Scattering of Co-Current Surface Waves on an Analogue Black Hole

Abstract

We have realized a stationary transcritical flow of water in a flume that possesses the analogue of a black hole horizon for long-wavelength surface waves. The horizon has been probed via the scattering of an incident co-current wave, which partially scatters into counter-current waves on either side of the horizon, yielding three outgoing waves (of which one is anomalous) rather than two in the absence of transcriticality. The measured scattering coefficients are in good agreement with the predictions of the non-dispersive theory, where the kinematical description in terms of an effective spacetime metric is exact. We also show the emergence of characteristic peaks in the two-point correlation function of free surface deformations, one of which indicates the presence of a horizon and in quantum settings would be used to demonstrate the presence of the thermal analogue Hawking effect.Recent years have witnessed an explosion of interest in Analogue Gravity [1]. This newly emergent field builds on Unruh's insight of 1981 [2] that, using the mathematical equivalence between wave propagation in curved spacetimes and in condensed matter systems, one can realize in the laboratory an analogue black hole and test Hawking's prediction that black holes emit radiation [3,4]. An analogue black hole is represented by an accelerating flow which at some point becomes transcritical. The point where the flow reaches the speed of sound is the (acoustic) horizon for sound waves, in analogy with the (event) horizon for light in a gravitational black hole. Because wave propagation close to the horizon is equivalent in the two cases, the Hawking effect, which is derived from purely kinematical considerations, should be present also in the analogue system. This effect has been the main driving force behind the Analogue Gravity program since its inception, with experimental studies in a wide range of physical systems, including photons in nonlinear optics [5,6], exciton-polaritons [7] and Bose-Einstein condensates [8,9].Surface waves on fluids are describable in this way, and there have been a number of experiments aimed at realizing and probing the analogue spacetimes [10][11][12][13][14][15][16]. Most, however, have focused on analogue white hole-like flows, which are time-reversed analogue black hole-like flows. While wave scattering on transcritical flows has been studied from a theoretical perspective [17,18], such flows have not yet been realized in experiment, not even of the white hole type. Moreover, white hole horizons are associated with the production of a stationary undulation [19,20] which complicates the effective spacetime and the scattering it induces.In this paper we report on the first-known scattering experiment of surface waves on a 1 + 1-dimensional analogue black hole, namely an effectively one-dimensional transcritical water flow inducing an effective spacetime metric with a black-hole horizon. We show how the waves available in each of the asymptotic regions indicate that a horizon is present. We also report on ...

The next generation of analogue gravity experiments

“…In 2015, a new analogue gravity platform emerged: fluids of light realized with polaritons in semiconductor microcavities, in which sonic horizons were demonstrated [20]. In the following year, the open channel-flow experiments made significant advances by extracting Hawking correlations from classical noise on the air-water interface surrounding an effective white hole horizon [21], leading to [22]. In the same year, the first study on the extraction of Hawking correlations from an effective black hole horizon in an atomic Bose-Einstein condensate appeared [23].…”

Analytical description of quantum emission in optical analogs to gravity

“…These horizons scatter waves and are predicted to spontaneously emit quanta by the Hawking effect [2,3]. Recently, experiments in many different 'fluid' systems, such as Bose-Einstein condensates [4,5], water [6][7][8][9][10][11] and polariton microcavities [12] have demonstrated horizons and studied the behaviour of waves in their vicinity. New developments in quantum fluids of light [13][14][15] may soon enable novel analogue gravity experiments.…”

Polariton fluids for analogue gravity physics

“…It has been theoretically predicted that it is possible to observe the Hawking effect at these sonic horizons [11,41]. We propose to begin by seeding the scattering process with a classical, low-amplitude excitation (similar to [5]). This can be done with a probe beam with an energy higher than that of the polariton flow, propagating in the downstream supersonic region towards the horizon.…”

“…Thus, analogue gravity enables the study of classical and semi-classical effects of fields on curved space–times, such as the Hawking effect at static horizons [15,16] or superradiance [17–20] and the quasi-normal modes of rotating geometries [21]. These effects were recently observed in water experiments [5,13,14]. In these seminal experiments, excitations were seeded by a classical state to stimulate the emission.…”