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Ski resorts are important tourist resources in mountain areas. They have a high impact on the environment but also on the development of the territory. Corporate social responsibility management plays an important role in influencing consumer purchasing behavior. This research seeks to understand the behavior of ski tourists and to classify them. Approximately 50,000 online purchases of tourist packages to ski resorts in Spain and Andorra are analyzed in order to describe the different segments of demand in these resorts through a latent class model. The tourists’ age and previous experience, the type of accommodation, and the season—among other variables—are considered to clarify the different classes. Six different segments were found. Some relevant results for the online ski-package market are highlighted, such as the influence of skier’s expenditure on service valuation. Managerial implications, limitations of this study and recommendations for future research are also discussed. This research was funded by the Spanish Ministry of Economy, Industry and Competitiveness through grants ECO2017-88984-R (E.C.-F. and B.F.-R.) and FIS2017-86497-C2-1 (G.J.).

This chapter, by Jesús Barreal Pernas and Gil Jannes, deals with gastro-tourism in Spain and studies the international tourist profile in the Spanish wine and olive oil Protected Destinations of Origin (PDOs). Specifically, the study focuses on regions that have such PDOs and examines the differences and similarities among the profiles of international tourists that visit them. The study concludes with a classification of four clusters according to demographic, travel and motivational characteristics of tourists in the particular area.

La gamificacion consiste en ludificar los contenidos de una determinada materia o asignatura a traves de dinamicas, mecanicas y componentes. Con este proceso se puede lograr una mayor motivacion e implicacion del alumno, asi como fomentar el desarrollo de competencias y del aprendizaje autonomo. Asi, el presente texto describe el desarrollo de esta herramienta para la asignatura “Estadistica Aplicada al Sector Turistico” del Grado en Turismo con el objetivo de lograr presentar un proceso formativo dinamico y atractivo para el alumno. No obstante, se elabora desde un punto transversal que permita su implementacion en asignaturas similares implementadas en ensenanzas superiores de grado. En mas detalle, se desarrolla una narrativa entorno a la promocion profesional del alumno dentro de una empresa hotelera ficticia donde ira alcanzando mayor rango profesional a medida que avanza el juego y va superando una serie de pruebas/eventos. Esta narrativa fomenta el interes intrinseco en la actividad por parte del alumno, al mismo tiempo que refuerza la utilidad del propio juego de cara a futuras experiencias profesionales.

11 págs.; 5 figs., In quantum field theory in curved backgrounds, one typically distinguishes between objective, tensorial quantities such as the renormalized stress-energy tensor (RSET) and subjective, nontensorial quantities such as Bogoliubov coefficients which encode perception effects associated with the specific trajectory of a detector. In this work, we propose a way to treat both objective and subjective notions on an equal tensorial footing. For that purpose, we define a new tensor which we will call the perception renormalized stress-energy tensor (PeRSET). The PeRSET is defined as the subtraction of the RSET corresponding to two different vacuum states. Based on this tensor, we can define perceived energy densities and fluxes. The PeRSET helps us to have a more organized and systematic understanding of various results in the literature regarding quantum field theory in black hole spacetimes. We illustrate the physics encoded in this tensor by working out various examples of special relevance. © 2016 American Physical Society, Financial support was provided by the Spanish MINECO through the Projects No. FIS2011-30145-C03-01, No. FIS2011-30145-C03-02, No. FIS2014-54800-C2-1, No. FIS2014-54800-C2-2 (with FEDER contribution), and by the Junta de Andalucía through Project No. FQM219.

We use the tunneling formalism to calculate the Hawking radiation of massive particles. For E>=m, we recover the traditional result, identical to the massless case. But E, Comment: 9 pages, 4 figures. v2: minor clarification about validity of semiclassical approach; 4 references added. To appear in JETP Letters

I discuss some problems related to extreme mathematical realism, focusing on a recently proposed "shut-up-and-calculate" approach to physics (arXiv:0704.0646, arXiv:0709.4024). I offer arguments for a moderate alternative, the essence of which lies in the acceptance that mathematics is (at least in part) a human construction, and discuss concrete consequences of this--at first sight purely philosophical--difference in point of view., 11 pages

MG14, University of Rome "La Sapienza" - Rome, July 12-18, 2015; http://www.icra.it/mg/mg14/, Current proposals for regularizing the classical singularity of black holes present longlived trapping horizons, with enormous inaccessible evaporation lifetimes. We propose an alternative regularization model, inspired in condensed matter gravitational analogues, in which the collapse of a stellar object would result in a genuine time-symmetric bounce. In geometrical terms this amounts to the connection of a black-hole geometry with a white-hole geometry in a regular manner. 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. The duration of the bounce as seen by external observers is very brief. This motivates the search for new forms of stellar equilibrium., Financial support was provided by the Spanish MICINN through Projects No. FIS2011-30145-C03-01 and FIS2011-30145-C03-02 (with FEDER contribution), and by the Junta de Andalucía through Project No. FQM219. R. C-R. acknowledges support from CSIC through the JAE-predoc program, cofunded by FSE.

