computations and few new refs are adde
Prevalent morphometric vertebral fractures in professional male rugby players
There is an ongoing concern about the risk of injury to the spine in professional rugby players. The objective of this study was to investigate the prevalence of vertebral fracture using vertebral fracture assessment (VFA) dual energy X-ray absorptiometry (DXA) imaging in professional male rugby players. Ninety five professional rugby league (n = 52) and union (n = 43) players (n = 95; age 25.9 (SD 4.3) years; BMI: 29.5 (SD 2.9) kg.m2) participated in the research. Each participant received one VFA, and one total body and lumbar spine DXA scan (GE Lunar iDXA). One hundred and twenty vertebral fractures were identified in over half of the sample by VFA. Seventy four were graded mild (grade 1), 40 moderate (grade 2) and 6 severe (grade 3). Multiple vertebral fractures (≥2) were found in 37 players (39%). There were no differences in prevalence between codes, or between forwards and backs (both 1.2 v 1.4; p>0.05). The most common sites of fracture were T8 (n = 23), T9 (n = 18) and T10 (n = 21). The mean (SD) lumbar spine bone mineral density Z-score was 2.7 (1.3) indicating high player bone mass in comparison with age- and sex-matched norms. We observed a high number of vertebral fractures using DXA VFA in professional rugby players of both codes. The incidence, aetiology and consequences of vertebral fractures in professional rugby players are unclear, and warrant timely, prospective investigation
Open timelike curves violate Heisenberg's uncertainty principle
Toy models for quantum evolution in the presence of closed timelike curves
(CTCs) have gained attention in the recent literature due to the strange
effects they predict. The circuits that give rise to these effects appear quite
abstract and contrived, as they require non-trivial interactions between the
future and past which lead to infinitely recursive equations. We consider the
special case in which there is no interaction inside the CTC, referred to as an
open timelike curve (OTC), for which the only local effect is to increase the
time elapsed by a clock carried by the system. Remarkably, circuits with access
to OTCs are shown to violate Heisenberg's uncertainty principle, allowing
perfect state discrimination and perfect cloning of coherent states. The model
is extended to wave-packets and smoothly recovers standard quantum mechanics in
an appropriate physical limit. The analogy with general relativistic
time-dilation suggests that OTCs provide a novel alternative to existing
proposals for the behaviour of quantum systems under gravity
Calculating the local-type fNL for slow-roll inflation with a non-vacuum initial state
Single-field slow-roll inflation with a non-vacuum initial state has an
enhanced bispectrum in the local limit. We numerically calculate the local-type
fNL signal in the CMB that would be measured for such models (including the
full transfer function and 2D projection). The nature of the result depends on
several parameters, including the occupation number N_k, the phase angle
\theta_k between the Bogoliubov parameters, and the slow-roll parameter
\epsilon. In the most conservative case, where one takes \theta_k \approx
\eta_0 k (justified by physical reasons discussed within) and \epsilon\lesssim
0.01, we find that 0 < fNL < 1.52 (\epsilon/0.01), which is likely too small to
be detected in the CMB. However, if one is willing to allow a constant value
for the phase angle \theta_k and N_k=O(1), fNL can be much larger and/or
negative (depending on the choice of \theta_k), e.g. fNL \approx 28
(\epsilon/0.01) or -6.4 (\epsilon/0.01); depending on \epsilon, these scenarios
could be detected by Planck or a future satellite. While we show that these
results are not actually a violation of the single-field consistency relation,
they do produce a value for fNL that is considerably larger than that usually
predicted from single-field inflation.Comment: 8 pages, 1 figure. v2: Version accepted for publication in PRD. Added
greatly expanded discussion of the phase angle \theta_k; this allows the
possibility of enhanced fNL, as mentioned in abstract. More explicit
comparisons with earlier wor
Quantum Communication with an Accelerated Partner
An unsolved problem in relativistic quantum information research is how to
model efficient, directional quantum communication between localised parties in
a fully quantum field theoretical framework. We propose a tractable approach to
this problem based on solving the Heisenberg evolution of localized field
observables. We illustrate our approach by analysing, and obtaining approximate
analytical solutions to, the problem of communicating coherent states between
an inertial sender, Alice and an accelerated receiver, Rob. We use these
results to determine the efficiency with which continuous variable quantum key
distribution could be carried out over such a communication channel.Comment: Additional explanatory text and typo in Eq.17 correcte
- …