Showing papers by "University of Cambridge published in 1983"

Thermodynamics of black holes in anti-de Sitter space

Abstract: The Einstein equations with a negative cosmological constant admit black hole solutions which are asymptotic to anti-de Sitter space. Like black holes in asymptotically flat space, these solutions have thermodynamic properties including a characteristic temperature and an intrinsic entropy equal to one quarter of the area of the event horizon in Planck units. There are however some important differences from the asymptotically flat case. A black hole in anti-de Sitter space has a minimum temperature which occurs when its size is of the order of the characteristic radius of the anti-de Sitter space. For larger black holes the red-shifted temperature measured at infinity is greater. This means that such black holes have positive specific heat and can be in stable equilibrium with thermal radiation at a fixed temperature. It also implies that the canonical ensemble exists for asymptotically anti-de Sitter space, unlike the case for asymptotically flat space. One can also consider the microcanonical ensemble. One can avoid the problem that arises in asymptotically flat space of having to put the system in a box with unphysical perfectly reflecting walls because the gravitational potential of anti-de Sitter space acts as a box of finite volume.

Release of Ca2+ from a nonmitochondrial intracellular store in pancreatic acinar cells by inositol-1,4,5-trisphosphate.

Abstract: Activation of receptors for a wide variety of hormones and neurotransmitters leads to an increase in the intracellular level of calcium. Much of this calcium is released from intracellular stores but the link between surface receptors and this internal calcium reservoir is unknown. Hydrolysis of the phosphoinositides, which is another characteristic feature of these receptors, has been implicated in calcium mobilization. The primary lipid substrates for the receptor mechanism seem to be two polyphosphoinositides, phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P2), which are rapidly hydrolysed following receptor activation in various cells and tissues. The action of phospholipase C on these polyphosphoinositides results in the rapid formation of the water-soluble products inositol 1,4-bisphosphate (Ins1,4P2) and inositol 1,4,5-trisphosphate (Ins1,4,5P3). In the insect salivary gland, where changes in Ins1,4P2 and Ins1,4,5P2 have been studied at early time periods, increases in these inositol phosphates are sufficiently rapid to suggest that they might mobilize internal calcium. We report here that micromolar concentrations of Ins1,4,5P3 release Ca2+ from a nonmitochondrial intracellular Ca2+ store in pancreatic acinar cells. Our results strongly suggest that this is the same Ca2+ store that is released by acetylcholine.

Finite Elements for Electrical Engineers

Abstract: 1. Finite elements in one dimension 2. First-order triangular elements for potential problems 3. Electromagnetics of finite elements 4. Simplex elements for the scalar Helmholtz equation 5. Differential operators in ferromagnetic materials 6. Finite elements for integral operators 7. Curvilinear, vectorial and unbounded elements 8. Time and frequency domain problems in bounded systems 9. Unbounded radiation and scattering 10. Numerical solution of finite element equations References Appendices Index.

Seismic Wave Propagation in Stratified Media

Abstract: Publisher Summary This chapter focuses on elastic wave propagation in stratified media. The development of the theory of elastic wave propagation in stratified media has been strongly influenced by the problems of seismic wave propagation and the nature of the seismograms recorded from earthquakes. For purely analytic developments of elastic wave propagation, the level of manageable algebraic complexity is reached in a model with one or two uniform layers overlying a uniform half space. This chapter shows how the excitation of elastic waves, within a horizontally stratified structure, can be conveniently developed in terms of reflection and transmission matrices. This procedure has allowed the construction of the full response of the medium or approximations with desired properties so that theoretical seismograms may be calculated for realistic distributions of elastic parameters. Although this development has been for isotropic media, nearly all the results apply directly to the case of full anisotropy if 3 × 3 reflection and transmission matrices allowing coupling between all wave types are employed. This development of the wavefield for both source and receiver within the stratification may be used for other classes of wave propagation.

Continuous deformations in random networks

Abstract: We examine some current ideas concerning the differences between covalent random networks with high and low average coordination. These ideas can be made rigorous by considering the number of continuous deformations (i.e. zero frequency modes) allowed within the network. In the transition from one kind of network to another, rigidity percolates through the system. This leads to a picture in which random networks with low average coordination (polymeric glasses) have large floppy or spongy regions with a few rigid inclusions. On the other hand in random networks with high average coordination (amorphous solids) the rigid regions have percolated to form a rigid solid with a few floppy or spongy inclusions.

The Development of the Family and Marriage in Europe

Abstract: Around 300 A.D. European patterns of marriage and kinship were turned on their head. What had previously been the norm - marriage to close kin - became the new taboo. The same applied to adoption, the obligation of a man to marry his brother's widow and a number of other central practices. With these changes Christian Europe broke radically from its own past and established practices which diverged markedly from those of the Middle East, North Africa and Asia. In this highly original and far-reaching work Jack Goody argues that from the fourth century there developed in the northern Mediterranean a distinctive but not undifferentiated kinship system, whose growth can be attributed to the role of the Church in acquiring property formerly held by domestic groups. He suggests that the early Church, faced with the need to provide for people who had left their kin to devote themselves to the life of the Church, regulated the rules of marriage so that wealth could be channelled away from the family and into the Church. Thus the Church became an 'interitor', acquiring vast tracts of property through the alienation of familial rights. At the same time, the structure of domestic life was changed dramatically, the Church placing more emphasis on individual wishes, on conjugality, and on spiritual rather than natural kinship. Tracing the consequences of this change through to the present day, Jack Goody challenges some fundamental assumptions about the making of western society, and provides an alternative focus for future study of the European family, kinship structures and marriage patterns. The questions he raises will provoke much interest and discussion amongst anthropologists, sociologists and historians.