Mach's principle applied to rotation can be correct if one takes into account the rotation of the quantum vacuum together with the Universe. Whether one can detect the rotation of the vacuum or not depends on its properties. If the vacuum is fully relativistic at all scales, Mach's principle should work and one cannot distinguish the rotation: in the rotating Universe+vacuum, the co-rotating bucket will have a flat surface (not concave). However, if there are "quantum gravity" effects which violate Lorentz invariance at high energy, then the rotation will become observable. This is demonstrated by analogy in condensed-matter systems, which consist of two subsystems: superfluid background (analog of vacuum) and "relativistic" excitations (analog of matter). For the low-energy (long-wavelength) observer the rotation of the vacuum is not observable. In the rotating frame, the "relativistic" quasiparticles feel the background as a Minkowski vacuum, i.e. they do not feel the rotation. Mach's idea of the relativity of rotational motion does indeed work for them. But rotation becomes observable by high-energy observers, who can see the quantum gravity effects., Comment: 16 pages version submitted to JETP Letters

The white-hole sector of Kruskal's solution is almost never used in physical applications. However, it can provide a radically different take on the gravitational collapse process, avoiding the problems appearing within the standard paradigm. In this contribution we will try to draw attention to some bouncing geometries that make a democratic usage of the black and white sectors of Kruskal's solution. We will argue that this type of behaviour could be perfectly natural in some approaches to the next physical level beyond classical General Relativity. © Published under licence by IOP Publishing Ltd.

38 pags.; 4 figs.; Open Access funded by Creative Commons Attribution 3.0 licence., We give a detailed description of electrodynamics as an emergent theory from condensed-matter-like structures, not only per se but also as a warm-up for the study of the much more complex case of gravity. We 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 Maxwellʼs mechanical model for electrodynamics, where Maxwellʼs 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 only in the low-energy regime is this physical reality blurred in favor of gauge invariance, which in addition turns out to be secondary to effective Lorentz invariance. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft, Financial support was provided by the Spanish MICINN through projects FIS2011-30145-C03- 01 and FIS2011-30145-C03-02 (with FEDER contribution), and by the Junta de Andalucía through project FQM219. R.C-R acknowledges support from CSIC through the JAE-predoc program, cofunded by FSE.

We apply a homogenization process to the acoustic velocity potential wave equation. The study of various examples shows that the resulting effective properties are different from those of the homogenized pressure wave equation for the same underlying acoustic parameters. A careful analysis reveals that a given set of inhomogeneous parameters represents an entirely different physical system depending on the considered equation. Our findings unveil a different way of tailoring acoustic properties through gradients of the static pressure. In contrast to standard metafluids based on isobaric composites, this alternative kind of metafluids is suitable for the implementation of transformational devices designed via the velocity potential equation. This includes acoustic systems in a moving background or arising from general space-time transformations. As an example, we design a device able to cloak the acoustic velocity potential., 11 pages, 4 figures

A recently developed technique known as analogue transformation acoustics has allowed the extension of the transformational paradigm to general spacetime transformations under which the acoustic equations are not form invariant. In this paper, we review the fundamentals of analogue transformation acoustics and show how this technique can be applied to build a device that increases the density of events within a given spacetime region by simultaneously compressing space and time., This work was developed under the framework of the ARIADNA contract 4000104572/12/NL/KML of the European Space Agency. C.G.-M., J.S.-D., and A.M. also acknowledge support from Consolider project CSD2008-00066, A.M. from project TEC2011-28664-CO2-02, and C.B. and G.J. from the project FIS2011-30145-C03-01. J.S.-D. acknowledges support from the USA Office of Naval Research.

