Showing papers by "University of Cambridge published in 1982"

Black Hole Explosions

Abstract: QUANTUM gravitational effects are usually ignored in calculations of the formation and evolution of black holes. The justification for this is that the radius of curvature of space-time outside the event horizon is very large compared to the Planck length (Għ/c3)1/2 ≈ 10−33 cm, the length scale on which quantum fluctuations of the metric are expected to be of order unity. This means that the energy density of particles created by the gravitational field is small compared to the space-time curvature. Even though quantum effects may be small locally, they may still, however, add up to produce a significant effect over the lifetime of the Universe ≈ 1017 s which is very long compared to the Planck time ≈ 10−43 s. The purpose of this letter is to show that this indeed may be the case: it seems that any black hole will create and emit particles such as neutrinos or photons at just the rate that one would expect if the black hole was a body with a temperature of (κ/2π) (ħ/2k) ≈ 10−6 (M/M)K where κ is the surface gravity of the black hole1. As a black hole emits this thermal radiation one would expect it to lose mass. This in turn would increase the surface gravity and so increase the rate of emission. The black hole would therefore have a finite life of the order of 1071 (M/M)−3 s. For a black hole of solar mass this is much longer than the age of the Universe. There might, however, be much smaller black holes which were formed by fluctuations in the early Universe2. Any such black hole of mass less than 1015 g would have evaporated by now. Near the end of its life the rate of emission would be very high and about 1030 erg would be released in the last 0.1 s. This is a fairly small explosion by astronomical standards but it is equivalent to about 1 million 1 Mton hydrogen bombs. It is often said that nothing can escape from a black hole. But in 1974, Stephen Hawking realized that, owing to quantum effects, black holes should emit particles with a thermal distribution of energies — as if the black hole had a temperature inversely proportional to its mass. In addition to putting black-hole thermodynamics on a firmer footing, this discovery led Hawking to postulate 'black hole explosions', as primordial black holes end their lives in an accelerating release of energy.

The mechanics of three-dimensional cellular materials

Abstract: The mechanical properties (linear and nonlinear elastic and plastic) of two-dimensional cellular materials, or honeycombs, are analysed and compared with experiments. The properties are well described in terms of the bending, elastic buckling and plastic collapse of the beams that make up the cell walls.

Molecular drive: a cohesive mode of species evolution

Abstract: It is generally accepted that mutations may become fixed in a population by natural selection and genetic drift. In the case of many families of genes and noncoding sequences, however, fixation of mutations within a population may proceed as a consequence of molecular mechanisms of turnover within the genome. These mechanisms can be both random and directional in activity. There are circumstances in which the unusual concerted pattern of fixation permits the establishment of biological novelty and species discontinuities in a manner not predicted by the classical genetics of natural selection and genetic drift.

T-cell mitogens cause early changes in cytoplasmic free Ca2+ and membrane potential in lymphocytes.

Abstract: One of the best models for studying the control of mammalian cell growth and proliferation is the response of lymphocytes to mitogenic agents1–4, which stimulate growth, DNA synthesis and division. How mitogens work remains obscure, although it has been hypothesized that they increase cytoplasmic free calcium, [Ca2+]i, as a trigger for the cascade of intracellular processes necessary for proliferation3–6. However, lymphocyte [Ca2+]i has not previously been measurable. A new technique7 for loading a novel Ca2+-specific indicator8 into the cytoplasm of intact small cells has now made possible the first direct measurements of [Ca2+]i in mouse thymocytes and pig node lymphocytes. We show here that lectins known to stimulate T cells raise average [Ca2+]1 approximately twofold within a few minutes. Deprivation of external Ca2+or elevation of cyclic AMP, conditions known to inhibit mitogenesis, prevented the [Ca2+]i response. Rises in [Ca2+]i were accompanied by hyper-polarization of the membrane potential, apparently due to a Ca2+-activated K+conductance. The co-carcinogen 12-0-tetradecanoylphorbol-13-acetate (TPA) seems to stimulate cell functions normally activated by Ca2+.

The correlation averaging of a regularly arranged bacterial cell envelope protein

Abstract: An adaptation of the 'correlation averaging' method is described which allows reliable and almost fully automatic image averaging in the case of near-periodic structures notwithstanding the presence of substantial crystal imperfections; methods for assessing resolution and symmetry without reliance on crystallinity are also discussed. Electron micrographs of negatively stained and rotary shadowed preparations of the HPI-layer protein from the cell envelope of Micrococcus radiodurans have been averaged using the method, and the projected structure is described to a resolution of about 1.9 nm.

