The Need for Agricultural Data in South Africa: With Specific Reference to the Western Cape Province

In order for public and private decision-makers in the agricultural sector to use agricultural information for decision-making, solving problems or increasing their knowledge, the necessary data on agriculture must be available. As a result of the deregulation of the agricultural marketing sector in South Africa, the supply of this type of data has decreased. Also, the need for data on agriculture by the various decision-makers, i.e. the policy-makers, researchers, agricultural service industries as well as the farmers and extension officers, has changed. Since information systems are always based on the needs of the decision-makers, the need for data on agriculture should be determined before either existing methodologies are improved or new methodologies are introduced, to increase the supply of data on agriculture in South Africa. The objective of this paper is to identify the agricultural data needs and supply for South Africa with specific reference to the Western Cape Province. This was done by means of postal surveys and informal methods. Recommendations for an effective information system have also be concluded.Research and Development/Tech Change/Emerging Technologies,

The damping of gravitational waves in dust

We examine a simple model of interaction of gravitational waves with matter (primarily represented by dust). The aim is to investigate a possible damping effect on the intensity of gravitational wave when passing through media. This might be important for gravitational wave astronomy when the sources are obscured by dust or molecular clouds.Comment: 7 pages, accepted to Phys. Sc

On the Asymptotic Stability of De-Sitter Spacetime: a non-linear perturbative approach

We derive evolution and constraint equations for second order perturbations of flat dust homogeneous and isotropic solutions to the Einstein field equations using all scalar, vector and tensor perturbation modes. We show that the perturbations decay asymptotically in time and that the solutions converge to the De-Sitter solution. By induction, this result is valid for perturbations of arbitrary order. This is in agreement with the cosmic no-hair conjecture of Gibbons and Hawking.Comment: 11 pages, 2 figure

Nuclear effects in the proton-deuteron Drell-Yan process

We compute the nuclear corrections to the proton-deuteron Drell-Yan cross section for inclusive dilepton production, which, when combined with the proton-proton cross section, is used to determine the flavor asymmetry in the proton sea, dbar - ubar. In addition to nuclear smearing corrections that are known to be important at large values of the nucleon's parton momentum fraction x_N, we also consider dynamical off-shell nucleon corrections associated with the modifications of the bound nucleon structure inside the deuteron, which we find to be significant at intermediate and large x_N values. We also provide estimates of the nuclear corrections at kinematics corresponding to existing and planned Drell-Yan experiments at Fermilab and J-PARC which aim to determine the dbar/ubar ratio for x < 0.6.Comment: 26 pages, 6 figures; Fig. 2 and typos corrected, published versio

Incorporation of Spacetime Symmetries in Einstein's Field Equations

In the search for exact solutions to Einstein's field equations the main simplification tool is the introduction of spacetime symmetries. Motivated by this fact we develop a method to write the field equations for general matter in a form that fully incorporates the character of the symmetry. The method is being expressed in a covariant formalism using the framework of a double congruence. The basic notion on which it is based is that of the geometrisation of a general symmetry. As a special application of our general method we consider the case of a spacelike conformal Killing vector field on the spacetime manifold regarding special types of matter fields. New perspectives in General Relativity are discussed.Comment: 41 pages, LaTe

Global existence and future asymptotic behaviour for solutions of the Einstein-Vlasov-scalar field system with surface symmetry

We prove in the cases of plane and hyperbolic symmetries a global in time existence result in the future for comological solutions of the Einstein-Vlasov-scalar field system, with the sources generated by a distribution function and a scalar field, subject to the Vlasov and wave equations respectively. The spacetime is future geodesically complete in the special case of plane symmetry with only a scalar field. Causal geodesics are also shown to be future complete for homogeneous solutions of the Einstein-Vlasov-scalar field system with plane and hyperbolic symmetry.Comment: 14 page

A WKB formalism for multicomponent fields and its application to gravitational and sound waves in perfect fluids

We review the WKB method for multicomponent fields obeying hyperbolic linear partial differential equations and derive a general necessary and sufficient condition for the formalism to provide transport equations. We apply the method to linearized perturbations of perfect fluid solutions to Einstein's equation and show that the gravitational and sound wave modes satisfy this condition, whereas a zero-frequency, non-propagating matter mode does not. We derive the transport equations for the wave amplitudes in leading order; they exhibit in particular the influence of background curvature on the propagation of gravitational waves