A recently proposed analogue transformation method has allowed the extension of transformation acoustics to general space-time transformations. We analyze here in detail the differences between this new analogue transformation acoustics (ATA) method and the standard one (STA). We show explicitly that STA is not suitable for transformations that mix space and time. ATA takes as starting point the acoustic equation for the velocity potential, instead of that for the pressure as in STA. This velocity-potential equation by itself already allows for some transformations mixing space and time, but not all of them. We explicitly obtain the entire set of transformations that leave its form invariant. It is for the rest of transformations that ATA shows its true potential, allowing for building a transformation acoustics method that enables the full range of space-time transformations. We provide an example of an important transformation which cannot be achieved with STA. Using this transformation, we design and simulate an acoustic frequency converter via the ATA approach. Furthermore, in those cases in which one can apply both the STA and ATA approaches, we study the different transformational properties of the corresponding physical quantities., This work was developed under the framework of the ARIADNA contracts 4000104572/11/NL/KML and 4000104572/12/NL/KML of the European Space Agency. C.G.-M., J.S.-D., and A.M. also acknowledge support from Consolider EMET project (CSD2008-00066), A.M. from project TEC2011-28664-C02-02, C.B. and GJ. from the project FIS2008-06078-C03-01, and J.S.-D. from the Office of Naval Research (Grant No. N00014-12-1-0216).

19 pags.; 2 figs., © 2015 IOP Publishing Ltd. It is logically possible that regularly evaporating black holes (REBHs) 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 REBHs 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., Financial support was provided by the Spanish MICINN through projects FIS2011-30145- C03-01 and FIS2011-30145-C03-02 (with FEDER contribution), and by the Junta de Andalucía through project FQM219. RC-R acknowledges support from CSIC through the JAE-predoc program, cofunded by FSE.

Parte II, 5 pags., We describe how the avoidance of the trans-Planckian problem of Hawking radiation can be used as a guiding principle in searching for a compelling scenario for the evaporation of black holes or black-hole-like objects. We argue that there exist only three possible scenarios, depending on whether the classical notion of long-lived horizon is preserved by high-energy physics and on whether the dark and compact astrophysical objects that we observe have long-lived horizons in the first place. Some specific findings along the way are (a) that a theory with high-energy superluminal signaling and a long-lived trapping horizon would be extremely unstable in astrophysical terms and (b) that stellar pulsations of objects hovering right outside, but extremely close to their gravitational radius, can result in a mechanism for Hawking emission.

Transformation optics has shaped up a revolutionary electromagnetic design paradigm, enabling scientists to build astonishing devices such as invisibility cloaks. Unfortunately, the application of transformation techniques to other branches of physics is often constrained by the structure of the field equations. We develop here a complete transformation method using the idea of analogue spacetimes. The method is general and could be considered as a new paradigm for controlling waves in different branches of physics, from acoustics in quantum fluids to graphene electronics. As an application, we derive an >analogue transformation acoustics> formalism that naturally allows the use of transformations mixing space and time or involving moving fluids, both of which were impossible with the standard approach. To demonstrate the power of our method, we give explicit designs of a dynamic compressor, a spacetime cloak for acoustic waves and a carpet cloak for a moving aircraft., This work was developed under the framework of the ARIADNA contract 4000104572/11/NL/KML of the European Space Agency. A. M. and J. S.-D. also acknowledge support from Consolider EMET project (CSD2008-00066), A. M. from project TEC2011-28664-C02-02, J.S.-D. from US Office of Naval Research, and C. B. and G. J. from the project FIS2008-06078-C03-01.

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.

Massive particles are radiated from black holes through the Hawking mechanism together with the more familiar radiation of massless particles. For $E >= m$, the emission rate is identical to the massless case. But $E < m$ particles can also tunnel across the horizon. A study of the dispersion relation and wave packet simulations show that their classical trajectory is similar to that of a boomerang. The tunneling formalism is used to calculate the probability for detecting such $E < m$ particles, for a Schwarzschild black hole of astrophysical size or in an analogue gravity experiment, as a function of the distance from the horizon and the energy of the particle., 4 pages, 2 figures. Proceedings of Spanish Relativity Meeting (ERE2011)

