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Search Results: 1 - 10 of 200561 matches for " ?yvind Gr?n "
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Electrodynamics of Radiating Charges
yvind Grn
Advances in Mathematical Physics , 2012, DOI: 10.1155/2012/528631
Abstract: The theory of electrodynamics of radiating charges is reviewed with special emphasis on the role of the Schott energy for the conservation of energy for a charge and its electromagnetic field. It is made clear that the existence of radiation from a charge is not invariant against a transformation between two reference frames that has an accelerated motion relative to each other. The questions whether the existence of radiation from a uniformly accelerated charge with vanishing radiation reaction force is in conflict with the principle of equivalence and whether a freely falling charge radiates are reviewed. It is shown that the resolution of an electromagnetic “perpetuum mobile paradox” associated with a charge moving geodetically along a circular path in the Schwarzschild spacetime requires the so-called tail terms in the equation of motion of a charged particle. 1. Introduction The nonrelativistic version of the equation of motion of a radiating charged particle was discussed already more than a hundred years ago by Lorentz [1], where is the ordinary (Newtonian) acceleration of the particle. The time is of the same order of magnitude as the time taken by light to move a distance equal to the classical electron radius, that is, ≈ 10?23 seconds. The general solution of the equation is Hence, the charge performs a runaway motion, that is, it accelerates away even when unless one chooses the initial condition By combining (1.1) and (1.3), one obtains This equation shows that the acceleration of the charge at a point of time is determined by the future force. Hence, when runaway motion is removed preacceleration appears. The relativistic generalization of the equation was originally found by Abraham in 1905 [2]. A new deduction of the Lorentz covariant equation of motion was given by Dirac in 1938 [3]. This equation is therefore called the Lorentz-Abraham-Dirac equation, or for short, the LAD equation. In the 4-vector notation invented by Minkowski in 1908, and referring to an inertial frame in flat spacetime, the equation of motion of a radiating charged particle takes the form where is the 4-velocity of the particle and the dot denotes the ordinary differentiation with respect to the proper time of the particle. Here, is called the Abraham 4-force, is the proper acceleration of the charged particle with respect to an inertial frame, and is the 4-acceleration of the particle. Let the components of the 4-acceleration be . Fritz Rohrlich called the spatial component of the Abraham 4-force, , for the field reaction force, and separated it in two forces, ,
Entropy and Gravity
yvind Grn
Entropy , 2012, DOI: 10.3390/e14122456
Abstract: The effect of gravity upon changes of the entropy of a gravity-dominated system is discussed. In a universe dominated by vacuum energy, gravity is repulsive, and there is accelerated expansion. Furthermore, inhomogeneities are inflated and the universe approaches a state of thermal equilibrium. The difference between the evolution of the cosmic entropy in a co-moving volume in an inflationary era with repulsive gravity and a matter-dominated era with attractive gravity is discussed. The significance of conversion of gravitational energy to thermal energy in a process with gravitational clumping, in order that the entropy of the universe shall increase, is made clear. Entropy of black holes and cosmic horizons are considered. The contribution to the gravitational entropy according to the Weyl curvature hypothesis is discussed. The entropy history of the Universe is reviewed.
Universe Models with Negative Bulk Viscosity
Iver Brevik,yvind Grn
Physics , 2013, DOI: 10.1007/s10509-013-1525-z
Abstract: The concept of negative temperatures has occasionally been used in connection with quantum systems. A recent example of this sort is reported in the paper of S. Braun et al. [Science 339,52 (2013)], where an attractively interacting ensemble of ultracold atoms is investigated experimentally and found to correspond to a negative-temperature system since the entropy decreases with increasing energy at the high end of the energy spectrum. As the authors suggest, it would be of interest to investigate whether a suitable generalization of standard cosmological theory could be helpful, in order to elucidate the observed accelerated expansion of the universe usually explained in terms of a positive tensile stress (negative pressure). In the present note we take up this basic idea and investigate a generalization of the standard viscous cosmological theory, not by admitting negative temperatures but instead by letting the bulk viscosity take negative values. Evidently, such an approach breaks standard thermodynamics, but may actually be regarded to lead to the same kind of bizarre consequences as the standard approach of admitting the equation-of-state parameter w to be less than -1. In universe models dominated by negative viscosity we find that the fluid's entropy decreases with time, as one would expect. Moreover, we find that the fluid transition from the quintessence region into the phantom region (thus passing the phantom divide w=-1) can actually be reversed. Also in generalizations of the LCDM-universe models with a fluid having negative bulk viscosity we find that the viscosity decreases the expansion of the universe.
