10,608 research outputs found
Energy Density-Flux Correlations in an Unusual Quantum State and in the Vacuum
In this paper we consider the question of the degree to which negative and
positive energy are intertwined. We examine in more detail a previously studied
quantum state of the massless minimally coupled scalar field, which we call a
``Helfer state''. This is a state in which the energy density can be made
arbitrarily negative over an arbitrarily large region of space, but only at one
instant in time. In the Helfer state, the negative energy density is
accompanied by rapidly time-varying energy fluxes. It is the latter feature
which allows the quantum inequalities, bounds which restrict the magnitude and
duration of negative energy, to hold for this class of states. An observer who
initially passes through the negative energy region will quickly encounter
fluxes of positive energy which subsequently enter the region. We examine in
detail the correlation between the energy density and flux in the Helfer state
in terms of their expectation values. We then study the correlation function
between energy density and flux in the Minkowski vacuum state, for a massless
minimally coupled scalar field in both two and four dimensions. In this latter
analysis we examine correlation functions rather than expectation values.
Remarkably, we see qualitatively similar behavior to that in the Helfer state.
More specifically, an initial negative energy vacuum fluctuation in some region
of space is correlated with a subsequent flux fluctuation of positive energy
into the region. We speculate that the mechanism which ensures that the quantum
inequalities hold in the Helfer state, as well as in other quantum states
associated with negative energy, is, at least in some sense, already
``encoded'' in the fluctuations of the vacuum.Comment: 21 pages, 7 figures; published version with typos corrected and one
added referenc
The evolution of sperm morphometry in pheasants
Postcopulatory sexual selection is thought to be a potent evolutionary force driving the
diversification of sperm shape and function across species. In birds, insemination and
fertilisation are separated in time and sperm storage increases the duration of sperm
female interaction and hence the opportunity for sperm competition and cryptic female
choice. We performed a comparative study of 24 pheasant species (Phasianidae,
Galliformes) to establish the relative importance of sperm competition and the duration of
sperm storage for the evolution of sperm morphometry (i.e. size of different sperm traits).
We found that sperm size traits were negatively associated with the duration of sperm
storage but were independent of the risk of sperm competition estimated from relative
testis mass. Our study emphasises the importance of female reproductive biology for the
evolution of sperm morphometry particularly in sperm storing taxa
Weighted Radon transforms for which the Chang approximate inversion formula is precise
We describe all weighted Radon transforms on the plane for which the Chang
approximate inversion formula is precise. Some subsequent results, including
the Cormack type inversion for these transforms, are also given
Wrapping an adhesive sphere with a sheet
We study the adhesion of an elastic sheet on a rigid spherical substrate.
Gauss'Theorema Egregium shows that this operation necessarily generates metric
distortions (i.e. stretching) as well as bending. As a result, a large variety
of contact patterns ranging from simple disks to complex branched shapes are
observed as a function of both geometrical and material properties. We describe
these different morphologies as a function of two non-dimensional parameters
comparing respectively bending and stretching energies to adhesion. A complete
configuration diagram is finally proposed
Josephson junction between anisotropic superconductors
The sin-Gordon equation for Josephson junctions with arbitrary misaligned
anisotropic banks is derived. As an application, the problem of Josephson
vortices at twin planes of a YBCO-like material is considered. It is shown that
for an arbitrary orientation of these vortices relative to the crystal axes of
the banks, the junctions should experience a mechanical torque which is
evaluated. This torque and its angular dependence may, in principle, be
measured in small fields, since the flux penetration into twinned crystals
begins with nucleation of Josephson vortices at twin planes.Comment: 6 page
Bernoulli type polynomials on Umbral Algebra
The aim of this paper is to investigate generating functions for modification
of the Milne-Thomson's polynomials, which are related to the Bernoulli
polynomials and the Hermite polynomials. By applying the Umbral algebra to
these generating functions, we provide to deriving identities for these
polynomials
The averaged null energy condition and difference inequalities in quantum field theory
Recently, Larry Ford and Tom Roman have discovered that in a flat cylindrical
space, although the stress-energy tensor itself fails to satisfy the averaged
null energy condition (ANEC) along the (non-achronal) null geodesics, when the
``Casimir-vacuum" contribution is subtracted from the stress-energy the
resulting tensor does satisfy the ANEC inequality. Ford and Roman name this
class of constraints on the quantum stress-energy tensor ``difference
inequalities." Here I give a proof of the difference inequality for a minimally
coupled massless scalar field in an arbitrary two-dimensional spacetime, using
the same techniques as those we relied on to prove ANEC in an earlier paper
with Robert Wald. I begin with an overview of averaged energy conditions in
quantum field theory.Comment: 20 page
Feasibility of sulfur concrete for martian constructions
A significant step in space exploration during the 21st century will be the human settlement on Mars. Instead of transporting all the construction materials from Earth to the red planet with incredibly high cost, using Martian soil to construct a site on Mars is a superior choice. Mars has long been considered a “sulfur-rich planet”. Studies of Martian meteorites suggest elevated sulfur concentrations in the interior, and Martian surface deposits contain high levels of sulfur (SO3 up to ~37 wt%, average ~6 wt%), likely in the form of sulfate salts1. To let the thoughts become facts, a new construction material using simulated Martian soil and molten sulfur is developed. In fact, sulfur concrete is not a new concept. The utilization of sulfur as a molten bonding agent can be traced back to prehistoric times2. Sulfur concretes are being produced by first hot-mixing sulfur (or modified sulfur) and aggregates, which allows the sulfur binder crystalize as monoclinic sulfur (Sβ), then letting the mixture cool down while sulfur transform to the stable orthorhombic polymorph (Sα) to achieve a reliable building material. In addition to the raw material availability, the utilization of sulfur concrete has many advantages compared to conventional Portland cement concrete. The strength reaches similar levels while the fatigue life, low temperature sustainability, and the curing time are superior, a minimum of 70-80 % of the ultimate compressive strength is reached within 24 hours
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