We use the avoidance of the trans-Planckian problem of Hawking radiation as a guiding principle in searching for a compelling scenario for the evaporation of black holes or black-hole-like objects. We argue that there exist only three possible scenarios, depending on whether the classical notion of long-lived horizon is preserved by high-energy physics and on whether the dark and compact astrophysical objects that we observe have longlived horizons in the first place. Along the way, we find that i) a theory with high-energy superluminal signalling and a long-lived trapping horizon would be extremely unstable in astrophysical terms and that ii) stellar pulsations of objects hovering right outside but extremely close to their gravitational radius can result in a mechanism for Hawkinglike emission. © SISSA 2011., Financial support was provided by the Spanish MICINN through the projects FIS2008-06078-C03-01 and FIS2008-06078-C03-03 and by the Junta de Andaluc a through the project FQM219. G.J. is supported by a FECYT postdoctoral mobility contract of the Spanish MEC/MICINN, and also acknowledges the Academy of Finland (Centers of Excellence Programme 2006-2011, grant 218211) and the EU 7th Framework Programme (FP7/2007-2013, grant 228464 Microkelvin).

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 EB (related to the Planck scale) and the much higher scale ELsignalling the breaking of Lorentz symmetry. We suggest a natural interpretation for these two scales: EL is the energy scale below which a special relativistic spacetime emerges, EB is the scale below which this spacetime geometry becomes curved. This implies that the first 'quantum' gravitational effect around EB 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 EL.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. © 2011 Springer Science+Business Media, LLC., Financial support was provided by the Spanish MICINN through the projects FIS2008-06078-C03-01 and FIS2008-06078-C03-03 and by Junta de Andaluc a through the projects FQM2288 and FQM219

Topological matter with Weyl points, such as superfluid 3He-A, provide an explicit example where there is a direct connection between the properly determined vacuum energy and the cosmological constant of the effective gravity emerging in condensed matter. This is in contrast to the acoustic gravity emerging in Bose-Einstein condensates, where the "value of this constant cannot be easily predicted by just looking at the ground state energy of the microscopic system from which spacetime and its dynamics should emerge" (S. Finazzi, S. Liberati and L. Sindoni, The cosmological constant: a lesson from Bose-Einstein condensates, Phys. Rev. Lett. 108, 071101 (2012)). The advantage of topological matter is that the relativistic fermions and gauge bosons emerging near the Weyl point obey the same effective metric and thus the effective gravity is more closely related to real gravity. We study this connection in the bi-metric gravity emerging in 3He-A, and its relation to the graviton masses, by comparison with a fully relativistic bi-metric theory of gravity. This shows that the parameter \lambda, which in 3He-A is the bi-metric generalization of the cosmological constant, coincides with the difference in the proper energy of the vacuum in two states (the nonequilibrium state without gravity and the equilibrium state in which gravity emerges) and is on the order of the characteristic Planck energy scale of the system. Although the cosmological constant \lambda\ is huge, the cosmological term itself is naturally non-constant and vanishes in the equilibrium vacuum, as dictated by thermodynamics. This suggests that the equilibrium state of any system including the final state of the Universe is not gravitating., Comment: 7 pages, no figures

We discuss the behaviour of massive modes near a horizon based on a study of the dispersion relation and wave packet simulations of the Klein-Gordon equation. We point out an apparent paradox between two (in principle equivalent) pictures of black hole evaporation through Hawking radiation. In the picture in which the evaporation is due to the emission of positive-energy modes, one immediately obtains a threshold for the emission of massive particles. In the picture in which the evaporation is due to the absorption of negative-energy modes, such a threshold apparently does not exist. We resolve this paradox by tracing the evolution of the positive-energy massive modes with an energy below the threshold. These are seen to be emitted and move away from the black hole horizon, but they bounce back at a "red horizon" and are re-absorbed by the black hole, thus compensating exactly for the difference between the two pictures. For astrophysical black holes, the consequences are curious but do not affect the terrestrial constraints on observing Hawking radiation. For analogue gravity systems with massive modes, however, the consequences are crucial and rather surprising., Comment: 12 pages, 7 figures. v2: clarifications on waves vs particles, detectability, and UV behaviour in analogue systems. Published version

