Mathisson's helical motions demystified

The motion of spinning test particles in general relativity is described by Mathisson-Papapetrou-Dixon equations, which are undetermined up to a spin supplementary condition, the latter being today still an open question. The Mathisson-Pirani (MP) condition is known to lead to rather mysterious helical motions which have been deemed unphysical, and for this reason discarded. We show that these assessments are unfounded and originate from a subtle (but crucial) misconception. We discuss the kinematical explanation of the helical motions, and dynamically interpret them through the concept of hidden momentum, which has an electromagnetic analogue. We also show that, contrary to previous claims, the frequency of the helical motions coincides exactly with the zitterbewegung frequency of the Dirac equation for the electron.Comment: To appear in the Proceedings of the Spanish Relativity Meeting 2011 (ERE2011), "Towards new paradigms", Madrid 29 August - 2 September 201

Introductory Note

  The present dossier of Revista Lusófona de Educação results from a peer-review selection of the papers presented at the MILT conference - Media Literacy for Living Together: the future of media and learning in participation, held in June 26-28, 2019 at Lusófona University in Lisbon

Cinema forges the event Filmmaking and the case of Thomas Harlan’s Torre Bela

The film Red Line and accompanying thesis revisit the Documentary Torre Bela (1975) by Thomas Harlan and its memory, reflecting upon the role cinema played in revolutionary process in Portugal in 1975 and its enduring significance to the collective memory of this event

Calibração de sensores do ATLASCAR2 por otimização global

In autonomous vehicles, it is often necessary to install a large number of sensors on board. Thus, the extrinsic calibration of these multi-sensory systems is a problem of high relevance for the development algorithms of autonomous driving or of assistance to the driving. This work proposes a tool to automatically calibrate simultaneously multiple cameras. In the process, aruco markers are used, which allows establishing a graph from which the geometric transformations between the various cameras and a global reference are extracted. Initially, markers are detected in the images using an OpenCV tool. Subsequently, the graph is established where the nodes are cameras or markers and the edges are the transformations between them. Then an initial estimate of the extrinsic parameters of all cameras is calculated based on the detections of the markers and the paths obtained from the graph. In the end, an optimization of the parameters is done, where the reprojection error is minimized. In order to demonstrate the process, several datasets were created in order to validate the obtained results.Em veículos autónomos é frequente a necessidade de instalar um grande número de sensores a bordo. Assim, a calibração extrínseca destes sistemas multi-sensoriais é um problema de grande relevância para o desenvolvimento de algoritmos de condução autónoma ou de apoio á condução. Este trabalho propõe um mecanismo capaz de fazer uma calibração automática em simultaâneo de várias câmaras. No processo são usados marcadores aruco, o que permite estabelecer um grafo de onde se extraem as transformações geométricas entre as várias câmaras e um referencial global. Inicialmente, os marcadores são detetados nas imagens usando uma ferramenta do OpenCV. Posteriormente é construido o grafo em que os nós são câmaras ou marcadores, e as ligações entre nós são transformações geométricas em pares câmara aruco. Em seguida é calculada uma estimativa inicial dos parâmetros extrínsecos de todas as câmaras, baseada nas deteções dos marcadores e nos caminhos obtidos do grafo. No fim, é feita uma otimização dos parâmetros, onde é minimizado o erro de reprojeção. Para demonstrar o processo foram criados vários "datasets", de modo a validar os resultados obtidos.Mestrado em Engenharia Mecânic

ANIMATION DOCUMENTARIES AND REALITY CROSS-BOUNDARIES

Filmmakers aim to deliver some emotional and aesthetic coin to their works, which makes it possible that the boundaries between fiction and documentary genres could be diluted artistically. The documentary is a recognized genre in film studies that is considered to move on one side of the boundary between fact and fiction. Michael Rabiger defends the objectivity and fairness of documentaries due to the expectation of the viewer to accept the photographic image as true (Rabiger, 1998, p. 6). Bill Nichols reinforces this idea evoking that the documentaries that best witness a certain theme are those in which filmmakers “don’t interfere", classifying them as "observational documentaries", as the examples from the images captured in World War II or the political TV news (Nichols, 1991, p. 38). Therefore, the challenge that we propose aims to relate the documentary live action characteristics with the animated images, where the veracity is just an animation reflection. To better illustrate the curious fusion of animation and documentary image, we will examine several emergent examples of this new “film genre”.