Book Review of Joshua M. Epstein & Robert Axtell, Growing Artificial Societies: Social Science from the Bottom Up (MIT Press 1996)

Review of the book Joshua M. Epstein & Robert Axtell, Growing Artificial Societies: Social Science from the Bottom Up (MIT Press 1996). Acknowledgements, appendices, index, introduction references. LC 96-25332; ISBN 0-262-05053-6. [208 pp. Cloth $39.95. 55 Hayward St., Cambridge MA 02142.

Gravitational radiation reaction and second order perturbation theory

A point particle of small mass m moves in free fall through a background vacuum spacetime metric g_ab and creates a first-order metric perturbation h^1ret_ab that diverges at the particle. Elementary expressions are known for the singular m/r part of h^1ret_ab and for its tidal distortion determined by the Riemann tensor in a neighborhood of m. Subtracting this singular part h^1S_ab from h^1ret_ab leaves a regular remainder h^1R_ab. The self-force on the particle from its own gravitational field adjusts the world line at O(m) to be a geodesic of g_ab+h^1R_ab. The generalization of this description to second-order perturbations is developed and results in a wave equation governing the second-order h^2ret_ab with a source that has an O(m^2) contribution from the stress-energy tensor of m added to a term quadratic in h^1ret_ab. Second-order self-force analysis is similar to that at first order: The second-order singular field h^2S_ab subtracted from h^2ret_ab yields the regular remainder h^2R_ab, and the second-order self-force is then revealed as geodesic motion of m in the metric g_ab+h^1R+h^2R.Comment: 7 pages, conforms to the version submitted to PR

Self-force via a Green's function decomposition

The gravitational field of a particle of small mass \mu moving through curved spacetime is naturally decomposed into two parts each of which satisfies the perturbed Einstein equations through O(\mu). One part is an inhomogeneous field which, near the particle, looks like the \mu/r field distorted by the local Riemann tensor; it does not depend on the behavior of the source in either the infinite past or future. The other part is a homogeneous field and includes the ``tail term''; it completely determines the self force effects of the particle interacting with its own gravitational field, including radiation reaction. Self force effects for scalar, electromagnetic and gravitational fields are all described in this manner.Comment: PRD, in press. Enhanced emphasis on the equivalence principl

Regularization of fields for self-force problems in curved spacetime: foundations and a time-domain application

We propose an approach for the calculation of self-forces, energy fluxes and waveforms arising from moving point charges in curved spacetimes. As opposed to mode-sum schemes that regularize the self-force derived from the singular retarded field, this approach regularizes the retarded field itself. The singular part of the retarded field is first analytically identified and removed, yielding a finite, differentiable remainder from which the self-force is easily calculated. This regular remainder solves a wave equation which enjoys the benefit of having a non-singular source. Solving this wave equation for the remainder completely avoids the calculation of the singular retarded field along with the attendant difficulties associated with numerically modeling a delta function source. From this differentiable remainder one may compute the self-force, the energy flux, and also a waveform which reflects the effects of the self-force. As a test of principle, we implement this method using a 4th-order (1+1) code, and calculate the self-force for the simple case of a scalar charge moving in a circular orbit around a Schwarzschild black hole. We achieve agreement with frequency-domain results to ~ 0.1% or better.Comment: 15 pages, 12 figures, 1 table. More figures, extended summar

High isolation RF signal selection switches

A selection switch with high isolation between RF signal input terminals is achieved with a gated Schmitt trigger circuit feeding into a control NAND gate in each signal switching channel. The control NAND gates of the separate signal channels are coupled to an output terminal by a single NAND gate. The schmitt trigger circuits and all gates are implemented with Schottky transistor-transistor logic circuits having input clamping diodes. Each Schmitt trigger circuit includes two cascaded NAND gates and a feedback isolation Schottky diode between one input terminal connected to receive an RF input and another input terminal connected to receive a feedback signal from the second of the two cascaded NAND gates. Both NAND gates of the Schmitt trigger circuits are enabled by the same switch control signal which enables the control gates

Power-over-Tether UAS Leveraged for Nearly Indefinite Meteorological Data Acquisition In the Platte River Basin

The integration of unmanned aerial systems (UASs) has increased in the field of agriculture. These systems can provide data that was previously difficult to obtain to help increase efficiency and production. Typical commercial off the shelf (COTS) UASs have significant limitations in the form of small payloads, and short flight times which inhibit their ability to provide significant quantities of useful data. We present the development of a novel power-over-tether UAS that leverages the physical presence of the tether to integrate sensors at multiple altitudes along the tether. The UAS can acquire data nearly indefinitely to sense atmospheric conditions and gradients along the tether. We present the development of the prototyped system, along with the results of field experiments where we demonstrate 6 hours of continuous flight at 50 feet altitude, and a 1 hour flight at sunset to acquire atmospheric temperature from an array of sensors. An evaluation of the systems performance is presented along with a discussion of the systems future implications

Self-force with (3+1) codes: a primer for numerical relativists

Prescriptions for numerical self-force calculations have traditionally been designed for frequency-domain or (1+1) time-domain codes which employ a mode decomposition to facilitate in carrying out a delicate regularization scheme. This has prevented self-force analyses from benefiting from the powerful suite of tools developed and used by numerical relativists for simulations of the evolution of comparable-mass black hole binaries. In this work, we revisit a previously-introduced (3+1) method for self-force calculations, and demonstrate its viability by applying it to the test case of a scalar charge moving in a circular orbit around a Schwarzschild black hole. Two (3+1) codes originally developed for numerical relativity applications were independently employed, and in each we were able to compute the two independent components of the self-force and the energy flux correctly to within $< 1$. We also demonstrate consistency between $t$-component of the self-force and the scalar energy flux. Our results constitute the first successful calculation of a self-force in a (3+1) framework, and thus open opportunities for the numerical relativity community in self-force analyses and the perturbative modeling of extreme-mass-ratio inspirals.Comment: 23 pages, 13 figure

Why Inaccessibility? Despite progressive tone, attacks on academics’ lack of clarity can be profoundly regressive.

It has become popular to denounce academic writing as elitist and unhelpful. Eric Detweiler argues that inaccessibility may be a more complex issue. “Inaccessible” writing may be the result of an author trying to do things with language that conventional, “clear” uses of language cannot. Furthermore, these critiques are often launched at marginalised fields that are writing in non-standard ways in an attempt to critique staunch elitism. Academics tossing blame at each other can be part of the problem rather than the solution