Insect Pests of Christmas Trees

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Geodesics in the static Mallett spacetime

Mallett has exhibited a cylindrically symmetric spacetime containing closed timelike curves produced by a light beam circulating around a line singularity. I analyze the static version of this spacetime obtained by setting the intensity of the light to zero. Some null geodesics can escape to infinity, but all timelike geodesics in this spacetime originate and terminate at the singularity. Freely falling matter originally at rest quickly attains relativistic velocity inward and is destroyed at the singularity.Comment: 5 page

Novel structure formation of a phase separating colloidal fluid in a ratchet potential

Based on Dynamical Density Functional Theory (DDFT) we investigate a binary mixture of interacting Brownian particles driven over a substrate via a one-dimensional ratchet potential. The particles are modeled as soft spheres where one component carries a classical Heisenberg spin. In the absence of a substrate field, the system undergoes a first-order fluid-fluid demixing transition driven by the spin-spin interaction. We demonstrate that the interplay between the intrinsic spinodal decomposition and time-dependent external forces leads to a novel dynamical instability where stripes against the symmetry of the external potential form. This structural transition is observed for a broad range of parameters related to the ratchet potential. Moreover, we find intriguing effects for the particle transport.Comment: 6 pages, 4 figure

Noncommutative spaces and matrix embeddings on flat R^{2n+1}

We conjecture an embedding operator which assigns, to any 2n+1 hermitian matrices, a 2n-dimensional hypersurface in flat (2n + 1)-dimensional Euclidean space. This corresponds to precisely defining a fuzzy D(2n)-brane corresponding to N D0-branes. Points on the emergent hypersurface correspond to zero eigenstates of the embedding operator, which have an interpretation as coherent states underlying the emergent noncommutative geometry. Using this correspondence, all physical properties of the emergent D(2n)-brane can be computed. We apply our conjecture to noncommutative flat and spherical spaces. As a by-product, we obtain a construction of a rotationally symmetric flat noncommutative space in 4 dimensions.Comment: 14 pages, no figures. v2: added references and a clarificatio

Tunable structures of mixtures of magnetic particles in liquid-crystalline matrices

We investigate the self-organization of a binary mixture of similar sized rods and dipolar soft spheres by means of Monte-Carlo simulations. We model the interparticle interactions by employing anisotropic Gay-Berne, dipolar and soft-sphere interactions. In the limit of vanishing magnetic moments we obtain a variety of fully miscible liquid crystalline phases including nematic, smectic and lamellar phases. For the magnetic mixture, we find that the liquid crystalline matrix supports the formation of orientationally ordered ferromagnetic chains. Depending on the relative size of the species the chains align parallel or perpendicular to the director of the rods forming uniaxial or biaxial nematic, smectic and lamellar phases. As an exemplary external perturbation we apply a homogeneous magnetic field causing uniaxial or biaxial ordering to an otherwise isotropic state.Comment: 11 pages, 12 figure

Measuring the star formation rate with gravitational waves from binary black holes

A measurement of the history of cosmic star formation is central to understand the origin and evolution of galaxies. The measurement is extremely challenging using electromagnetic radiation: significant modeling is required to convert luminosity to mass, and to properly account for dust attenuation, for example. Here we show how detections of gravitational waves from inspiraling binary black holes made by proposed third-generation detectors can be used to measure the star formation rate of massive stars with high precision up to redshifts of ~10. Depending on the time-delay model, the predicted detection rates ranges from ~1400 to ~16000 per month with the current measurement of local merger rate density. With three months of observations, parameters describing the volumetric star formation rate can be constrained at the few percent level, and the volumetric merger rate can be directly measured to 3% at z~2. Given a parameterized star formation rate, the characteristic delay time between binary formation and merger can be measured to ~60%.Comment: 7 pages, 1 table, 4 fig

"If We Don't Produce, Bring Another:" Work Organization and Tomato Worker Health.

Objectives: Specific work processes and management structures that contribute to high rates of occupational illness and injury in agricultural industries are not well described in academic literature. This qualitative study of work organization in the U.S. fresh tomato industry investigates how work processes and management structures impact tomato workers' occupational health. Methods: After conducting literature review and key informant interviews, semi-structured interviews and focus groups were conducted with 36 individuals with experience working in the U.S. fresh tomato industry. Interviews and focus groups were audio-recorded, transcribed, coded, and analyzed using a modified grounded theory approach. Results: These data indicate that participants endured income insecurity and hazardous supervisory practices, including wage theft, retaliation, intimidation, and humiliation, that put them at risk of preventable illness and injury. Support from workers' organizations and health-conscious supervisory practices helped mitigate some of these occupational hazards. Conclusion: Participants' adverse work experiences may be considered sequelae of workers' lack of job control and positions of socioeconomic structural vulnerability. Other aspects of tomato work organization, including health-conscious supervisory practices and the involvement of workers' organizations, indicate that modifying work organization to better safeguard health is possible. Such modifications present compelling opportunities for employers, employees, organizations, community and government leaders, and health care professionals to help create healthier occupational environments for tomato workers