Electrodynamic Radiation Reaction and General Relativity

We argue that the well-known problem of the instabilities associated with the self-forces (radiation reaction forces) in classical electrodynamics are possibly stabilized by the introduction of gravitational forces via general relativity

Mechanisms for Lasing with Cold Atoms as the Gain Medium

We realize a laser with a cloud of cold rubidium atoms as gain medium, placed in a low-finesse cavity. Three different regimes of laser emission are observed corresponding respectively to Mollow, Raman and Four Wave Mixing mechanisms. We measure an output power of up to 300 $\mu$W and present the main properties of these different lasers in each regime

Fe I Oscillator Strengths for the Gaia-ESO Survey

The Gaia-ESO Public Spectroscopic Survey (GES) is conducting a large-scale study of multi-element chemical abundances of some 100 000 stars in the Milky Way with the ultimate aim of quantifying the formation history and evolution of young, mature and ancient Galactic populations. However, in preparing for the analysis of GES spectra, it has been noted that atomic oscillator strengths of important Fe I lines required to correctly model stellar line intensities are missing from the atomic database. Here, we present new experimental oscillator strengths derived from branching fractions and level lifetimes, for 142 transitions of Fe I between 3526 {\AA} and 10864 {\AA}, of which at least 38 are urgently needed by GES. We also assess the impact of these new data on solar spectral synthesis and demonstrate that for 36 lines that appear unblended in the Sun, Fe abundance measurements yield a small line-by-line scatter (0.08 dex) with a mean abundance of 7.44 dex in good agreement with recent publications.Comment: Accepted for publication in Mon. Not. R. Astron. So

The influence of electron collisions on non-LTE Li line formation in stellar atmospheres

The influence of the uncertainties in the rate coefficient data for electron-impact excitation and ionization on non-LTE Li line formation in cool stellar atmospheres is investigated. We examine the electron collision data used in previous non-LTE calculations and compare them to recent calculations that use convergent close-coupling (CCC) techniques and to our own calculations using the R-matrix with pseudostates (RMPS) method. We find excellent agreement between rate coefficients from the CCC and RMPS calculations, and reasonable agreement between these data and the semi-empirical data used in non-LTE calculations up to now. The results of non-LTE calculations using the old and new data sets are compared and only small differences found: about 0.01 dex (~ 2%) or less in the abundance corrections. We therefore conclude that the influence on non-LTE calculations of uncertainties in the electron collision data is negligible. Indeed, together with the collision data for the charge exchange process Li(3s) + H Li^+ + H^- now available, and barring the existence of an unknown important collisional process, the collisional data in general is not a source of significant uncertainty in non-LTE Li line formation calculations.Comment: 8 pages, accepted by Astronomy and Astrophysics; Replaced with minor corrections following proof

Voice Flows To And Around Leaders: Understanding When Units Are Helped Or Hurt By Employee Voice

In two studies, we develop and test theory about the relationship between speaking up, one type of organizational citizenship behavior, and unit performance by accounting for where employee voice is flowing. Results from a qualitative study of managers and professionals across a variety of industries suggest that voice to targets at different formal power levels (peers or superiors) and locations in the organization (inside or outside a focal unit) differs systematically in terms of its usefulness in generating actions to a unit's benefit on the issues raised and in the likely information value of the ideas expressed. We then theorize how distinct voice flows should be differentially related to unit performance based on these core characteristics and test our hypotheses using time-lagged field data from 801 employees and their managers in 93 units across nine North American credit unions. Results demonstrate that voice flows are positively related to a unit's effectiveness when they are targeted at the focal leader of that unitwho should be able to take actionwhether from that leader's own subordinates or those in other units, and negatively related to a unit's effectiveness when they are targeted at coworkers who have little power to effect change. Together, these studies provide a structural framework for studying the nature and impact of multiple voice flows, some along formal reporting lines and others that reflect the informal communication structure within organizations. This research demonstrates that understanding the potential performance benefits and costs of voice for leaders and their units requires attention to the structure and complexity of multiple voice flows rather than to an undifferentiated amount of voice.Business Administratio

Entropy and the variational principle for actions of sofic groups

Recently Lewis Bowen introduced a notion of entropy for measure-preserving actions of a countable sofic group on a standard probability space admitting a generating partition with finite entropy. By applying an operator algebra perspective we develop a more general approach to sofic entropy which produces both measure and topological dynamical invariants, and we establish the variational principle in this context. In the case of residually finite groups we use the variational principle to compute the topological entropy of principal algebraic actions whose defining group ring element is invertible in the full group C*-algebra.Comment: 44 pages; minor changes; to appear in Invent. Mat

New approaches to model and study social networks

We describe and develop three recent novelties in network research which are particularly useful for studying social systems. The first one concerns the discovery of some basic dynamical laws that enable the emergence of the fundamental features observed in social networks, namely the nontrivial clustering properties, the existence of positive degree correlations and the subdivision into communities. To reproduce all these features we describe a simple model of mobile colliding agents, whose collisions define the connections between the agents which are the nodes in the underlying network, and develop some analytical considerations. The second point addresses the particular feature of clustering and its relationship with global network measures, namely with the distribution of the size of cycles in the network. Since in social bipartite networks it is not possible to measure the clustering from standard procedures, we propose an alternative clustering coefficient that can be used to extract an improved normalized cycle distribution in any network. Finally, the third point addresses dynamical processes occurring on networks, namely when studying the propagation of information in them. In particular, we focus on the particular features of gossip propagation which impose some restrictions in the propagation rules. To this end we introduce a quantity, the spread factor, which measures the average maximal fraction of nearest neighbors which get in contact with the gossip, and find the striking result that there is an optimal non-trivial number of friends for which the spread factor is minimized, decreasing the danger of being gossiped.Comment: 16 Pages, 9 figure