Quantum Non-Gravity and Stellar Collapse

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 $E_B$ (related to the Planck scale) and the much higher scale $E_L$ signalling the breaking of Lorentz symmetry. We suggest a natural interpretation for these two scales: $E_L$ is the energy scale below which a special relativistic spacetime emerges, $E_B$ is the scale below which this spacetime geometry becomes curved. This implies that the first `quantum' gravitational effect around $E_B$ 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 $E_L$. 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.Comment: 6 pages, 2 figures. v2: Partially restructured; potentially observable consequence added. Several clarifications + 3 new references. To appear in Found. of Phy

Some aspects of dispersive horizons: lessons from surface waves

Hydrodynamic surface waves propagating on a moving background flow experience an effective curved space-time. We discuss experiments with gravity waves and capillary-gravity waves in which we study hydrodynamic black/white-hole horizons and the possibility of penetrating across them. Such possibility of penetration is due to the interaction with an additional "blue" horizon, which results from the inclusion of surface tension in the low-frequency gravity-wave theory. This interaction leads to a dispersive cusp beyond which both horizons completely disappear. We speculate the appearance of high-frequency "superluminal" corrections to be a universal characteristic of analogue gravity systems, and discuss their relevance for the trans-Planckian problem. We also discuss the role of Airy interference in hybridising the incoming waves with the flowing background (the effective spacetime) and blurring the position of the black/white-hole horizon.Comment: 29 pages. Lecture Notes for the IX SIGRAV School on "Analogue Gravity", Como (Italy), May 201

Quasi-normal mode analysis in BEC acoustic black holes

We perform a quasi-normal mode analysis of black hole configurations in Bose-Einstein condensates (BEC). In this analysis we use the full Bogoliubov dispersion relation, not just the hydrodynamic or geometric approximation. We restrict our attention to one-dimensional flows in BEC with step-like discontinuities. For this case we show that in the hydrodynamic approximation quasi-normal modes do not exist. The full dispersion relation, however, allows the existence of quasi-normal modes. Remarkably, the spectrum of these modes is not discrete but continuous.Comment: 7 pages, 3 figure

Menafsir Masyarakat Adat Hatuhaha Amarima Dengan Pendekatan Hermeneutika Paul Ricoeur Dan Signifikansinya Untuk Hidup Dalam Persaudaraan

Hermeneutics is no longer focused on the written text, but has grown and that concerns to the spoken texts. The development of hermeneutics of interpretation of written text into spoken text is intended to express the meaning behind human actions are not mentioned in the article. Phenomenology indigenous of Hatuhaha Amarima with the culture's through meaningful when interpreted with Paul Ricoeur's hermeneutic approach. Deepest meaning of phenomenology indigenous of Hatuhaha Amarima useful in establishing a life together in religious differences. Religious differences to provide awareness for indigenous peoples to survive in there al brotherhood became everyone's expectations

Electromagnetism as an emergent phenomenon: a step-by-step guide

We give a detailed description of electrodynamics as an emergent theory from condensed-matter-like structures, not only {\it per se} but also as a warm-up for the study of the much more complex case of gravity. We will concentrate on two scenarios that, although qualitatively different, share some important features, with the idea of extracting the basic generic ingredients that give rise to emergent electrodynamics and, more generally, to gauge theories. We start with Maxwell's mechanical model for electrodynamics, where Maxwell's equations appear as dynamical consistency conditions. We next take a superfluid $^3$He-like system as representative of a broad class of fermionic quantum systems whose low-energy physics reproduces classical electrodynamics (Dirac and Maxwell equations as dynamical low-energy laws). An important lesson that can be derived from both analyses is that the vector potential has a microscopic physical reality and that it is only in the low-energy regime that this physical reality is blurred in favour of gauge invariance, which in addition turns out to be secondary to effective Lorentz invariance.Comment: 41 pages, 4 figures; v2: references added, version accepted for publicatio

