Note on scalars, perfect fluids, constrained field theories, and all that

The relation of a scalar field with a perfect fluid has generated some debate along the last few years. In this paper we argue that shift-invariant scalar fields can describe accurately the potential flow of an isentropic perfect fluid, but, in general, the identification is possible only for a finite period of time. After that period in the evolution the dynamics of the scalar field and the perfect fluid branch off. The Lagrangian density for the velocity-potential can be read directly from the expression relating the pressure with the Taub charge and the entropy per particle in the fluid, whereas the other quantities of interest can be obtained from the thermodynamic relations.Comment: 5 pages, 1 figure. Accepted for publication in Phys. Lett.

El verano contra el invierno. Mimesis y subversión ritual en la religiosidad popular

Apegado al ritmo de las estaciones, el campesino experimenta el tiempo de manera concreta, cíclica y bipolar, otorgando un antagónico sentido mitopoético a la primavera-verano y al invierno. Frente a los rituales y fiestas del buen tiempo, imitativos con la naturaleza, la temposensitividad invernal sugiere el desorden y la subversión carnavalesca, conjura simbólica al tiempo nefasto del frío, la noche y la muerte.Nekazariak, urtaroen erritmoei loturik, denbora era zehatzean sentitzen du, ziklikoa eta bipolarra, zentzu mitopoetiko antagonikoa ematen diela udaberri-udari eta neguari. Eguraldi onaren erritu eta jaien aurrean, naturaren imitaziozkoak diren horien aurrean, desordena eta inauteriko subertsioa iradokitzen du neguko denborasentsitibitateak, zorigaiztoko hotz, gau eta heriotza denboraren aurkako konjuru sinboliko gisa.Attaché au rythme des saisons, le paysan expérimente le temps de façon concrète, cyclique et bipolaire, octroyant un sens mythopoétique antagonique au printemps et en hiver. Face aux rites et aux fêtes du beau temps, imitations de la nature, la «temposensitivité» hivernale suggère le désordre et la subvention carnavalesque, conjuration symbolique au temps néfaste du froid, de la nuit et de la mort.Connected as they are to the rhythm of the seasons, peasants experience the weather in a concrete, cyclical and bipolar manner, attributing an antagonic mythological-poetic sense to spring-summer and to winter. In contrast with the good weather rituals and festivities, that imitate nature, their winter weather-sensibility suggests Carnival disorder and subversion, a symbolic spell against the extremely bad weather of cold, night and death