1,432 research outputs found
Algebraic Rainich theory and antisymmetrisation in higher dimensions
The classical Rainich(-Misner-Wheeler) theory gives necessary and sufficient
conditions on an energy-momentum tensor to be that of a Maxwell field (a
2-form) in four dimensions. Via Einstein's equations these conditions can be
expressed in terms of the Ricci tensor, thus providing conditions on a
spacetime geometry for it to be an Einstein-Maxwell spacetime. One of the
conditions is that is proportional to the metric, and it has previously
been shown in arbitrary dimension that any tensor satisfying this condition is
a superenergy tensor of a simple -form. Here we examine algebraic Rainich
conditions for general -forms in higher dimensions and their relations to
identities by antisymmetrisation. Using antisymmetrisation techniques we find
new identities for superenergy tensors of these general (non-simple) forms, and
we also prove in some cases the converse; that the identities are sufficient to
determine the form. As an example we obtain the complete generalisation of the
classical Rainich theory to five dimensions.Comment: 16 pages, LaTe
Two dimensional Sen connections and quasi-local energy-momentum
The recently constructed two dimensional Sen connection is applied in the
problem of quasi-local energy-momentum in general relativity. First it is shown
that, because of one of the two 2 dimensional Sen--Witten identities, Penrose's
quasi-local charge integral can be expressed as a Nester--Witten integral.Then,
to find the appropriate spinor propagation laws to the Nester--Witten integral,
all the possible first order linear differential operators that can be
constructed only from the irreducible chiral parts of the Sen operator alone
are determined and examined. It is only the holomorphy or anti-holomorphy
operator that can define acceptable propagation laws. The 2 dimensional Sen
connection thus naturally defines a quasi-local energy-momentum, which is
precisely that of Dougan and Mason. Then provided the dominant energy condition
holds and the 2-sphere S is convex we show that the next statements are
equivalent: i. the quasi-local mass (energy-momentum) associated with S is
zero; ii.the Cauchy development is a pp-wave geometry with pure
radiation ( is flat), where is a spacelike hypersurface
whose boundary is S; iii. there exist a Sen--constant spinor field (two spinor
fields) on S. Thus the pp-wave Cauchy developments can be characterized by the
geometry of a two rather than a three dimensional submanifold.Comment: 20 pages, Plain Tex, I
Arterio-ureteral Fistula – a Systematic Review
AbstractObjective: to review published reports on arterio-ureteral fistula. Method: literature search. Results: eighty cases were identified. Primary fistulas were mainly seen in combination with aortoiliac aneurysmal disease. Secondary fistulas were seen after pelvic cancer surgery, often with radiation, fibrosis and ureteral stenting or after vascular surgery with synthetic grafting. The dominating symptom is massive haematuria, often with circulatory impairment. The clue to a rapid and correct diagnosis is a high degree of suspicion. Most frequently diagnosis has been obtained through angiography or pyelography. When there is a ureteral stent manipulation it will often provoke bleeding and lead to diagnosis. The fistula must be excluded and a vascular reconstruction made. Most frequently this has been obtained through occlusion of the fistula and an extra-anatomic reconstruction (femoro-femoral crossover). Recently stent-grafting has been successfully used but follow-up is short. Conclusion: arterio-ureteral fistula is rare and should be suspected in patients with complicated pelvic surgery and massive haematuria, especially where rigid ureteral stents have been placed
Conserved Matter Superenergy Currents for Orthogonally Transitive Abelian G2 Isometry Groups
In a previous paper we showed that the electromagnetic superenergy tensor,
the Chevreton tensor, gives rise to a conserved current when there is a
hypersurface orthogonal Killing vector present. In addition, the current is
proportional to the Killing vector. The aim of this paper is to extend this
result to the case when we have a two-parameter Abelian isometry group that
acts orthogonally transitive on non-null surfaces. It is shown that for
four-dimensional Einstein-Maxwell theory with a source-free electromagnetic
field, the corresponding superenergy currents lie in the orbits of the group
and are conserved. A similar result is also shown to hold for the trace of the
Chevreton tensor and for the Bach tensor, and also in Einstein-Klein-Gordon
theory for the superenergy of the scalar field. This links up well with the
fact that the Bel tensor has these properties and the possibility of
constructing conserved mixed currents between the gravitational field and the
matter fields.Comment: 15 page
A Note on Matter Superenergy Tensors
We consider Bel-Robinson-like higher derivative conserved two-index tensors
H_\mn in simple matter models, following a recently suggested Maxwell field
version. In flat space, we show that they are essentially equivalent to the
true stress-tensors. In curved Ricci-flat backgrounds it is possible to
redefine H_\mn so as to overcome non-commutativity of covariant derivatives,
and maintain conservation, but they become model- and dimension- dependent, and
generally lose their simple "BR" form.Comment: 3 page
Magnon softening in a ferromagnetic monolayer: a first-principles spin dynamics study
We study the Fe/W(110) monolayer system through a combination of first
principles calculations and atomistic spin dynamics simulations. We focus on
the dispersion of the spin waves parallel to the [001] direction. Our results
compare favorably with the experimental data of Prokop et al. [Phys. Rev. Lett.
