973 research outputs found

    Effective Constraints for Relativistic Quantum Systems

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    Determining the physical Hilbert space is often considered the most difficult but crucial part of completing the quantization of a constrained system. In such a situation it can be more economical to use effective constraint methods, which are extended here to relativistic systems as they arise for instance in quantum cosmology. By side-stepping explicit constructions of states, such tools allow one to arrive much more feasibly at results for physical observables at least in semiclassical regimes. Several questions discussed recently regarding effective equations and state properties in quantum cosmology, including the spreading of states and quantum back-reaction, are addressed by the examples studied here.Comment: 27 pages, 2 figures; v2: new appendix comparing effective constraints and physical coherent states by an exampl

    Particle Weights and their Disintegration I

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    The notion of Wigner particles is attached to irreducible unitary representations of the Poincare group, characterized by parameters m and s of mass and spin, respectively. However, the Lorentz symmetry is broken in theories with long-range interactions, rendering this approach inapplicable (infraparticle problem). A unified treatment of both particles and infraparticles via the concept of particle weights can be given within the framework of Local Quantum Physics. They arise as temporal limits of physical states in the vacuum sector and describe the asymptotic particle content. In this paper their definition and characteristic properties are worked out in detail. The existence of the temporal limits is established by use of suitably defined seminorms which are also essential in proving the characteristic features of particle weights.Comment: 33 pages, amslatex, mathptm, minor corrections including numbering schem

    Automated Ground Truth Estimation For Automotive Radar Tracking Applications With Portable GNSS And IMU Devices

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    Baseline generation for tracking applications is a difficult task when working with real world radar data. Data sparsity usually only allows an indirect way of estimating the original tracks as most objects' centers are not represented in the data. This article proposes an automated way of acquiring reference trajectories by using a highly accurate hand-held global navigation satellite system (GNSS). An embedded inertial measurement unit (IMU) is used for estimating orientation and motion behavior. This article contains two major contributions. A method for associating radar data to vulnerable road user (VRU) tracks is described. It is evaluated how accurate the system performs under different GNSS reception conditions and how carrying a reference system alters radar measurements. Second, the system is used to track pedestrians and cyclists over many measurement cycles in order to generate object centered occupancy grid maps. The reference system allows to much more precisely generate real world radar data distributions of VRUs than compared to conventional methods. Hereby, an important step towards radar-based VRU tracking is accomplished.Comment: 10 pages, 9 figures, accepted paper for 2019 20th International Radar Symposium (IRS), Ulm, Germany, June 2019. arXiv admin note: text overlap with arXiv:1905.1121

    Low Cost Propulsion Development for Small Satellites at the Surrey Space Centre

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    The Surrey Space Centre (SSC) has led the way in demonstrating the utility of microsatellite size spacecraft for research, humanitarian, commercial, and military applications. SSC recognises that cost effective propulsion technology for small spacecraft is an enabling technology for expanding the utility of these assets and has been actively researching this field since 1993. This paper provides an overview of propulsion research and development at the Surrey Space Centre. The paper will summarise SSC goals for small spacecraft propulsion technology and link them to areas of propulsion research past, present and future. A review of Surrey\u27s propulsion history to include hybrid, monopropellant, cold gas and resistojet technology is presented. Design and integration of SSC cold gas and resistojet technologies on flight spacecraft will also be covered with an emphasis on the SSC low cost approach to qualification, integration and operation of these systems. These topics will be followed by a discussion of areas that are currently being investigated for near term research, specifically, H202 long term storage, expulsion, catalysis, Green monopropellant and hybrid technology utilising both N20 and H202. One topic covered in detail is a novel alternative geometry hybrid rocket motor. This motor is currently under development to provide a low-cost, intrinsically-safe and easy to integrate orbital upper-stage for small spacecraft. A prototype motor has been constructed and test results are presented

    Effective Constraints and Physical Coherent States in Quantum Cosmology: A Numerical Comparison