Breaking planetary waves in the stratosphere

Abstract: Satellite-borne IR radiometers are turning the Earth's stratosphere into one of the best available outdoor laboratories for observing the large-scale dynamics of a rotating, heterogeneous fluid under gravity. New insight is being gained not only into stratospheric dynamics as such, with its implications for pollutant behaviour and the ozone layer, but also indirectly into the dynamics of the troposphere, with its implications for weather forecasting. Similar dynamical regimes occur in the oceans and in stellar interiors. A key development has been the construction of coarse-grain maps of the large-scale distribution of potential vorticity in the middle stratosphere. Potential vorticity is a conservable quantity which has a central role in the dynamical theory, but is difficult to calculate accurately from observational data. We present the first mid-stratospheric potential vorticity maps which appear good enough to make visible the ‘breaking’ of planetary or Rossby waves, a phenomenon ubiquitous in nature and arguably one of the most important dynamical processes affecting the stratosphere as a whole.

Darwin's Plots: Evolutionary Narrative in Darwin, George Eliot and Nineteenth-Century Fiction

Abstract: Foreword George Levine Preface to second edition Introduction Part I. Darwin's Language: 1. 'Pleasure like a tragedy': imagination and the material world 2. Fit and misfitting: anthropomorphism and the natural order Part II. Darwin's Plots: 3. Analogy, metaphor and narrative in The Origin 4. Darwinian myths 5. George Eliot: Middlemarch 6. George Eliot: Daniel Deronda and the idea of a future life 7. Descent and sexual selection: women in narrative 8. Finding a scale for the human: plot and writing in Hardy's novels 9. Darwin and the consciousness of others Select bibliography of primary works.

Teaching the spoken language

Abstract: Those familiar with the work of Gillian Brown and George Yule on Discourse Analysis might be expecting a book which deals with the applications of such analysis to teaching English. Insofar as this is true, the title of the book is misleading : discourse analysis is an ambiguous term which has been used to refer to (i) conversational analysis, or (ii) the process of producing language as opposed to ’text’, the product (cf Robinson, 1980). Brown and Yule opt for the latter definition i.e. p. 57 &dquo;discourse is text interpreted in context&dquo;. Thus, the arguments in this book could well hold for written language, especially those

Mechanics of Fracture

Abstract: Calculations of the theoretical strengths of crystalline solids are usually based on idealised forms of atomic force-displacement curves, in which the force is defined as the differential with respect to distance of the inter-atomic or inter-ionic energy. The energy curve is similar to that for a diatomic molecule in that it represents the resultant of inter-atomic repulsions and attractions; the nature and strength of the attractive forces depending on the bond type: ionic, covalent, metallic, or Van der Waals. Differences in character between a lattice and a molecule occur at separations of order one lattice spacing, when Friedel oscillations in the screening charge cause the long range component of the interaction potential in the lattice to undergo a damped oscillation about zero. For small displacements, the atomic force/displacement curve is linear, having a slope equivalent to Young’s modulus, E. A lattice also has shear stiffness, denoted by the shear modulus μ. Both E and μ are defined macroscopically, usually for randomly-oriented polycrystals which are assumed to be isotropic. In single crystals, both the tensile stiffness and the shear stiffness vary with the orientation of the crystal with respect to the tensile axis and these variations can be substantial: in iron, for example, the minimum value of E is in the [100]direction (132 GPa at room temperature) and the maximum value is in the [111] direction (260 GPa).

A new technique for calculations of binary stellar evolution, with application to magnetic braking

Abstract: The development of appropriate computer programs has made it possible to conduct studies of stellar evolution which are more detailed and accurate than the investigations previously feasible. However, the use of such programs can also entail some serious drawbacks which are related to the time and expense required for the work. One approach for overcoming these drawbacks involves the employment of simplified stellar evolution codes which incorporate the essential physics of the problem of interest without attempting either great generality or maximal accuracy. Rappaport et al. (1982) have developed a simplified code to study the evolution of close binary stellar systems composed of a collapsed object and a low-mass secondary. The present investigation is concerned with a more general, but still simplified, technique for calculating the evolution of close binary systems with collapsed binaries and mass-losing secondaries.

Molecular Genetics of the Bithorax Complex in Drosophila melanogaster

Abstract: The bithorax complex in Drosophila melanogaster is a cluster of homeotic genes that specify developmental pathways for many of the body segments of the fly. The DNA of the bithorax complex has been isolated, and a region of 195,000 base pairs that covers the left half of the complex is described here. The lesions associated with many of the bithorax complex mutants have been identified, and most are due to DNA rearrangements. Most of the spontaneous mutants have insertions of a particular mobile element named "gypsy." This element affects the functions of sequences removed from the site of insertion. Mutant lesions for a given phenotypic class are distributed over large DNA distances of up to 73,000 base pairs.

