Enabling a High Throughput Real Time Data Pipeline for a Large Radio Telescope Array with GPUs

The Murchison Widefield Array (MWA) is a next-generation radio telescope currently under construction in the remote Western Australia Outback. Raw data will be generated continuously at 5GiB/s, grouped into 8s cadences. This high throughput motivates the development of on-site, real time processing and reduction in preference to archiving, transport and off-line processing. Each batch of 8s data must be completely reduced before the next batch arrives. Maintaining real time operation will require a sustained performance of around 2.5TFLOP/s (including convolutions, FFTs, interpolations and matrix multiplications). We describe a scalable heterogeneous computing pipeline implementation, exploiting both the high computing density and FLOP-per-Watt ratio of modern GPUs. The architecture is highly parallel within and across nodes, with all major processing elements performed by GPUs. Necessary scatter-gather operations along the pipeline are loosely synchronized between the nodes hosting the GPUs. The MWA will be a frontier scientific instrument and a pathfinder for planned peta- and exascale facilities.Comment: Version accepted by Comp. Phys. Com

Apparent Superluminal Behavior

The apparent superluminal propagation of electromagnetic signals seen in recent experiments is shown to be the result of simple and robust properties of relativistic field equations. Although the wave front of a signal passing through a classically forbidden region can never move faster than light, an attenuated replica of the signal is reproduced ``instantaneously'' on the other side of the barrier. The reconstructed signal, causally connected to the forerunner rather than the bulk of the input signal, appears to move through the barrier faster than light.Comment: 8 pages, no figure

International graduates’ experiences of reflection in postgraduate training : a cross-sectional survey

Background: Reflection is a key component of postgraduate training in general practice (GP). International medical graduates (IMG) are thought to be less familiar with reflection, with international medical schools favouring more didactic methods of education. Aim: To explore IMGs’ experiences of reflection prior to and during GP training, and the support available for developing skills in reflection. Design and setting: A cross sectional survey was sent to IMGs undertaking GP training in 12 of the 14 UK regions from March to April 2021. Method: A pre-tested self-administered on-line questionnaire was used to collect data on experiences of reflection both prior to and during GP training, and support available for developing skills in reflection. Results: 485 of 3413 IMG trainees completed the questionnaire (14.2% response rate, representative of national demographics). 79.8% of participants reported no experience of reflection as an undergraduate and 36.9% reported no formal training in reflection during GP training. 69.7% of participants agreed that reflection was beneficial for their training and 58.3% reported that the best support in reflection came from their supervisors. Experience of reflection, opinions on the benefits, and best sources of support all varied by where respondents' primary medical qualification was obtained (all p-values <.01). Conclusion: Most IMGs have not experienced reflection prior to commencing UK GP training. There is diversity in experience and culture within this group which must be considered when tailoring educational interventions to support IMGs in their transition to UK GP training

Lorentz Invariant Superluminal Tunneling

It is shown that superluminal optical signalling is possible without violating Lorentz invariance and causality via tunneling through photonic band gaps in inhomogeneous dielectrics of a special kind.Comment: 10 pages revtex, no figure, more discussions added, submitted to Phys. Rev.

Investigating event-specific drought attribution using self-organizing maps

Previous studies evaluating anthropogenic influences on the meteorological drivers of drought have found mixed results owing to (1) the complex physical mechanisms which lead to the onset of drought, (2) differences in the characteristics and time scales of drought for different regions of the world, and (3) different approaches to the question of attribution. For a midlatitude, temperate climate like New Zealand, strongly modulated by oceanic influences, summer droughts last on the order of 3 months, and are less strongly linked to persistent temperature anomalies than continental climates. Here we demonstrate the utility of a novel approach for characterizing the meteorological conditions conducive to extreme drought over the North Island of New Zealand, using the January–March 2013 event as a case study. Specifically, we consider the use of self‐organizing map techniques in a multimember coupled climate model ensemble to capture changes in daily circulation, between two 41 year periods (1861–1901 and 1993–2033). Comparisons are made with seasonal pressure and precipitation indices. Our results demonstrate robust (>99% confidence) increases in the likelihood of observing circulation patterns like those of the 2013 drought in the recent‐climate simulations when compared with the early‐climate simulations. Best guess estimates of the fraction of attributable risk range from 0.2 to 0.4, depending on the metric used and threshold considered. Contributions to uncertainty in these attribution statements are discussed