Fluctuations around a Bose-Einstein condensate can be described by means of Bogolubov theory leading to the notion of quasiparticle and antiquasiparticle familiar to nonrelativistic condensed-matter practitioners. On the other hand, we already know that these perturbations evolve according to a relativistic Klein-Gordon equation in the long-wavelength approximation. For shorter wavelengths, we show that this equation acquires nontrivial corrections which modify the Klein-Gordon product. In this approach, quasiparticles can also be defined (up to the standard ambiguities due to observer dependence). We demonstrate that-in the low-energy as well as in the high-energy regimes-both concepts of quasiparticle are actually the same, regardless of the formalism (Bogolubov or Klein-Gordon) used to describe them. These results also apply to any barotropic, inviscid, irrotational fluid, with or without quantum potential. Finally, we illustrate how the quantization of these systems of quasiparticles proceeds by analyzing a stationary configuration containing an acoustic horizon. We show that there are several possible choices of a regular vacuum state, including a regular generalization of the Boulware vacuum. Issues such us Hawking radiation crucially depend on this vacuum choice. © 2010 The American Physical Society., Financial support by the Spanish MICINN through Projects No. FIS2008-06078-C03-01 and No. FIS2008-06078-C03-03 and Consolider-Ingenio 2010 Program CPAN (CSD2007-00042) and by the Junta de Andalucía through Projects No. FQM2288 and No. FQM219.

It is widely hoped that quantum gravity will shed light on the question of the origin of time in physics. The currently dominant approaches to a candidate quantum theory of gravity have naturally evolved from general relativity, on the one hand, and from particle physics, on the other hand. A third important branch of 20th century `fundamental' physics, condensed-matter physics, also offers an interesting perspective on quantum gravity, and thereby on the problem of time. The bottomline might sound disappointing: to understand the origin of time, much more experimental input is needed than what is available today. Moreover it is far from obvious that we will ever find out the true origin of physical time, even if we become able to directly probe physics at the Planck scale. But we might learn some interesting lessons about time and the structure of our universe in the process. A first lesson is that there are probably several characteristic scales associated with "quantum gravity" effects, rather than the single Planck scale usually considered. These can differ by several orders of magnitude, and thereby conspire to hide certain effects expected from quantum gravity, rendering them undetectable even with Planck-scale experiments. A more tentative conclusion is that the hierarchy between general relativity, special relativity and Newtonian physics, usually taken for granted, might have to be interpreted with caution., v1: 9 pages. Fourth juried prize in FQXi essay contest on "the Nature of Time" (2008). v2: 2015 update, partially rewritten and extended for Foundations of Physics. arXiv admin note: substantial text overlap with arXiv:0810.0613

13 pages, 7 figures.-- PACS nrs.: 04.70.Dy; 04.60.Bc; 04.62.+v.-- ArXiv pre-print available at: http://arxiv.org/abs/0807.4147, We analyze the Hawking radiation process due to collapsing configurations in the presence of superluminal modifications of the dispersion relation. With such superluminal dispersion relations, the horizon effectively becomes a frequency-dependent concept. In particular, at every moment of the collapse, there is a critical frequency above which no horizon is experienced. We show that, as a consequence, the late-time radiation suffers strong modifications, both quantitative and qualitative, compared to the standard Hawking picture. Concretely, we show that the radiation spectrum becomes dependent on the measuring time, on the surface gravities associated with different frequencies, and on the critical frequency. Even if the critical frequency is well above the Planck scale, important modifications still show up., Financial support was provided by the Spanish MEC through the projects FIS2005-05736-C03-01, FIS2005-05736-C03-02, and FIS2006-26387-E.

We study selected aspects of quantum gravity phenomenology inspired by the gravitational analogy in Bose--Einstein condensates (BECs). We first review the basic ideas and formalism of analogue gravity in BECs, with particular emphasis on the possibility of simulating black holes. The non-relativistic, 'superluminal' modifications of the dispersion relation in a BEC beyond the hydrodynamic limit make it a particularly interesting model for many scenarios of quantum gravity phenomenology which consider a possible violation of local Lorentz invariance at high energies. In particular, these modifications allow the study of kinematical corrections that such quantum gravity scenarios could impose on general relativity. A simple (1+1)-dimensional acoustic black hole configuration in a BEC is presented, and its dynamical stability and quasinormal mode spectrum are studied. Then, an analysis is performed of the Hawking radiation for a collapsing geometry in which a black hole is created. It is seen that the superluminality of the dispersion relation leads to a frequency-dependence of the horizon, which can cause strong qualitative and quantitative modifications with respect to the standard (Lorentz-invariant) Hawking spectrum. We end with some considerations related to the possibility of constructing a serious toy model for Planck-scale gravity understood as an emergent phenomenon, based on the condensed matter analogy. In particular, we discuss the problem of diffeomorphism invariance in such a seemingly background-dependent approach and indicate some possible ideas for how to recover the Einstein equations in the adequate limit., Comment: PhD thesis; 144 pages

