Many condensed matter systems are such that their collective excitations at low energies can be described by fields satisfying equations of motion formally indistinguishable from those of relativistic field theory. The finite speed of propagation of the disturbances in the effective fields (in the simplest models, the speed of sound) plays here the role of the speed of light in fundamental physics. However, these apparently relativistic fields are immersed in an external Newtonian world (the condensed matter system itself and the (...) laboratory can be considered Newtonian, since all the velocities involved are much smaller than the velocity of light) which provides a privileged coordinate system and therefore seems to destroy the possibility of having a perfectly defined relativistic emergent world. In this essay we ask ourselves the following question: In a homogeneous condensed matter medium, is there a way for internal observers, dealing exclusively with the low-energy collective phenomena, to detect their state of uniform motion with respect to the medium? By proposing a thought experiment based on the construction of a Michelson-Morley interferometer made of quasi-particles, we show that a real Lorentz-FitzGerald contraction takes place, so that internal observers are unable to find out anything about their ‘absolute’ state of motion. Therefore, we also show that an effective but perfectly defined relativistic world can emerge in a fishbowl world situated inside a Newtonian (laboratory) system. This leads us to reflect on the various levels of description in physics, in particular regarding the quest towards a theory of quantum gravity. (shrink)

Se editan, traducen y comentan dos poemas estróficos en árabe dialectal andalusí, escritos en un volumen misceláneo encontrado en Cútar (Málaga). Su contenido alude a los reveses de los últimos tiempos de dominio musulmán en la Península Ibérica. Se comentan aspectos sobre el posible autor de los versos, la biografía del copista y su contexto religioso.

Esta investigación se centra en un texto árabe pintado sobre un elemento inusual: una balsa de agua para riego en Bédar (Almería). Junto al epígrafe que la data, hay dibujados dos animales cuadrúpedos que quizá representen una escena de caza y una cenefa decorativa; conserva asimismo otros signos gráficos y diseños muy degradados que no ha sido posible descifrar totalmente. Como hasta ahora no se había determinado su cronología, en este estudio he seguido el método habitual en Epigrafía Árabe: dibujar (...) los trazos visibles y restituir después las partes de pintura desaparecida o muy borrada. Como resultado de este estudio queda de relieve el valor de este destacado documento de la cultura popular andalusí, una alberca de dimensiones similares a las que se han hallado en otros yacimientos de época califal. Gracias a la pintura se puede afirmar que la balsa y los elementos que la componen se realizaron en esta zona rural bajo el gobierno del califa al-Ḥakam II, el año 355/966. Es urgente proceder a una excavación profesional del monumento. (shrink)

Observational indications combined with analyses of analogue and emergent gravity in condensed matter systems support the possibility that there might be two distinct energy scales related to quantum gravity: the scale that sets the onset of quantum gravitational effects $E_{\rm B}$ (related to the Planck scale) and the much higher scale $E_{\rm L}$ signalling the breaking of Lorentz symmetry. We suggest a natural interpretation for these two scales: $E_{\rm L}$ is the energy scale below which a special relativistic spacetime emerges, (...) $E_{\rm B}$ is the scale below which this spacetime geometry becomes curved. This implies that the first ‘quantum’ gravitational effect around $E_{\rm B}$ could simply be that gravity is progressively switched off, leaving an effective Minkowski quantum field theory up to much higher energies of the order of $E_{\rm L}$ . This scenario may have important consequences for gravitational collapse, inasmuch as it opens up new possibilities for the final state of stellar collapse other than an evaporating black hole. (shrink)