Dispersive properties of quasi-phase-matched optical parametric amplifiers

The dispersive properties of non-degenerate optical parametric amplification in quasi-phase-matched (QPM) nonlinear quadratic crystals with an arbitrary grating profile are theoretically investigated in the no-pump-depletion limit. The spectral group delay curve of the amplifier is shown to be univocally determined by its spectral power gain curve through a Hilbert transform. Such a constraint has important implications on the propagation of spectrally-narrow optical pulses through the amplifier. In particular, it is shown that anomalous transit times, corresponding to superluminal or even negative group velocities, are possible near local minima of the spectral gain curve. A possible experimental observation of such effects using a QPM Lithium-Niobate crystal is suggested.Comment: submitted for publicatio

A Knob for Changing Light Propagation from Subluminal to Superluminal

We show how the application of a coupling field connecting the two lower metastable states of a lambda-system can produce a variety of new results on the propagation of a weak electromagnetic pulse. In principle the light propagation can be changed from subluminal to superluminal. The negative group index results from the regions of anomalous dispersion and gain in susceptibility.Comment: 6 pages,5 figures, typed in RevTeX, accepted in Phys. Rev.

Wave reflection at the origin of a first-generation branch artery and target organ protection: the AGES-Reykjavik study

Excessive pressure and flow pulsatility in first-generation branch arteries are associated with microvascular damage in high-flow organs like brain and kidneys. However, the contribution of local wave reflection and rereflection to microvascular damage remains controversial. Aortic flow, carotid pressure, flow and hydraulic power, brain magnetic resonance images, and cognitive scores were assessed in AGES-Reykjavik study participants without history of stroke, transient ischemic attack, or dementia (N=668, 378 women, 69-93 years of age). The aorta-carotid interface was generalized as a markedly asymmetrical bifurcation, with a large parent vessel (proximal aorta) branching into small (carotid) and large (distal aorta) daughter vessels. Local reflection coefficients were computed from aortic and carotid characteristic impedances. The bifurcation reflection coefficient, which determines pressure amplification in both daughter vessels, was low (0.06 +/- 0.03). The carotid flow transmission coefficient was low (0.11 +/- 0.04) and associated with markedly lower carotid versus aortic flow pulsatility (waveform SD, 7.2 +/- 2.0 versus 98.7 +/- 21.8 mL/s, P<0.001), pulsatility index (1.8 +/- 0.5 versus 4.5 +/- 0.6, P<0.001), and pulsatile power percentage (10 +/- 4% versus 25 +/- 5%, P<0.001). Transmitted as compared to incident pulsatile power (19.0 +/- 9.8 versus 35.9 +/- 17.8 mW, P<0.001) was further reduced by reflection (-4.3 +/- 2.7 mW) and rereflection (-12.5 +/- 8.1 mW) within the carotid. Higher carotid flow pulsatility correlated with lower white matter volume (R=-0.130, P<0.001) and lower memory scores (R=-0.161, P<0.001). Marked asymmetry of characteristic impedances at aorta-branch artery bifurcations limits amplification of pressure, markedly reduces absolute and relative pulsatility of transmitted flow and hydraulic power into first-generation branch arteries, and thereby protects the downstream local microcirculation from pulsatile damage.Neuro Imaging Researc

Shortest Path Problems on a Polyhedral Surface

We develop algorithms to compute shortest path edge sequences, Voronoi diagrams, the Fréchet distance, and the diameter for a polyhedral surface