The Sacred and the Profane Day

Abstract

The day is one of the fundamental experiences of our natural existence. The obvious cycle of the sun, the alternation of sleep and being awake provide a link between the life of the body and the great regularity that assigns their successive moments to light and to darkness. Only a simplified abstraction allows us to consider time lived as an homogeneous flow. Our existence, in its proper substance and in its larger environment, is dominated by the rhythm of days and nights. Our very experience of the reality of objects is subject to it: the universe of things depends on the light of day that reveals it. It shrivels and becomes uncertain when night falls, with terror and dreams taking its place. The evidence that appears with the clarity of day is not of the same order as apparitions that arise from the depth of darkness.

Superradiance from hydrodynamic vortices: A numerical study

“…The flow field induced by the vortex is associated with an acoustic metric with two crucial ingredients of black hole physics: an event horizon and an ergosphere. The former is a spatial surface which allows only one-way propagation of physical signals (from the outside into the vortex), while the latter is a region from which part of the the vortex energy can be extracted via the mechanism of superradiance.Such a phenomenon was first studied by Zel'dovich [4] with regard to the generation of waves by a rotating body * Electronic address: fr.federici@sns.it and was then analysed as stimulated emission in blackhole radiance [5,6,7]. Superresonance is an acousticwave version of the Penrose [8] process, whereby a planewave solution of a scalar massless field in the black hole background is scattered from the ergosphere with an amplification at the expenses of the rotational energy of the black hole.…”

The History and Present Status of Quantum Field Theory in Curved Spacetime

“…This suggested that for a rotating black hole, "spontaneous emission"-i.e., spontaneous particle creation from the vacuum-should occur. This was noted by Starobinski [6] and confirmed by Unruh [7]. The fact that spontaneous particle creation occurs near rotating black holes did not cause much surprise or excitement.…”

“…Not long after Penrose's discovery, Misner (unpublished) and Zel'dovich and Starobinski [5,6] realized that there is a wave analog of the Penrose energy extraction process. Instead of sending in a classical particle and having it break up into two fragments, one can simply have a classical wave impinge upon a rotating black hole.…”

Perturbação de spin zero no espaço-tempo de Kerr-Randall-Sundrum

“…Esta instabilidade vem do fato que a massa do campo escalar age como se fosse um espelho, e se somarmos a isso o efeito do espalhamento super-radiante, que ocorre em buracos negros em rotação, teremos a amplitude da perturbação escalar crescendo indefinidamente [48]. Este cenárioé o que obtemos no caso de uma perturbação escalar não massiva no espaço-tempo de Kerr Este coeficiente de reflexão difere da fórmula encontrada por Starobinski [25] para o caso da solução de Kerr padrão pelo fator Q n que aparece no termo (ω 2 − Q 2 n ) L+1/2 . De fato, isto nãoé apenas uma frequência recalibrada pelo fato Q 2 n , pois o termo χ definido por (4.46) contém ω. Esta influência da dimensão extra, representada pelos autovalores Q n , no espalhamento super-radiante será importante apenas para o regime em que Q né da mesma ordem que ω. Estes resultados foram publicados em [49].…”

“…45) sendo F (a, b, c, z) e F (a − c + 1, b − c + 1, 2 − c, z) funções hipergeométricas,χ = (ω − mΩ − H) r r − r + r − r − . (4.47)Segundo Starobinski[25] para grandes valores de r a solução acima tem a seguinte forma assintóticaR ∼ AΓ(1 − 2iχ) (r + − r − ) −L Γ(2L + 1)r L Γ(L + 1)Γ(L + 1 − 2iχ) + (r + − r − ) L+1 Γ(−2L − 1)r −L−1 Γ(−L)Γ(−L − 2iχ) . (4.48)Na região distante r − r + ≫ M a equação de onda possui a forma d 2 dr 2 (rR) + [ω 2 − Q 2 dada pela seguinte combinação de funções de BesselR ∼ r −1/2 [aJ L+1/2 ( ω 2 − Q 2 n ) + bJ −L−1/2 ( ω 2 − Q 2 n )] .…”