Ion-supported tori and the origin of radio jets

Abstract: While apparently supplying tremendous power to their extended radio-emitting regions, the nuclei of most radio galaxies emit little detectable radiation. It is proposed that at the centre of each is a spinning black hole surrounded by a torus of gas too hot and tenuous to radiate efficiently. The torus anchors magnetic fields which extract rotational energy from the hole in the form of two collimated beams of relativistic particles and fields. These in turn drive the observed radio jets and hot spots. A large supply of accreting gas is thus unnecessary and radio galaxies may be interpreted as starved quasars.

A thin viscous sheet model for continental deformation

Abstract: Summary. For the purposes of describing its large-scale and long-term deformation, the continental lithosphere is regarded as a continuum, obeying a Newtonian or a power law rheology. The flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible. A numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent-continent collision. The material flows in response to forces applied to its boundaries (for example, the indenting of one continent by another) and to forces in its interior arising from gradients in crustal thickness. The horizontal divergence of the flow produces changes in the crustal thickness and hence a time-dependent form to the flow itself. For a given set of boundary conditions, the flow depends on the stress exponent in the power law rheology, n, and on the Argand number& which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates. When the effective viscosity of the medium is very high (Ar-+O), crustal thickness variations do not influence the flow. If the material is Newtonian (n = l), the deformation associated with an influx of material (approximating an indenter) is of much greater lateral dimension than the width of the indenter, whereas when material has a power law rheology (n = 3, 5 are used), the deformation is confined to a region of lateral extent comparable to that of the indenter. As Ar increases, the forces arising from crustal thickness contrasts exert more influence on the flow, and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates. In the limit of a very weak medium (Ar> 10) the lithosphere is unable to sustain appreciable crustal elevation contrasts. The results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

Grapevine: an exercise in distributed computing

Abstract: Grapevine is a multicomputer system on the Xerox research internet. It provides facilities for the delivery of digital messages such as computer mail; for naming people, machines, and services; for authenticating people and machines; and for locating services on the internet. This paper has two goals: to describe the system itself and to serve as a case study of a real application of distributed computing. Part I describes the set of services provided by Grapevine and how its data and function are divided among computers on the internet. Part II presents in more detail selected aspects of Grapevine that illustrate novel facilities or implementation techniques, or that provide insight into the structure of a distributed system. Part III summarizes the current state of the system and the lesson learned from it so far.

Serotonin and octopamine in the nematode Caenorhabditis elegans

Abstract: The biogenic amines serotonin and octopamine are present in the nematode Caenorhabditis elegans. Serotonin, detected histochemically in whole mounts, is localized in two pharyngeal neurons that appear to be neurosecretory. Octopamine, identified radioenzymatically in crude extracts, probably is also localized in a few neurons. Exogenous serotonin and octopamine elicit specific and opposite behavioral responses in Caenorhabditis elegans, suggesting that these compounds function physiologically as antagonists.

Variations in the sensitivity of instrumental responding to reinforcer devaluation

Abstract: In five experiments hungry rats were trained to make a lever press response for a sucrose reinforcer. That sucrose was subsequently devalued by conditioning a food-aversion to it, and the ability of the rats to integrate knowledge about the instrumental contingency with that gained from aversion training was assessed in an extinction test. Experiment I showed successful integration following limited but not extended instrumental training. Experiment II suggested that the crucial factor was the spacing of training; successful integration was seen after massed but not distributed training. The third experiment implicated distributed experience with the reinforcer, rather than distributed response practice, in failures of integration. Experiment IV showed that if the distribution of food-aversion learning was dissimilar to that of instrumental training then a failure of integration could result; this finding was able to account for the distribution of training effects seen in previous studies, but not the ef...

Cooking, Cuisine and Class: A Study in Comparative Sociology

Abstract: The preparation, serving and eating of food are common features of all human societies, and have been the focus of study for numerous anthropologists - from Sir James Frazer onwards - from a variety of theoretical and empirical perspectives. It is in the context of this previous anthropological work that Jack Goody sets his own observations on cooking in West Africa. He criticises those approaches which overlook the comparative historical dimension of culinary, and other, cultural differences that emerge in class societies, both of which elements he particularly emphasises in this book. The central question that Professor Goody addresses here is why a differentiated 'haute cuisine' has not emerged in Africa, as it has in other parts of the world. His account of cooking in West Africa is followed by a survey of the culinary practices of the major Eurasian societies throughout history - ranging from Ancient Egypt, Imperial Rome and medieval China to early modern Europe - in which he relates the differences in food preparation and consumption emerging in these societies to differences in their socio-economic structures, specifically in modes of production and communication. He concludes with an examination of the world-wide rise of 'industrial food' and its impact on Third World societies, showing that the ability of the latter to resist cultural domination in food, as in other things, is related to the nature of their pre-existing socio-economic structures. The arguments presented here will interest all social scientists and historians concerned with cultural history and social theory.