The lifetime problem of evaporating black holes: mutiny or resignation

It is logically possible that regularly evaporating black holes exist in nature. In fact, the prevalent theoretical view is that these are indeed the real objects behind the curtain in astrophysical scenarios. There are several proposals for regularizing the classical singularity of black holes so that their formation and evaporation do not lead to information-loss problems. One characteristic is shared by most of these proposals: these regularly evaporating black holes present long-lived trapping horizons, with absolutely enormous evaporation lifetimes in whatever measure. Guided by the discomfort with these enormous and thus inaccessible lifetimes, we elaborate here on an alternative regularization of the classical singularity, previously proposed by the authors in an emergent gravity framework, which leads to a completely different scenario. In our scheme the collapse of a stellar object would result in a genuine time-symmetric bounce, which in geometrical terms amounts to the connection of a black-hole geometry with a white-hole geometry in a regular manner. The two most differential characteristics of this proposal are: i) the complete bouncing geometry is a solution of standard classical general relativity everywhere except in a transient region that necessarily extends beyond the gravitational radius associated with the total mass of the collapsing object; and ii) the duration of the bounce as seen by external observers is very brief (fractions of milliseconds for neutron-star-like collapses). This scenario motivates the search for new forms of stellar equilibrium different from black holes. In a brief epilogue we compare our proposal with a similar geometrical setting recently proposed by Haggard and Rovelli.Comment: 20 pages, 2 figures; v2: published version, references adde

Rotterdam in the 21st century:From ‘sick man’ to ‘capital of cool’

This City Profile presents a multi-disciplinary perspective on the development of Rotterdam, analysing its transformation from a “sick man” to the “capital of cool” between 1995-current. Our profile integrates insights from five policy domains and presents them as a new framework. First, Rotterdam witnessed the rise of the populist right and established a new safety regime through a zero-tolerance mentality. Second, Rotterdam's superdiversity initially triggered anti-migration sentiments, but has more recently been normalised. Third, state-led gentrification policies have uplifted Rotterdam's status and provided space for middle-class households, thereby restricting access for working-class households. Fourth, the local administration has initiated large-scale urban regeneration projects as new flagships in former port areas and the city centre. Fifth, the city has been using water safety improvements to guide urban development and to create an attractive city. Overall, these developments have contributed to Rotterdam's new, hip image. However, we argue this image is Janus-faced. The populist and repressive form of urban disadvantage management is highly politicised and considered discriminatory, whereas the new flagships and water-led urban development are depoliticised and technocratic. These two sides often operate autonomously from each other, but together they contribute to new divisions in Rotterdam.</p

Horizon effects for surface waves in wave channels and circular jumps

Surface waves in classical fluids experience a rich array of black/white hole horizon effects. The dispersion relation depends on the characteristics of the fluid (in our case, water and silicon oil) as well as on the fluid depth and the wavelength regime. In some cases, it can be tuned to obtain a relativistic regime plus high-frequency dispersive effects. We discuss two types of ongoing analogue white-hole experiments: deep water waves propagating against a counter-current in a wave channel and shallow waves on a circular hydraulic jump.Comment: 4 pages, 2 figs. To appear in: Proceedings of the Spanish Relativity Meeting (ERE2010

Stability analysis of sonic horizons in Bose-Einstein condensates

We examine the linear stability of various configurations in Bose-Einstein condensates with sonic horizons. These configurations are chosen in analogy with gravitational systems with a black hole horizon, a white hole horizon and a combination of both. We discuss the role of different boundary conditions in this stability analysis, paying special attention to their meaning in gravitational terms. We highlight that the stability of a given configuration, not only depends on its specific geometry, but especially on these boundary conditions. Under boundary conditions directly extrapolated from those in standard General Relativity, black hole configurations, white hole configurations and the combination of both into a black hole--white hole configuration are shown to be stable. However, we show that under other (less stringent) boundary conditions, configurations with a single black hole horizon remain stable, whereas white hole and black hole--white hole configurations develop instabilities associated to the presence of the sonic horizons.Comment: 14 pages, 7 figures (reduced resolution