An increasingly popular approach to quantum gravity rests on the idea that gravity (and maybe electromagnetism and the other gauge fields) might be an 'emergent phenomenon', in the sense of representing a collective behaviour resulting from a very different microscopic physics. A prominent example of this approach is the condensed matter scheme for quantum gravity, which considers the possibility that gravity emerges as an effective low-energy phenomenon from the quantum vacuum in a way similar to the emergence of collective excitations in condensed matter systems. This condensed matter view of the quantum vacuum clearly hints that, while the term 'ether' has been discredited for about a century, quantum gravity holds many (if not all) of the characteristics that have led people in the past to label various hypothetical substances with the term 'ether'. Since the last burst of enthusiasm for an ether, at the end of the 19th century, was brought to the grave in part by the performance of a series of important experiments in interferometry, the suggestion then naturally arises that maybe interferometry could also play a role in the current discussion on quantum gravity. We will highlight some aspects of this suggestion in the context of the condensed matter scheme for emergent gravity., Comment: 11 pages; to appear in: F. Columbus (ed.), "Interferometers: Research, Technology and Applications". v2: reference added, minor corrections

Submitted on 31 May 2007 (v1), last revised 4 Mar 2008 (this version, v2).-- 6 pages, no figures. Minor changes reflect published version.-- Final version of the paper in: http://www.springer.com/physics/journal/10701., 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., This work has been funded by the Spanish MEC under project FIS2005-05736-C03-01. G.J. was also supported by CSIC grant I3P-BPD2005 of the I3P programme, cofinanced by the European Social Fund.

7 pages, 3 figures.-- PACS nrs.: 04.70.Dy; 03.75.Kk; 04.80.-y., We perform a quasinormal mode analysis of black-hole configurations in Bose-Einstein condensates (BECs). 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 BECs with steplike discontinuities. For this case we show that in the hydrodynamic approximation quasinormal modes do not exist. The full dispersion relation, however, allows the existence of quasinormal modes. Remarkably, the spectrum of these modes is not discrete but continuous., C.B. has been funded by the Spanish MEC under Project No. FIS2005-05736-C03-01 with a partial FEDER contribution. G. J. was supported by CSIC Grant No. I3P-BPD2005 of the I3P programme, cofinanced by the European Social Fund, and by the Spanish MEC under Project No. FIS2005-05736-C03-01. L.G. was supported by the Spanish MEC under Project No. FIS2005-05736-C03-02. The authors also acknowledge support from the Spanish MEC Project No. FIS2006-26387-E.

11 pags., 2 figs. -- Proceedings, Workshop on From quantum to emergent gravity: Theory and phenomenology (QG-Ph) : Trieste, Italy, June 11-15, 2007, The object of this contribution is to provide an illustration of the impact of modified dispersion relations with respect to relativistic or hydrodynamic ones. The example studied here is the quasinormal mode (QNM) spectrum of a toy model for an acoustic black hole in a (1+1)-dimensional flow of a Bose–Einstein condensate (BEC). Acoustic black holes in the hydrodynamic limit, just like general relativistic black holes, have no quasinormal modes in 1+1 dimensions, whereas in 3+1 a discrete QNM spectrum appears. However, when using the full or modified (Bogoliubov) dispersion relation, these (1+1)-dimensional BEC black holes present a QNM spectrum consisting of a continuous region in the complex frequency plane. A straightforward speculation about similar effects in high-energy physics scenarios with modified dispersion relations will briefly be discussed., This work has been supported by the European Science Foundation network programme “Quantum Geometry and Quantum Gravity”.

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., 14 pages, 7 figures (reduced resolution)

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)

Fluctuations around a Bose‐Einstein condensate can be described by means of Bogolubov theory leading to the notion of quasiparticle and antiquasiparticle familiar to non‐relativistic condensed matter practitioners. On the other hand, we already know that these perturbations evolve according to a relativistic Klein‐Gordon equation in the long wavelength approximation. For shorter wavelengths, we show that this equation acquires nontrivial corrections which modify the Klein‐Gordon product. In this approach, quasiparticles can also be defined (up to the standard ambiguities due to observer‐dependence). We demonstrate that—in the low as well as in the high energy regimes—both concepts of quasiparticle are actually the same, regardless of the formalism (Bogolubov or Klein‐Gordon) used to describe them. These results also apply to any barotropic, inviscid, irrotational fluid, with or without quantum potential.