The unpredictability of quantum gravity

Abstract: Quantum gravity seems to introduce a new level of unpredictability into physics over and above that normally associated with the uncertainty principle. This is because the metric of spacetime can fluctuate from being globally hyperbolic. In other words, the evolution is not completely determined by Cauchy data at past or future infinity. I present a number of axioms that the asymptotic Green functions should obey in any reasonable theory of quantum gravity. These axioms are the same as for ordinary quantum field theory in flat spacetime, except that one axiom, that of asymptotic completeness, is omitted. This allows pure quantum states to decay into mixed states. Calculations with simple models of topologically non-trivial spacetime indicate that such loss of quantum coherence will occur but that the effect will be very small except for fundamental scalar particles, if any such exist.

Fast quantizing and decoding and algorithms for lattice quantizers and codes

Abstract: For each of the lattices A_{n}(n \geq 1), D_{n}(n \geq 2), E_{6}, E_{7}, E_{8} , and their duals a very fast algorithm is given for finding the closest lattice point to an arbitrary point. If these lattices are used for vector quantizing of uniformly distributed data, the algorithm finds the minimum distortion lattice point. If the lattices are used as codes for a Gaussian channel, the algorithm performs maximum likelihood decoding.

Writers' cramp—a focal dystonia

Abstract: We have examined 29 subjects with writers' cramp (and 4 with typists' and one with pianists' cramp) and have noted two major groupings, simple and dystonic. We have observed spread from one to the other. We have seen repeatedly, in patients with isolated simple writers' cramp certain subtle physical signs which are found also in other basal ganglia diseases. We have noted also the frequent association of other features of segmental and generalized dystonia in patients with dystonic writers' cramp. We have demonstrated that patients with isolated writers' cramp have no higher an incidence of psychiatric disturbance, as judged by formal Present State Examination, than the normal population. We conclude that isolated writers' cramp is a physical illness rather than a psychological disturbance, and that it is a focal dystonia.

Sedimentation in a dilute polydisperse system of interacting spheres. Part 1. General theory

Abstract: Small rigid spherical partials are settling under gravity through Newtonian fluid, and the volume fraction of the particles (ϕ) is small although sufficiently large for the effects of interactions between pairs of particles to be significant. Two neighbouring particles interact both hydrodynamically (with low-Reynolds-number flow about each particle) and through the exertion of a mutual force of molecular or electrical origin which is mainly repulsive; and they also diffuse relatively to each other by Brownian motion. The dispersion contains several species of particle which differ in radius and density.The purpose of the paper is to derive formulae for the mean velocity of the particles of each species correct to order ϕ, that is, with allowance for the effect of pair interactions. The method devised for the calculation of the mean velocity in a monodisperse system (Batchelor 1972) is first generalized to give the mean additional velocity of a particle of species i due to the presence of a particle of species j in terms of the pair mobility functions and the probability distribution pii(r) for the relative position of an i and a j particle. The second step is to determine pij(r) from a differential equation of Fokker-Planck type representing the effects of relative motion of the two particles due to gravity, the interparticle force, and Brownian diffusion. The solution of this equation is investigated for a range of special conditions, including large values of the Peclet number (negligible effect of Brownian motion); small values of the Ptclet number; and extreme values of the ratio of the radii of the two spheres. There are found to be three different limits for pij(r) corresponding to different ways of approaching the state of equal sphere radii, equal sphere densities, and zero Brownian relative diffusivity.Consideration of the effect of relative diffusion on the pair-distribution function shows the existence of an effective interactive force between the two particles and consequently a contribution to the mean velocity of the particles of each species. The direct contributions to the mean velocity of particles of one species due to Brownian diffusion and to the interparticle force are non-zero whenever the pair-distribution function is non-isotropic, that is, at all except large values of the Peclet number.The forms taken by the expression for the mean velocity of the particles of one species in the various cases listed above are examined. Numerical values will be presented in Part 2.

The Dynamic Response of Railway Track to High Frequency Vertical Excitation

Abstract: Two new dynamic models of railway track are presented, one continuous and the other incorporating the discrete mass of the sleepers. These models include the effect of the railpads which exist betw...

Preferences for cousins in Japanese quail

Abstract: Early experience affects the mating preferences of many birds and mammals1,2. The plasticity of behaviour is such that individuals reared with a member of another species may subsequently choose to mate with that species3–6. Recent evidence suggests that the most strongly preferred mates are slightly different from individuals that are familiar from early life7–9. The implication of these findings is that an individual is able to strike an optimal balance between inbreeding and outbreeding by learning about its immediate kin and mating with a member of the opposite sex that is slightly different from its immediate kin1,10. What such a balance might amount to in practice has previously been uncertain. I report here that Japanese quail of both sexes, having been reared with their siblings, subsequently prefer a first cousin of the opposite sex.