An electromagnetic perpetuum mobile?
yvind Grn,Sigurd Kirkevold N?ss
Physics , 2008,
Abstract: A charge moving freely in orbit around the Earth radiates according to Larmor's formula. If the path is closed, it would constitute a perpetuum mobile. The solution to this energy paradox is found in an article by C. M. DeWitt and B. DeWitt from 1964. The main point is that the equation of motion of a radiating charge is modified in curved spacetime. In the present article we explain the physics behind this modification, and use the generalized equation to solve the perpetuum mobile paradox.
Gravitational Entropy and Inflation
?ystein Elgar?y,yvind Grn
Entropy , 2013, DOI: 10.3390/e15093620
Abstract: The main topic of this paper is a description of the generation of entropy at the end of the inflationary era. As a generalization of the present standard model of the Universe dominated by pressureless dust and a Lorentz invariant vacuum energy (LIVE), we first present a flat Friedmann universe model, where the dust is replaced with an ideal gas. It is shown that the pressure of the gas is inversely proportional to the fifth power of the scale factor and that the entropy in a comoving volume does not change during the expansion. We then review different measures of gravitational entropy related to the Weyl curvature conjecture and calculate the time evolution of two proposed measures of gravitational entropy in a LIVE-dominated Bianchi type I universe, and a Lemaitre-Bondi-Tolman universe with LIVE. Finally, we elaborate upon a model of energy transition from vacuum energy to radiation energy, that of Bonanno and Reuter, and calculate the time evolution of the entropies of vacuum energy and radiation energy. We also calculate the evolution of the maximal entropy according to some recipes and demonstrate how a gap between the maximal entropy and the actual entropy opens up at the end of the inflationary era.
Experimental limits to the density of dark matter in the solar system
yvind Grn,Harald H. Soleng
Physics , 1995, DOI: 10.1086/176669
Abstract: On the scales of galaxies and beyond there is evidence for unseen dark matter. In this paper we find the experimental limits to the density of dark matter bound in the solar system by studying its effect upon planetary motion.
Social and biological pathways in childhood and consequent adult mortality risk: A review of evidence and some research implications
yvind N?ss
Norsk Epidemiologi , 2009,
Abstract: Artikkelen diskuterer den prinsipielle betydningen av sosiale og biologiske faktorer i barndommen for d delighetsrisikoen i voksen alder. Ulike modeller av livsl pet blir presentert; kritisk og sensitiv periode, akkumulering av risiko og til sist en forl psmodell. Disse modellene blir diskutert i lys av empiriske studier hvor sosiale og biologiske faktorer i barndommen er vist ha en assosiasjon med d delighet i voksen alder. En slik livsl psmodell har potensial til forklare ulikhet i helse p en biologisk og sosialt plausibel m te. Men den stiller ogs sp rsm l ved om rsaksfaktorer fra tidligere i livet kan hevdes v re uavhengige f r vi kjenner hvordan de biologiske og sosiale kjedene av risiko henger sammen og har utviklet seg over tid. Det blir derfor hevdet av en del forskere at for forst kroniske sykdommers etiologi fullt ut m b de sosiale og biologiske rsakskjeder inng i modellen.
