729 research outputs found
Classical electrodynamics of point charges
A simple mathematical procedure is introduced which allows redefining in an
exact way divergent integrals and limits that appear in the basic equations of
classical electrodynamics with point charges. In this way all divergences are
at once removed without affecting the locality and the relativistic covariance
of the theory, and with no need for mass renormalization. The procedure is
first used to obtain a finite expression for the electromagnetic
energy-momentum of the system. We show that the relativistic Lorentz-Dirac
equation can be deduced from the conservation of this electromagnetic
energy-momentum plus the usual mechanical term. Then we derive a finite
lagrangian, which depends on the particle variables and on the actual
electromagnetic potentials at a given time. From this lagrangian the equations
of motion of both particles and fields can be derived via Hamilton's
variational principle. The hamiltonian formulation of the theory can be
obtained in a straightforward way. This leads to an interesting comparison
between the resulting divergence-free expression of the hamiltonian functional
and the standard renormalization rules for perturbative quantum
electrodynamics.Comment: 46 pages, REVTeX . Revised version with added comments and reference
IT-toepassingen in de gezondheidszorg - Bewezen procesverbeteringen in het ziekenhuis.
In this thesis we tried to answer the following research question: âWhat are the proven effects of IT applications with respect to how they increase efficiency in health care processes?"
To answer this question we searched for IT applications which claim to increase the efficiency of health care processes. Today, many IT applications are used in hospitals, but not all of them were suitable to study in this thesis. Therefore we selected four IT applications (EHR, CDSS, CPOE and Scheduling) to investigate. Our research consisted of two parts, first an intensive literature study. Second, we interviewed several IT application suppliers.
We compared the results of those two studies to come up with final conclusions. The main problem we tried to investigate in this study is the strength of proof which can be given for some of the claimed effects of the selected IT applications which are applied in hospital environments.
From our literature study it appears that only a small amount of effects have been reasonably proven, most of them are bared on just a meager amount of prove. This thesis gives an indication of the current state of the art with respect to available research in this area.
The main finding from this thesis is that the increase of efficiency of health care processes due to the use of IT applications has not been proven in literature. Only a few effects have been reasonably proven according to the literature study, most effects have a poor amount of prove. However, the fact that there is no strong proof doesn't mean that the concerning effects will not occur at all, there is just not enough evidence to draw the conclusion that they do occur.
The results from this study are, among others, that the impression has been given that health care in general hasn't much interest in checking what the results are from IT applications. This is strange considering that these IT applications get more and more functions
which no longer concern just making letters, but also involve critical processes and sensitive information
Hamiltonian Formulation of Two Body Problem in Wheeler-Feynman electrodynamics
A Hamiltonian formulation for the classical problem of electromagnetic
interaction of two charged relativistic particles is found.Comment: 22 pages, 8 Uuencoded Postscript figure
A Review of Numerical Simulation Strategies for Hydraulic Fracturing, Natural Fracture Reactivation and Induced Microseismicity Prediction
Hydraulic fracturing, natural fracture reactivation and resulting induced microseismicity are interconnected phenomena
involved in shale gas exploitation. Due to their multi-physics and their complexity, deep understanding of these phenomena as well
as their mutual interaction require the adoption of coupled mechanical and fluid flow approaches. Modeling these systems is a
challenging procedure as the involved processes take place on different scales of space and also require adequate multidisciplinary
knowledge. An extensive literature review is presented here to provide knowledge on the modeling approaches adopted for these
coupled problems. The review is intended as a guide to select effective modeling approaches for problems of different complexit
Physics-based monitoring of subsurface processes using seismic ambient noise
Understanding subsurface processes is essential for resource management, hazard mitigation, and environmental protection. Passive seismic methods using ambient seismic noise have emerged as a promising tool for subsurface monitoring and imaging. These methods offer non-invasive, large-scale, and continuous measurements, providing valuable insights into geological structures, geophysical properties, and subsurface processes. However, these methods are often based on empirical relationships between seismic velocity variations and dynamic subsurface properties, and the physical mechanisms underlying these relationships have not yet been fully exploited. This doctoral research contributes to advancing the understanding of seismic velocity changes and leverages this knowledge to enhance the monitoring of subsurface dynamics. It addresses the physical mechanisms behind seismic velocity variations, offering a fundamental understanding that goes beyond empirical relationships. Physics-based approaches allow for more accurate and comprehensive interpretations, predictive modeling, and technological advancements in subsurface monitoring and imaging. The research begins by constructing a physics-based model that connects seismic velocity changes to variations in pore pressure and vertical stress, related to fluctuations in groundwater level. The model utilizes established relationships between seismic velocities and induced stress, coupled with wave propagation theory, basic hydrology and geomechanics, to establish direct links between seismic velocity variations and specific dynamic subsurface properties, namely fluctuations in pore pressure and vertical compressional stress. For pore pressure fluctuations, the modelâs validity is confirmed using passive image interferometry on seismic ambient