102, 177206], and correctly capture a drastic softening of the magnon spectrum,
with respect to bulk bcc Fe. The suggested shortcoming of the itinerant
electron model, in particular that given by density functional theory, is
refuted. We also demonstrate that finite temperature effects are significant,
and that atomistic spin dynamics simulations represent a powerful tool with
which to include these.Comment: v1: 11 pages, 3 figures. v2: double column, 5 pages, 3 figures, typos
corrected, references adde
Framing Effects in Museum Narratives: Objectivity in Interpretation Revisited
Museums establish specific contexts, framings, which distinguish them from viewing the world face-to-face. One striking aspect of exhibition in so-called participatory museums is that it echoes and transforms the limits of its own frame as a public space. I argue that it is a mistake to think of the meaning of an exhibit as either determined by the individual viewer's narrative or as determined by the conception as presented in the museum's ‘authoritative’ narrative. Instead I deploy the concept of a model of comparison to illuminate the philosophical significance of perspective in understanding the idea of objectivity in museum narratives
Corticotropin-Releasing Hormone (CRH) gene family duplications in Lampreys correlate with two early vertebrate genome doublings
The ancestor of gnathostomes (jawed vertebrates) is generally considered to have undergone two rounds of whole genome duplication (WGD). The timing of these WGD events relative to the divergence of the closest relatives of the gnathostomes, the cyclostomes, has remained contentious. Lampreys and hagfishes are extant cyclostomes whose gene families can shed light on the relationship between the WGDs and the cyclostome-gnathostome divergence. Previously, we have characterized in detail the evolution of the gnathostome corticotropin-releasing hormone (CRH) family and found that its five members arose from two ancestral genes that existed before the WGDs. The two WGDs resulted, after secondary losses, in one triplet consisting of CRH1, CRH2, and UCN1, and one pair consisting of UCN2 and UCN3. All five genes exist in representatives for cartilaginous fishes, ray-finned fishes, and lobe-finned fishes. Differential losses have occurred in some lineages. We present here analyses of CRH-family members in lamprey and hagfish by comparing sequences and gene synteny with gnathostomes. We found five CRH-family genes in each of two lamprey species (Petromyzon marinus and Lethenteron camtschaticum) and two genes in a hagfish (Eptatretus burgeri). Synteny analyses show that all five lamprey CRH-family genes have similar chromosomal neighbors as the gnathostome genes. The most parsimonious explanation is that the lamprey CRH-family genes are orthologs of the five gnathostome genes and thus arose in the same chromosome duplications. This suggests that lampreys and gnathostomes share the same two WGD events and that these took place before the lamprey-gnathostome divergence.Portuguese Foundation for Science and Technology: UIDB/04326/2020info:eu-repo/semantics/publishedVersio
Two dimensional Sen connections in general relativity
The two dimensional version of the Sen connection for spinors and tensors on
spacelike 2-surfaces is constructed. A complex metric on the spin
spaces is found which characterizes both the algebraic and extrinsic
geometrical properties of the 2-surface . The curvature of the two
dimensional Sen operator is the pull back to of the
anti-self-dual part of the spacetime curvature while its `torsion' is a boost
gauge invariant expression of the extrinsic curvatures of . The difference
of the 2 dimensional Sen and the induced spin connections is the anti-self-dual
part of the `torsion'. The irreducible parts of are shown to be the
familiar 2-surface twistor and the Weyl--Sen--Witten operators. Two Sen--Witten
type identities are derived, the first is an identity between the 2 dimensional
twistor and the Weyl--Sen--Witten operators and the integrand of Penrose's
charge integral, while the second contains the `torsion' as well. For spinor
fields satisfying the 2-surface twistor equation the first reduces to Tod's
formula for the kinematical twistor.Comment: 14 pages, Plain Tex, no report numbe
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