Communication and empathy in an emergency setting involving persons in crisis
Halvor Nordby,yvind N?hr
Scandinavian Journal of Trauma, Resuscitation and Emergency Medicine , 2008, DOI: 10.1186/1757-7241-16-5
Abstract: This article presents results from a qualitative research project designed to study and understand interpersonal relations in an emergency setting involving persons in crisis. The study has focused on the interaction between paramedics and parents of children whose deaths were later attributed to Sudden Infant Death Syndrome (SIDS). This interaction is very dramatic and difficult. The paramedics normally encounter the parents with the knowledge that they have a lifeless child, and they also know that the parents are undergoing extreme emotional and cognitive stress. Before the arrival of the paramedics and medical doctors, the parents' only contact with the health system is interactive, by way of medical emergency telephone.The aim of our research has been to understand how challenges related to communication, care and empathy are experienced in the interaction between paramedics and parents. The next section clarifies this aim of the project and its wider significance to studies of communication in emergency situations. Section three presents relevant theoretical assumptions and the qualitative method used in the semi-structured interviews with paramedics and parents. Section four presents the main results of the study, while the fifth and final section discusses implications of these results.Discussions of challenges in health personnel-patient-interaction in emergency situations often focus on the medical aspects of disease or injury [1,2]. This is understandable, since improving health and saving lives are the primary aims of the health services, and since skilled medical treatment is necessary for achieving these aims. It is nevertheless important to remember that emergency situations involve interpersonal relations, and that successful interaction between emergency personnel and their patients often depends on adequate communication [3,4]. Good communication in emergency situations is important for two kinds of reasons. Firstly, communication is a means of avo
Nonlinear Time-Domain Strip Theory Formulation for Low-Speed Manoeuvering and Station-Keeping
Thor I. Fossen,yvind N. Smogeli
Modeling, Identification and Control , 2004, DOI: 10.4173/mic.2004.4.1
Abstract: This paper presents a computer effective nonlinear time-domain strip theory formulation for dynamic positioning (DP) and low-speed manoeuvring. Strip theory or 2D potential theory, where the ship is divided in 20 to 30 cross sections, can be used to compute the potential coefficients (added mass and potential damping) and the exciting wave loads (Froude-Krylov and diffraction forces). Commercially available programs are ShipX (VERES) by Marintek (Fathi, 2004) and SEAWAY by Amarcon (Journée & Adegeest, 2003), for instance. The proposed method can easily be extended to utilize other strip theory formulations or 3-D potential programs like WAMIT (2004). The frequency dependent potential damping, which in classic theory results in a convolution integral not suited for real-time simulation, is compactly represented by using the state-space formulation of Kristiansen & Egeland (2003). The separation of the vessel model into a low-frequency model (represented by zerofrequency added mass and damping) and a wave-frequency model (represented by motion transfer functions or RAOs), which is commonly used for simulation, is hence made superfluous. Transformations of motions and coefficients between different coordinate systems and origins, i.e. data frame, hydrodynamic frame, body frame, inertial frame etc., are put into the rigid framework of Fossen (1994, 2002). The kinematic equations of motion are formulated in a compact nonlinear vector representation and the classical kinematic assumption that the Euler angles are small is removed. This is important for computation of accurate control forces at higher roll and pitch angles. The hydrodynamic forces in the steadily translating hydrodynamic reference frame (equilibrium axes) are, however, assumed tobe linear. Recipes for computation of retardation functions are presented and frequency dependent viscous damping is included. Emphasis is placed on numerical computations and representation of the data from VERES and SEAWAY in Matlab/Simulink. For this purpose a Simulink add-in to the Marine Systems Simulator (MSS) at the Norwegian University of Science and Technology has been developed (Fossen et al., 2004).
The selection hypothesis of social inequalities in health: The Oslo Mortality Study
Bj?rgulf Claussen,yvind N?ss
Norsk Epidemiologi , 2009,
Abstract: -
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