Pair creation of particles and black holes in external fields

It is well known that massive black holes may form through the gravitational collapse of a massive astrophysical body. Less known is the fact that a black hole can be produced by the quantum process of pair creation in external fields. These black holes may have a mass much lower than their astrophysical counterparts. This mass can be of the order of Planck mass so that quantum effects may be important. This pair creation process can be investigated semiclassically using non-perturbative instanton methods, thus it may be used as a theoretical laboratory to obtain clues for a quantum gravity theory. In this work, we review briefly the history of pair creation of particles and black holes in external fields. In order to present some features of the euclidean instanton method which is used to calculate pair creation rates, we study a simple model of a scalar field and propose an effective one-loop action for a two-dimensional soliton pair creation problem. This action is built from the soliton field itself and the soliton charge is no longer treated as a topological charge but as a Noether charge. The results are also valid straightforwardly to the problem of pair creation rate of domain walls in dimensions greater than 2.Comment: LaTeX file (World Scientific macros), no figures, 9 pages, talk given at Xth Portuguese Meeting on Astronomy and Astrophysics, (Lisbon, Portugal, 27-28 July 2000), to be published in Proc. Xth A & A meeting, edited by J. P. S. Lemos, A. Mourao, L. Teodoro, R. Ugoccioni, (World Scientific, 2001

Surface defects and temperature on atomic friction

We present a theoretical study of the effect of surface defects on atomic friction in the stick-slip dynamical regime of a minimalistic model. We focus on how the presence of defects and temperature change the average properties of the system. We have identified two main mechanisms which modify the mean friction force of the system when defects are considered. As expected, defects change locally the potential profile and thus affect the friction force. But the presence of defects also changes the probability distribution function of the tip slip length and thus the mean friction force. We corroborated both effects for different values of temperature, external load, dragging velocity and damping. We show also a comparison of the effects of surface defects and surface disorder on the dynamics of the system

Carbon markets, transaction costs and bioenergy

Payment for carbon sequestration by agriculture and forestry can provide incentives for adoption of sustainable agricultural practices. However, a project involving contracts with farmers may face high transaction costs in showing that net emission reductions are real and attributable to the project. This paper presents a model of project participation that includes transaction and abatement costs. A project feasibility frontier (PFF) is derived, which shows the minimum project size that is feasible for any given market price of carbon. The PFF is used to analyse how the design of a climate mitigation program may affect the feasibility of actual projects.Climate Policy, Greenhouse Effect, Carbon Sequestration, Agroforestry, Transaction Costs, Environmental Economics and Policy,

DYNAMIC MODELS, EXTERNALITIES AND SUSTAINABILITY IN AGRICULTURE

The goal of sustainability in the management of natural resources and agricultural systems has received increasing attention during the 1990's. The many dimensions of the problem have been extensively discussed in the literature and a recognition of the interaction between economic, biological and social objectives have led to an acceptance of its multidisciplinary nature. When studying sustainability in agriculture, two aspects which cannot be ignored are (i) any measure must include economic as well as biological criteria and (ii) the dynamic nature of the production system and the environment (both physical and economic) must be accounted for. The goal of sustainable agricultural practices at the microeconomic level is explored in this paper, in an attempt to link the individual producer behaviour to the regulatory environment. Particular attention is paid to the dynamic aspect in the context of a grazing system, where plant and animal populations interact with each other and are influenced by the environment. An optimal control formulation is used to discuss the alternative ways in which externalities (such as salinity, soil loss and fertiliser and chemical run-off) can be incorporated into a model. The problem of valuing externalities and the role of the discount rate on optimal management strategies are briefly discussed.sustainability, dynamic modelling, bioeconomics, Environmental Economics and Policy, Livestock Production/Industries, Resource /Energy Economics and Policy,

VALUING AGROFORESTRY IN THE PRESENCE OF LAND DEGRADATION

Agroforestry can help prevent land degradation while allowing continuing use of land to produce crops and livestock. A problem with the evaluation of agroforestry using long-run static models and traditional discounting techniques is that the present value of the forestry enterprise is generally much lower than that of other production activities. This problem is common with Australian native species which tend to have a high environmental value but a low market value. This paper presents an economic analysis of an agroforestry operation in land prone to degradation and in the presence of positive externalities provided by trees. The value of the land is estimated based on the present value of expected returns in perpetuity under optimal management. Simulation analysis is used to evaluate the loss in land value caused by dryland salinity. A nonlinear programming model is developed and used to study the effects of timber prices and forest planting costs on optimal forest area and the level of salinity. Elasticities of relevant variables with respect to prices and costs are derived and policy implications of results are discussed.agroforestry, land degradation, externalities, dryland salinity, Land Economics/Use,

Bounds on Correlation Functions of Quantum Rotators

We derive a McBryan-Spencer bound to the correlation function of a one-dimensional array of quantum rotators in the Villain approximation of the cosine interaction. We obtain the partition function of the system in the gas representation and establish a lower bound on the external charge correlation function. We also discuss the possible existence of a Kosterlitz-Thouless phase for the quantum rotator in the Villain approximation.Comment: Latex 7 page