Black hole collisions: how far can perturbation theory go?

The computation of gravitational radiation generated by the coalescence of inspiralling binary black holes is nowdays one of the main goals of numerical relativity. Perturbation theory has emerged as an ubiquitous tool for all those dynamical evolutions where the two black holes start close enough to each other, to be treated as single distorted black hole (close limit approximation), providing at the same time useful benchmarks for full numerical simulations. Here we summarize the most recent developments to study evolutions of perturbations around rotating (Kerr) black holes. The final aim is to generalize the close limit approximation to the most general case of two rotating black holes in orbit around each other, and thus provide reliable templates for the gravitational waveforms in this regime. For this reason it has become very important to know if these predictions can actually be trusted to larger separation parameters (even in the region where the holes have distinct event horizons). The only way to extend the range of validity of the linear approximation is to develop the theory of second order perturbations around a Kerr hole, by generalizing the Teukolsky formalism.Comment: 6 pages, Latex, uses moriond.sty, proceedings of the talk given at the Moriond 99' euroconferenc

Information Spreading on Almost Torus Networks

Epidemic modeling has been extensively used in the last years in the field of telecommunications and computer networks. We consider the popular Susceptible-Infected-Susceptible spreading model as the metric for information spreading. In this work, we analyze information spreading on a particular class of networks denoted almost torus networks and over the lattice which can be considered as the limit when the torus length goes to infinity. Almost torus networks consist on the torus network topology where some nodes or edges have been removed. We find explicit expressions for the characteristic polynomial of these graphs and tight lower bounds for its computation. These expressions allow us to estimate their spectral radius and thus how the information spreads on these networks

New Algebraic Formulation of Density Functional Calculation

This article addresses a fundamental problem faced by the ab initio community: the lack of an effective formalism for the rapid exploration and exchange of new methods. To rectify this, we introduce a novel, basis-set independent, matrix-based formulation of generalized density functional theories which reduces the development, implementation, and dissemination of new ab initio techniques to the derivation and transcription of a few lines of algebra. This new framework enables us to concisely demystify the inner workings of fully functional, highly efficient modern ab initio codes and to give complete instructions for the construction of such for calculations employing arbitrary basis sets. Within this framework, we also discuss in full detail a variety of leading-edge ab initio techniques, minimization algorithms, and highly efficient computational kernels for use with scalar as well as shared and distributed-memory supercomputer architectures

The Evolution of Quantum Field Theory, From QED to Grand Unification

In the early 1970s, after a slow start, and lots of hurdles, Quantum Field Theory emerged as the superior doctrine for understanding the interactions between relativistic sub-atomic particles. After the conditions for a relativistic field theoretical model to be renormalizable were established, there were two other developments that quickly accelerated acceptance of this approach: first the Brout-Englert-Higgs mechanism, and then asymptotic freedom. Together, these gave us a complete understanding of the perturbative sector of the theory, enough to give us a detailed picture of what is now usually called the Standard Model. Crucial for this understanding were the strong indications and encouragements provided by numerous experimental findings. Subsequently, non-perturbative features of the quantum field theories were addressed, and the first proposals for completely unified quantum field theories were launched. Since the use of continuous symmetries of all sorts, together with other topics of advanced mathematics, were recognised to be of crucial importance, many new predictions were pointed out, such as the Higgs particle, supersymmetry and baryon number violation. There are still many challenges ahead.Comment: 25 pages in total. A contribution to: The Standard Theory up to the Higgs discovery - 60 years of CERN - L. Maiani and G. Rolandi, ed

Body language, security and e-commerce

Security is becoming an increasingly more important concern both at the desktop level and at the network level. This article discusses several approaches to authenticating individuals through the use of biometric devices. While libraries might not implement such devices, they may appear in the near future of desktop computing, particularly for access to institutional computers or for access to sensitive information. Other approaches to computer security focus on protecting the contents of electronic transmissions and verification of individual users. After a brief overview of encryption technologies, the article examines public-key cryptography which is getting a lot of attention in the business world in what is called public key infrastructure. It also examines other efforts, such as IBM’s Cryptolope, the Secure Sockets Layer of Web browsers, and Digital Certificates and Signatures. Secure electronic transmissions are an important condition for conducting business on the Net. These business transactions are not limited to purchase orders, invoices, and contracts. This could become an important tool for information vendors and publishers to control access to the electronic resources they license. As license negotiators and contract administrators, librarians need to be aware of what is happening in these new technologies and the impact that will have on their operations

Numerical Relativity: A review

Computer simulations are enabling researchers to investigate systems which are extremely difficult to handle analytically. In the particular case of General Relativity, numerical models have proved extremely valuable for investigations of strong field scenarios and been crucial to reveal unexpected phenomena. Considerable efforts are being spent to simulate astrophysically relevant simulations, understand different aspects of the theory and even provide insights in the search for a quantum theory of gravity. In the present article I review the present status of the field of Numerical Relativity, describe the techniques most commonly used and discuss open problems and (some) future prospects.Comment: 2 References added; 1 corrected. 67 pages. To appear in Classical and Quantum Gravity. (uses iopart.cls