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Dos autores Juan Neirotti and Nestor Caticha. Publicado na Physical Review E | Editors' Suggestion

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Abstract

Inspired by studies of legislative-executive conflict in modern presidential democracies in South America, we present an agent-based statistical mechanics exploration of the collective, coordinated action of strategic political actors in the legislative chamber and the conditions that may result in premature changes in the executive officeholder, such as a president's impeachment or a motion of no confidence in a prime minister. The legislative actors are represented by information processing agents equipped with a neural network, and emit opinions about issues in the presidential agenda. We construct a Hamiltonian which is the sum of the costs for the agents to hold a specific set of political positions. We use replica methods for two types of disorder, in the space of weights and in the network of agents' interactions. We obtain the phase diagram of the model, where the control parameters may be loosely described as indices measuring the strategic legislative support, the presidential polling popularity and the volume of the presidential agenda under discussion. It shows an intermediate phase of coexistence of pro and con strategic behavior. This region is surrounded by pure phases where the strategic vote falls completely in the pro or con camps. Driven by external forces, the change of these indices may lead the system out of the coexistence region into the purely anti-executive phase, triggering a phase transition into a state opposing the executive leader and supporting their removal from office by constitutional means. We use data from Brazil, and show the presidential trajectories that led to impeachment or not during the democratic period which began in 1989. These trajectories ended in the region of the phase diagram in accordance to the president being removed or not from office.

O podcast Arte&Ciência CAST é uma iniciativa do grupo Arte&Ciência, liderado pelo Prof. Mikiya Muramatsu. A ideia é divulgar o tradicional projeto de extensão, bem como apresentar os monitores e professores envolvidos, os experimentos utilizados pelo gruo e curiosidades de divulgação científica no geral.

Do autor Jorge L. de Lyra. 
Publicado em General Relativity and Gravitation.

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Abstract

We examine the black-hole limits of the family of static and spherically symmetric solutions of the Einstein field equations for polytropic matter, that was presented in a previous paper. This exploration is done in the asymptotic sub-regions of the allowed regions of the parameter planes of that family of solutions, for a few values of the polytropic index n, with the limitation that n>1. These allowed regions were determined and discussed in some detail in another previous paper. The characteristics of these limits are examined and analyzed. We find that there are different types of black-hole limits, with specific characteristics involving the local temperature of the matter. We also find that the limits produce a very unexpected but specific type of spacetime geometry in the interior of the black holes, which we analyze in detail. Regarding the spatial part of the interior geometry, we show that in the black-hole limits there is a general collapse of all spatial distances to zero. Regarding the temporal part, there results an infinite overall red shift in the limits, with respect to the flat space at radial infinity, over the whole interior region. The analysis of the interior geometry leads to a very surprising connection with quantum-mechanical studies in the background metric of a naked Schwarzschild black hole. The nature of the solutions in the black-hole limits leads to the definition of a new type of singularity in General Relativity. We argue that the black-hole limits cannot actually be taken all the way to their ultimate conclusion, due to the fact that this would lead to the violation of some essential physical and mathematical conditions. These include questions of consistency of the solutions, questions involving infinite energies, and questions involving violations of the quantum behavior of matter. However, one can still approach these limiting situations to a very significant degree, from the physical standpoint, so that the limits can still be considered, at least for some purposes, as useful and simpler approximate representations of physically realizable configurations with rather extreme properties.

Homenagem a Luciano Portante
Do Departamento de Física Experimental, por Marcos N. Martins.

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Perdemos o Luciano (Periquito, ou ainda Piri, para os mais íntimos, provavelmente por conta do saliente nariz adunco, que portava com orgulho). 

Luciano foi contratado no Instituto como mecânico (mecânico de veículos, não torneiro mecânico), numa época em que isso ainda era possível e a improvisação era norma. Não sei bem como ele foi parar no Laboratório do Acelerador Linear, nem como foi a transição de mecânico de automóveis (que ele nunca deixou de ser) para torneiro mecânico, mas isso é irrelevante. O importante é saber, e lembrar, que Luciano era um artista, capaz de conceber e executar, com acabamento impecável, peças metálicas de grande complexidade. Sua habilidade nos vários tipos de solda também era notória, o que o fazia muito requisitado por vários laboratórios, quando precisavam soldar peças para alto vácuo. Mas, como muitos artistas, Piri tinha um temperamento bastante volátil, que, como muitos compostos voláteis, entrava em combustão com facilidade. Um preço módico a ser pago por sua disposição em ajudar e sua enorme habilidade em encontrar soluções para problemas mecânicos difíceis. 

Só para nos contrariar, Luciano se foi sem alarde, discretamente. Vamos sentir sua falta.

Foto: Luciano "Periquito" e seu filho. Acervo familiar.