On Radar Time and the Twin `Paradox'

In this paper we apply the concept of radar time (popularised by Bondi in his work on k-calculus) to the well-known relativistic twin `paradox'. Radar time is used to define hypersurfaces of simultaneity for a class of travelling twins, from the `Immediate Turn-around' case, through the `Gradual Turn-around' case, to the `Uniformly Accelerating' case. We show that this definition of simultaneity is independent of choice of coordinates, and assigns a unique time to any event (with which the travelling twin can send and receive signals), resolving some common misconceptions.Comment: 9 pages, 10 figures. Minor changes (includes minor corrections not in published version

Continuous Time Monte Carlo for Lattice QCD in the Strong Coupling Limit

We present results for lattice QCD in the limit of infinite gauge coupling, obtained from a worm-type Monte Carlo algorithm on a discrete spatial lattice but with continuous Euclidean time. This is obtained by sending both the anisotropy parameter gamma^2 \sim a/a_t and the number of time-slices N_\tau to infinity, keeping the ratio \gamma^2/N_\tau \sim aT fixed. The obvious gain is that no continuum extrapolation N_\tau -> \infty has to be carried out. Moreover, the algorithm is faster and the sign problem disappears. We compare our computations with those on discrete lattices. We determine the phase diagram as a function of temperature and baryon chemical potential.Comment: 4 pages, Proceedings for Quark Matter 2011 Conference, May 23-28, 2011, Annecy, Franc

Superfluid gap formation in a fermionic optical lattice with spin imbalanced populations

We investigate the attractive Hubbard model in infinite spatial dimensions at quarter filling. By combining dynamical mean-field theory with continuous-time quantum Monte Carlo simulations in the Nambu formalism, we directly deal with the superfluid phase in the population imbalanced system. We discuss the low energy properties in the polarized superfluid state and the pseudogap behavior in the vicinity of the critical temperature.Comment: 4 pages, 1 figure, To appear in J. Phys.: Conf. Ser. for SCES201

The 51.8 micron (0 3) line emission observed in four galactic H 2 regions

The (0 III) 51.8 microns line from four H II regions, M42, M17, W51 and NGC 6375A was detected. Respective line strengths are 7 x 10 to the minus 15 power, 1.0 x 10 to the minus 14 power, 2.1 x 10 to the minus 15 power and 2.6 x 10 to the minus 15 power watt cm/2. Observations are consistent with previously reported line position and place the line at 51.80 + or 0.05 micron. When combined with the 88.35 microns (0 III) reported earlier, clumping seems to be an important factor in NGC 6375A and M42 and to a lesser extent in W51 and M17. The combined data also suggest an (0 III) abundance of approximately 3 x 0.0001 sub n e' a factor of 2 greater than previously assumed

opendf - an implementation of the dual fermion method for strongly correlated systems

The dual fermion method is a multiscale approach for solving lattice problems of interacting strongly correlated systems. In this paper, we present the \texttt{opendf} code, an open-source implementation of the dual fermion method applicable to fermionic single-orbital lattice models in dimensions $D=1,2,3$ and $4$. The method is built on a dynamical mean field starting point, which neglects all local correlations, and perturbatively adds spatial correlations. Our code is distributed as an open-source package under the GNU public license version 2.Comment: 7 pages, 6 figures, 28th Annual CSP Workshop proceeding

Revisited abundance diagnostics in quasars: Fe II/Mg II ratios

Both the Fe II UV emission in the 2000- 3000 A region [Fe II (UV)] and resonance emission line complex of Mg II at 2800 A are prominent features in quasar spectra. The observed Fe II UV/ Mg II emission ratios have been proposed as means to measure the buildup of the Fe abundance relative to that of the alpha-elements C, N, O, Ne and Mg as a function of redshift. The current observed ratios show large scatter and no obvious dependence on redshift. Thus, it remains unresolved whether a dependence on redshift exists and whether the observed Fe II UV/ Mg II ratios represent a real nucleosynthesis diagnostic. We have used our new 830-level model atom for Fe+ in photoionization calculations, reproducing the physical conditions in the broad line regions of quasars. This modeling reveals that interpretations of high values of Fe II UV/ Mg II are sensitive not only to Fe and Mg abundance, but also to other factors such as microturbulence, density, and properties of the radiation field. We find that the Fe II UV/ Mg II ratio combined with Fe II (UV)/ Fe II (Optical) emission ratio, where Fe II (Optical) denotes Fe II emission in 4000 - 6000 A can be used as a reliable nucleosynthesis diagnostic for the Fe/Mg abundance ratios for the physical conditions relevant to the broad-line regions (BLRs) of quasars. This has extreme importance for quasar observations with the Hubble Space Telescope and also with the future James Webb Space Telescope.Comment: kverner.gzip, 9 pages, f1-5.eps; aastex.cls; aastexug.sty, ApJL in pres