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    A cosmological model with a cyclic interpretation is introduced, which is subject to quantum back-reaction and yet can be treated rather completely by physical coherent state as well as effective constraint techniques. By this comparison, the role of quantum back-reaction in quantum cosmology is unambiguously demonstrated. Also the complementary nature of strengths and weaknesses of the two procedures is illustrated. Finally, effective constraint techniques are applied to a more realistic model filled with radiation, where physical coherent states are not available.Comment: 32 pages, 25 figure

    On The KMS Condition for the critical Ising model

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    Using the KMS condition and exchange algebras we discuss the monodromy and modular properties of two-point KMS states of the critical Ising model.Comment: 8 pages, TcilaTe

    The Colour of Quarks

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    It is shown that colour can only be defined on gauge invariant states. Since the ability to associate colour with constituent quarks is an integral part of the constituent quark model, this means that, if we want to extract constituent quarks from QCD, we need to dress Lagrangian quarks with gluons so that the result is gauge invariant. We further prove that gauge fixings can be used to construct such dressings. Gauge invariant dressed quark states are presented and a direct approach to the interquark potential is discussed. Some further aspects of dressing quarks are briefly discussed.Comment: Extended version, to appear in Phys. Lett B., 7 pages, Te

    Topological States on the Gold Surface

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    Gold surfaces host special electronic states that have been understood as a prototype of Shockley surface states (SSs). These SSs are commonly employed to benchmark the capability of angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy. We find that these Shockley SSs can be reinterpreted as topologically derived surface states (TDSSs) of a topological insulator (TI), a recently discovered quantum state. Based on band structure calculations, the Z2 topological invariant can be well defined to characterize the nontrivial features of gold that we detect by ARPES. The same TDSSs are also recognized on surfaces of other well-known noble metals (e.g., silver, copper, platinum, and palladium). Besides providing a new understanding of noble metal SSs, finding topological states on late transition metals provokes interesting questions on the role of topological effects in surface-related processes, such as adsorption and catalysis.Comment: 21 pages, 3 figure

    Droit naturel et histoire dans la philosophie de Diderot

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    Parce qu’il glisse du concept de nature à celui de matière vivante et animée, à l’intérieur d’une philosophie qui refuse tout recours à la finalité, Diderot ne peut que faire coïncider le droit naturel avec le fait du devenir, tel qu’il est découvert et construit par les philosophies « rationnelles » et « expérimentales » de son temps. Dès lors, la contradiction qu’il ne cesse de dénoncer, entre le code naturel, d’une part, les « codes civil et religieux », d’autre part, se ramène à l’opposition du droit individuel et collectif à construire l’histoire, et d’une civilisation qui impose aux hommes l’idéal du repos, du renoncement, de l’ascétisme moral ou intellectuel, voire de l’obéissance passive. Dans le cadre de cet article, nous nous sommes bornés à étudier cette opposition sur le plan de l’histoire de la connaissance, les « génies » devenant exemplaires de la capacité de l’homme à résister au despotisme des codes civil et religieux, de telle sorte que Diderot invente, avec Nietzsche, une histoire monumentale des génies, histoire qui est au service de la vie, et qui préserve la possibilité d’un perfectionnement humain ou d’un devenir ascendant de la rationalité. Dans cette perspective, on peut désormais comprendre que le génie ne saurait être évalué à travers l’opposition unilatérale du bien et du mal, ou du vrai et du faux.Natural law and history in Diderot’s philosophyDiderot slides from the concept of nature to that of living animated matter in the context of a philosophy which refuses any recourse to finalism; thus is forced to equate natural law with possibility as it was discovered and invented by the ‘rational’ and ‘esperimental’ philosophies of his time. Henceforth the contradiction which he unceasingly denounces between the natural code on the one hand and the ‘civil and religious codes’ on the other coincides with an opposition between the individual and collective right to construct history and a civilisation which imposes on humans the ideal of inaction, renunciation, moral or intellectual asceticism and even passive obedience. This article studies this opposition in the field of the history of knowledge, where geniuses exemplify human capacity to resist the despotism of the civil and religious codes ; thus Diderot invents, before Nietzsche, a monumental history of geniuses which serves life and preserves the possibility of human perfection or a development of rationality. This enables us to understand that genius should not be judged in terms of a unilateral opposition between good and evil, true and false
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