Human visual pigments: microspectrophotometric results from the eyes of seven persons.

Abstract: The material for this work was obtained from seven eyes removed because of malignant growths. Foveal and parafoveal samples of the retinas were taken and transverse measurements were made of the absorbance spectra of the outer segments of the rods and cones, using a Liebman microspectrophotometer. Four kinds of spectra were obtained with absorbance peaks at the following wavelengths: rods, 496.3 +/- 2.3 nm (n = 39); red cones, 558.4 +/- 5.2 nm (n = 58); green cones, 530.8 +/- 3.5 nm (n = 45); blue cones, 419.0 +/- 3.6 nm (n = 5). The distribution of the peaks was unimodal for the rods. For the red and green cones, however, there was evidence for bimodal distributions, with sub-population maxima at 563.2 +/- 3.1 nm (n = 27) and 554.2 +/- 2.3 nm (n = 31) for the reds and at 533.7 +/- 2.1 nm (n = 23) and 527.8 +/- 1.8 nm (n = 22) for the greens. A substantial difference in mean spectral location of the red cones was observed between patient 1 (561 nm) and patient 4 (553 nm). Both patients were classified as normal trichromats by all clinical tests of colour vision but there was a clear difference in their relative sensitivities to long-wave fields. In both direction and magnitude, this difference proved to be that required by the microspectrophotometric results.

Gravity currents produced by instantaneous releases of a heavy fluid in a rectangular channel

Abstract: Results of laboratory experiments are presented in which a finite volume of homogeneous fluid was released instantaneously into another fluid of slightly lower density The experiments were performed in a channel of rectangular cross-section, and the two fluids used were salt water and fresh water As previously reported, the resulting gravity current, if viscous effects are negligible, passes through two distinct phases: an initial adjustment phase, during which the initial conditions are important, and an eventual self-similar phase, in which the front speed decreases as t−1/3 (where t is the time measured from release) The experiments reported herein were designed to emphasize the inviscid motion From our observations we argue that the current front moves steadily in the first phase, and that the transition to the inviscid self-similar phase occurs when a disturbance generated at the endwall (or plane of symmetry) overtakes the front If the initial depth of the heavy fluid is equal to or slightly less than the total depth of the fluid in the channel, the disturbance has the appearance of an internal hydraulic drop Otherwise, the disturbance is a long wave of depression Measurements of the duration of the initial phase and of the speed and depth of the front during this phase are presented as functions of the ratio of the initial heavy fluid depth to the total fluid depth These measurements are compared with numerical solutions of the shallow-water equations for a two-layer fluid

The accommodation of large strains in the upper lithosphere of the earth and other solids by self-similar fault systems: the geometrical origin of b-Value

Abstract: Plate tectonics has provided a method of visualizing the geometry of the deformation of the Earth’s lithosphere on a large scale. The description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes. The mechanics of the zones between the plates are less well understood, particularly in continental regions where large areas are subject to deformation. Both continuous and discontinuous models have been tried but both have obvious drawbacks.

Variable Metric Methods for Constrained Optimization

Abstract: Variable metric methods solve nonlinearly constrained optimization problems, using calculated first derivatives and a single positive definite matrix, which holds second derivative information that is obtained automatically. The theory of these methods is shown by analysing the global and local convergence properties of a basic algorithm, and we find that superlinear convergence requires less second derivative information than in the unconstrained case. Moreover, in order to avoid the difficulties of inconsistent linear approximations to constraints, careful consideration is given to the calculation of search directions by unconstrained minimization subproblems. The Maratos effect and relations to reduced gradient algorithms are studied briefly.

Activation energy spectra and relaxation in amorphous materials

Abstract: A theoretical model for relaxation in glassy materials, in particular metallic glasses, based on a spectrum of available processes distributed in activation energy is presented. The model is used to discuss “In t” kinetics, “reversibility” and “crossover” effects which have been observed experimentally. Where direct comparison is possible between the theory and experiment the agreement is good over all these various observed phenomena.

Mantle reservoirs and ocean island basalts

Abstract: The mantle part of the continental lithosphere possesses in part Sr and Nd isotopic compositions similar to those of ocean island basalts. Here we investigate circumstances under which this mantle material may become detached from continental lithosphere, incorporated into the convective flow and thereby providing a source for some ocean island basalts.

Positive mass theorems for black holes

Abstract: We extend Witten's proof of the positive mass theorem at spacelike infinity to show that the mass is positive for initial data on an asymptotically flat spatial hypersurface Σ which is regular outside an apparent horizonH. In addition, we prove that if a black hole has electromagnetic charge, then the mass is greater than the modulus of the charge. These results are also valid for the Bondi mass at null infinity. Finally, in the case of the Einstein equation with a negative cosmological constant, we show that a suitably defined mass is positive for data on an asymptotically anti-de Sitter surface Σ which is regular outside an apparent horizon.