noise data from Groningen, the Netherlands, demonstrating its ability to explain surface-wave phase-velocity variations caused by pore pressure fluctuations. The direct link between seismic velocity variations and pore pressure is subsequently exploited for four-dimensional space-time pore pressure monitoring using surface-wave phase-velocity changes. As such, I introduce pore pressure sensitivity kernels as a direct connection between depth-dependent pore pressure variations and frequency-dependent changes in surface-wave phase velocities, showcasing their utility in inferring pore pressure variations in the Groningen subsurface. The inferred pore pressure models align closely with independent pressure head measurements, highlighting the potential for quantitative pore pressure inferences. The sensitivity of surface-wave phase velocities to pore pressure changes was found to decrease much faster with depth than the sensitivity to changes in elastic parameters, limiting the monitoring approach in the Groningen subsurface to the shallowest 200 m for natural pore pressure variations. In contrast, pore pressure variations caused by human activities are significantly larger. Therefore, this research continues with a feasibility assessment of utilizing surface-wave phase-velocity changes to monitor anthropogenic pore pressure developments in deeper reservoirs, particularly in the Harlingen and Groningen gas reservoirs. It expands pore pressure sensitivity kernels to deeper depths and models surface-wave phase-velocity changes in response to hypothetical production scenarios. While monitoring the shallow (⌠1 km) Harlingen reservoir appears feasible, monitoring the deeper (⌠3 km) Groningen reservoir presents substantial challenges, as the existing measurement uncertainties for velocity changes must be significantly reduced. Finally, the research delves into the physics of temperature-induced seismic velocity changes in the shallow unconsolidated subsurface. It reconciles field and laboratory experiments by considering intrinsic temperature dependencies, thermally induced stress, and thermally induced strain. The study predicts seasonal temperature-induced seismic velocity variations and their implications for site amplification. The site-specific material properties determine whether site amplification is more pronounced during summer or winter. Overall, this research enhances our understanding of seismic velocity changes and benefits their utility in subsurface monitoring, thereby contributing to resource management, hazard assessment, and a sustainable environment. The physics-based approach facilitates amore comprehensive understanding of subsurface dynamics and supports the development of innovative monitoring and imaging techniques
Effects of fault transmissivity on the potential of fault reactivation and induced seismicity : Implications for understanding induced seismicity at Pohang EGS
Funding Information: The project leading to part of the results in this article received funding from the European Unionâs Horizon 2020 research and innovation programme under grant agreement No 691728 .Peer reviewedPublisher PD
Quantum-classical crossover in electrodynamics
A classical field theory is proposed for the electric current and the
electromagnetic field interpolating between microscopic and macroscopic
domains. It represents a generalization of the density functional for the
dynamics of the current and the electromagnetic field in the quantum side of
the crossover and reproduces standard classical electrodynamics on the other
side. The effective action derived in the closed time path formalism and the
equations of motion follow from the variational principle. The polarization of
the Dirac-see can be taken into account in the quadratic approximation of the
action by the introduction of the deplacement field strengths as in
conventional classical electrodynamics. Decoherence appears naturally as a
simple one-loop effect in this formalism. It is argued that the radiation time
arrow is generated from the quantum boundary conditions in time by decoherence
at the quantum-classical crossover and the Abraham-Lorentz force arises from
the accelerating charge or from other charges in the macroscopic or the
microscopic side, respectively. The functional form of quantum renormalization
group, the generalization of the renormalization group method for the density
matrix, is proposed to follow the scale dependence through the
quantum-classical crossover in a systematical manner.Comment: new references added, few sign errors fixed, to appear in Physical
Review
Self-organization in a phonon laser
We make an adaptation of laser modelling equations to describe the behavior
of a phonon laser (saser). Our saser consists of an AlGaAs/GaAs double barrier
heterostructure designed to generate an intense beam of transversal acoustic
(TA) phonons. To study our system, we begin with a Hamiltonian that describes
the decay of primary longitudinal optical phonons (LO_1) into secondary (LO_2)
and TA (LO_1 -> LO_2 + TA) and its inverse process (recombination). Using this
Hamiltonian, a set of coupled equations of motion for the phonons is obtained.
We also consider the interaction between the phonons and its reservoirs. These
interactions are introduced in the equations of motion leading to a set of
coupled Langevin equations. In order to obtain an expression to describe our
saser we apply, in the Langevin equations, an adiabatic elimination of some
variables of the subsystem. Following the method above we obtain the value of
the injection threshold for the operation of our phonon laser. At this
threshold occurs a phase transition from a disordered to a coherent state. It
is shown that it is not necessary a big "optical" pumping to get a sasing
region.Comment: 4 figure
Time evolution of relativistic d + Au and Au + Au collisions
The evolution of charged-particle production in collisions of heavy ions at
relativistic energies is investigated as function of centrality in a
nonequilibrium-statistical framework. Precise agreement with recent d + Au and
Au + Au data at sqrt(s_NN) = 200 GeV is found in a Relativistic Diffusion Model
with three sources for particle production. Only the midrapidity source comes
very close to local equilibrium, whereas the analyses of the overall
pseudorapidity distributions show that the systems remain far from statistical
equilibrium.Comment: 16 pages, 5 figures, 1 tabl
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