G eant 4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of functionality including tracking, geometry, physics models and hits. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting software engineering and object-oriented technology and implemented in the C++ programming language. It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical physics.

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Geant4—a simulation toolkit

Software radios are emerging as platforms for multiband multimode personal communications systems. Radio etiquette is the set of RF bands, air interfaces, protocols, and spatial and temporal patterns that moderate the use of the radio spectrum. Cognitive radio extends the software radio with radio-domain model-based reasoning about such etiquettes. Cognitive radio enhances the flexibility of personal services through a radio knowledge representation language. This language represents knowledge of radio etiquette, devices, software modules, propagation, networks, user needs, and application scenarios in a way that supports automated reasoning about the needs of the user. This empowers software radios to conduct expressive negotiations among peers about the use of radio spectrum across fluents of space, time, and user context. With RKRL, cognitive radio agents may actively manipulate the protocol stack to adapt known etiquettes to better satisfy the user's needs. This transforms radio nodes from blind executors of predefined protocols to radio-domain-aware intelligent agents that search out ways to deliver the services the user wants even if that user does not know how to obtain them. Software radio provides an ideal platform for the realization of cognitive radio.

Cognitive radio: making software radios more personal

The study of networks pervades all of science, from neurobiology to statistical physics. The most basic issues are structural: how does one characterize the wiring diagram of a food web or the Internet or the metabolic network of the bacterium Escherichia coli? Are there any unifying principles underlying their topology? From the perspective of nonlinear dynamics, we would also like to understand how an enormous network of interacting dynamical systems-be they neurons, power stations or lasers-will behave collectively, given their individual dynamics and coupling architecture. Researchers are only now beginning to unravel the structure and dynamics of complex networks.

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Exploring complex networks

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Brownian motion in a field of force and the diffusion model of chemical reactions

This chapter describes the possibility of simulating physics in the classical approximation, a thing which is usually described by local differential equations. But the physical world is quantum mechanical, and therefore the proper problem is the simulation of quantum physics. A computer which will give the same probabilities as the quantum system does. The present theory of physics allows space to go down into infinitesimal distances, wavelengths to get infinitely great, terms to be summed in infinite order, and so forth; and therefore, if this proposition is right, physical law is wrong. Quantum theory and quantizing is a very specific type of theory. The chapter talks about the possibility that there is to be an exact simulation, that the computer will do exactly the same as nature. There are interesting philosophical questions about reasoning, and relationship, observation, and measurement and so on, which computers have stimulated people to think about anew, with new types of thinking.

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Simulating physics with computers

The paper presents a new approach to discretizing flow in porous media via mixed finite element methods on non-matching multiblock grids. The velocity space along the interfaces is enhanced to give flux-continuous approximation. No additional matching conditions need to be imposed. The computational complexity of the resulting algebraic problem is comparable to the one for the single-block case. A priori error estimates for the pressure and the velocity and numerical experiments confirming the theory are presented.

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Enhanced Velocity Mixed Finite Element Methods for Flow in Multiblock Domains

The complications of calculating the photoabsorption cross section of atoms for frequencies in the far ultraviolet and soft x-ray regions motivate an analysis of this problem on the basis of the Bloch semiclassical model of hydrodynamic charge oscillation in the neutral Thomas-Fermi atom. The hydrodynamic modes of oscillation of the neutral Thomas-Fermi atom have a continuous frequency spectrum. The resulting photoabsorption cross section is a continuous function of frequency which scales with atomic number $Z$ and in this sense is a universal cross section, approximately applicable to all heavy atoms. Numerical solutions of the normal mode functions of dipole charge oscillation are used to calculate the photoabsorption cross section in a range of photon energies $0.816 \mathrm{Z}\mathrm{eV}l\ensuremath{\Elzxh}\ensuremath{\omega}l272 \mathrm{Z}\mathrm{eV}$, the range where the model is expected to be most realistic. In this range the semiclassical hydrodynamic cross section agrees with experimental data for the noble gases as well as could be expected for a cross section applicable to all atoms. The model cross section, extended to zero and infinite frequencies by analytical calculation, checks the sum rule to within 2%; but gives a value $I=4.95$ $Z$ eV for the logarithmic mean excitation energy of stopping power formulas. The unrealistically low value of $\frac{I}{Z}$ results because the Thomas-Fermi atom exaggerates the number of electrons which absorb at low frequencies. Use of the hydrodynamic cross section in the approximate range of validity of this model $\ensuremath{\Elzxh}\ensuremath{\omega}g0.816$ Z eV gives $I=12.4{e}^{\ensuremath{\beta}}$ Z eV, where $\ensuremath{\beta}$ depends upon the oscillator strength below $\ensuremath{\Elzxh}\ensuremath{\omega}=0.816$ Z eV which cannot be satisfactorily determined from the Bloch model. Used within its limitations the Bloch model of hydrodynamic oscillation in the statistical atom provides a useful method of estimating photoabsorption cross sections and could possibly be applied to other atomic processes.

Photoabsorption and Charge Oscillation of the Thomas—Fermi Atom

Spacetime physics. an introduction to special relativity

Is Physics Legislated by Cosmogony

kind of a universe do we live in? A strange one, yes. But where does the strangeness mostly lie? In the seen? Or in the unseen? Shall we fix our attention on the billiard balls as they now bat about the billiard table? Or shall we ask how the game began, and how it will end? Who will not choose first to look about a little at the game? If then deeper questions come to mind, who shall stop our asking them?

John Archibald Wheeler

Papers

Enhanced Velocity Mixed Finite Element Methods for Flow in Multiblock Domains

Photoabsorption and Charge Oscillation of the Thomas—Fermi Atom

Spacetime physics. an introduction to special relativity

Is Physics Legislated by Cosmogony

Our universe: the known and the unknown.