The Teleparallel Equivalent of Newton-Cartan Gravity

We construct a notion of teleparallelization for Newton-Cartan theory, and show that the teleparallel equivalent of this theory is Newtonian gravity; furthermore, we show that this result is consistent with teleparallelization in general relativity, and can be obtained by null-reducing the teleparallel equivalent of a five-dimensional gravitational wave solution. This work thus strengthens substantially the connections between four theories: Newton-Cartan theory, Newtonian gravitation theory, general relativity, and teleparallel gravity

The Teleparallel Equivalent of Newton-Cartan Gravity

We construct a notion of teleparallelization for Newton-Cartan theory, and show that the teleparallel equivalent of this theory is Newtonian gravity; furthermore, we show that this result is consistent with teleparallelization in general relativity, and can be obtained by null-reducing the teleparallel equivalent of a five-dimensional gravitational wave solution. This work thus strengthens substantially the connections between four theories: Newton-Cartan theory, Newtonian gravitation theory, general relativity, and teleparallel gravity.Comment: 7 pages, forthcoming in Classical and Quantum Gravity (letters

The Teleparallel Equivalent of Newton-Cartan Gravity

We construct a notion of teleparallelization for Newton-Cartan theory, and show that the teleparallel equivalent of this theory is Newtonian gravity; furthermore, we show that this result is consistent with teleparallelization in general relativity, and can be obtained by null-reducing the teleparallel equivalent of a five-dimensional gravitational wave solution. This work thus strengthens substantially the connections between four theories: Newton-Cartan theory, Newtonian gravitation theory, general relativity, and teleparallel gravity

The local validity of special relativity: the standard story articulated and defended

Morally, the `standard story' that special relativity is locally valid in general relativity is surely correct. However, rendering the standard story precise and unambiguous has proved to be a vexed issue. In this article, we articulate and defend the standard story from an effective field theory (EFT)-inspired perspective

Why surplus structure is not superfluous

The idea that gauge theory has `surplus' structure poses a puzzle: in one much discussed sense, this structure is redundant; but on the other hand, it is also widely held to play an essential role in the theory. In this paper, we employ category-theoretic tools to illuminate an aspect of this puzzle. We precisify what is meant by `surplus' structure by means of functorial comparisons with equivalence classes of gauge fields, and then show that such structure is essential for any theory that represents a rich collection of physically relevant fields which are `local' in nature

Rheumatoid arthritis : what do MRI and ultrasound show

Rheumatoid arthritis is the most common inflammatory arthritis, affecting approximately 1% of the world's population. Its pathogenesis has not been completely understood. However, there is evidence that the disease may involve synovial joints, subchondral bone marrow as well as intra-and extraarticular fat tissue, and may lead to progressive joint destruction and disability. Over the last two decades, significant improvement in its prognosis has been achieved owing to new strategies for disease management, the emergence of new biologic therapies and better utilization of conventional disease-modifying antirheumatic drugs. Prompt diagnosis and appropriate therapy have been recognized as essential for improving clinical outcomes in patients with early rheumatoid arthritis. Despite the potential of ultrasonography and magnetic resonance imaging to visualize all tissues typically involved in the pathogenesis of rheumatoid arthritis, the diagnosis of early disease remains difficult due to limited specificity of findings. This paper summarizes the pathogenesis phenomena of rheumatoid arthritis and describes rheumatoid arthritis-related features of the disease within the synovium, subchondral bone marrow and articular fat tissue on MRI and ultrasound. Moreover, the paper aims to illustrate the significance of MRI and ultrasound findings in rheumatoid arthritis in the diagnosis of subclinical and early inflammation, and the importance of MRI and US in the follow-up and establishing remission. Finally, we also discuss MRI of the spine in rheumatoid arthritis, which may help assess the presence of active inflammation and complications

Cross-aisle seismic performance of selective storage racks

© 2020 A series of single-axis shaking table tests were conducted on three full-scale selective storage racks in the cross-aisle direction. The uplifting and rocking behaviour of the racks was examined under three baseplate types: ductile, heavy duty, and unanchored. Each rack was subjected to a sequence of ground motions of increasing intensity up to failure, with a total of 29 tests conducted. At 1.5 times the respective design level ground motions, the heavy duty baseplates caused a foundation failure while the unanchored rack failed by overturning. The rack with ductile baseplates survived all tests up to 2.3 times the design level. For a given ground motion, the unanchored rack upright always had the smallest peak axial load. However, the unanchored rack had much larger sways under the Northbridge and Kobe ground motions. The NZS 1170.5 equivalent static method design loading was found to be overly conservative for racks with ductile and heavy duty baseplates, of which the upright design axial forces were better predicted using the refined equivalent static method

Distral: Robust Multitask Reinforcement Learning

Most deep reinforcement learning algorithms are data inefficient in complex and rich environments, limiting their applicability to many scenarios. One direction for improving data efficiency is multitask learning with shared neural network parameters, where efficiency may be improved through transfer across related tasks. In practice, however, this is not usually observed, because gradients from different tasks can interfere negatively, making learning unstable and sometimes even less data efficient. Another issue is the different reward schemes between tasks, which can easily lead to one task dominating the learning of a shared model. We propose a new approach for joint training of multiple tasks, which we refer to as Distral (Distill & transfer learning). Instead of sharing parameters between the different workers, we propose to share a "distilled" policy that captures common behaviour across tasks. Each worker is trained to solve its own task while constrained to stay close to the shared policy, while the shared policy is trained by distillation to be the centroid of all task policies. Both aspects of the learning process are derived by optimizing a joint objective function. We show that our approach supports efficient transfer on complex 3D environments, outperforming several related methods. Moreover, the proposed learning process is more robust and more stable---attributes that are critical in deep reinforcement learning

RasGRP1 Transduces Low-Grade TCR Signals which Are Critical for T Cell Development, Homeostasis, and Differentiation

AbstractTwo important Ras-guanyl nucleotide exchange factors, Sos and RasGRP1, control Ras activation in thymocytes. However, the relative contribution of these two exchange factors to Ras/ERK activation and their resulting impact on positive and negative selection is unclear. We have produced two lines of RasGRP1−/− TCR transgenic mice to determine the effect of RasGRP1 in T cell development under conditions of defined TCR signaling. Our results demonstrate that RasGRP1 is crucial for thymocytes expressing weakly selecting TCRs whereas those that express stronger selecting TCRs are more effective at utilizing RasGRP1-independent mechanisms for ERK activation and positive selection. Analysis of RasGRP1−/− peripheral T cells also revealed hitherto unidentified functions of RasGRP1 in regulating T cell homeostasis and sustaining antigen-induced developmental programming