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The next generation of analogue gravity experiments

Discussion meeting

Starts:

December
092019

09:00

Ends:

December
102019

17:00

Location

The Royal Society, London, 6-9 Carlton House Terrace, London, SW1Y 5AG

Overview

Scientific discussion meeting organised by Dr Maxime Jacquet, Dr Silke Weinfurtner and Dr Friedrich König.

Vortex in a water flow. Copyright Alex Wilkinson Media

The discussion meeting brought together scientists who reproduce effects of the interplay between general relativity and quantum physics in the laboratory. From the foundational ideas of the research programme to its latest experimental breakthroughs and their philosophical implications, participants discussed the present state of the field and possible routes to the future.

The schedule of talks and speaker biographies are available below. Speaker abstracts are also available below. Recorded audio of the presentations will be available on this page shortly. Meeting papers will be published in a future issue of Philosophical Transactions of the Royal Society A

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Schedule of talks

09 December

Session 1 09:00-12:30

Moving horizons

4 talks Show detail Hide detail

Chairs

Associate Professor Almut Beige, University of Leeds, UK

09:00-09:30 Hawking radiation in optics and beyond

Dr David Bermudez, Cinvestav, Mexico

Abstract

Almost 40 years ago Unruh proposed a fluid-mechanical analogy of Hawking radiation. In the decade thereafter, this work inspired a series of studies in the astrophysical case to understand the phenomenon from a more general point of view, by Jacobson and Visser. At the beginning of this millennium, there were several proposals to take this analogy from the blackboard to the laboratory in several systems, including phonons in Bose-Einstein condensates by Garay et al. and photons in optical fibers by Leonhardt et al.

The last decade has seen a rise of experiments related to analogue Hawking radiation, as those by the groups of Rousseaux, Weinfurtner, König, Faccio, Steinhauer, and Leonhardt. There is no doubt that the Hawking effect can indeed be generalised to other fields outside of astrophysics. Now, the experiments may lead the way into the actual treatment of the quantum field theory in the analogue systems. This analogy can be beneficial to both astrophysics and the analogue systems. For example, the use of negative frequencies in optics has already lead to the prediction and measurement of new nonlinear effects, the negative-frequency resonant radiation by Faccio's and the negative-frequency Hawking radiation by Leonhardt's group.

This talk discusses the analogy of Hawking radiation and the lessons learnt so far. Then Dr Bermudez will suggest a way forward with emphasis on the optical case, including a discussion on the physical origin of the analogue effect.

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09:30-09:45 Discussion

09:45-10:15 Light at the horizon: optical experiments with black holes made of light

Yuval Rosenberg, Weizmann Institute of Science, Israel

Abstract

In theory, realising Hawking radiation in optical analogues is almost trivial, but in reality it is not. It has taken a series of ideas and practical improvements for being able to measure stimulated Hawking radiation in such an optical analogue [Phys. Rev. Lett. 122, 010404 (2019)]. 

In this talk, Yuval Rosenberg will present measurements of the stimulated Hawking effect and the progress made to enable them. He will discuss general practical considerations and lessons learnt along the way, translating essential features in the theory of Hawking radiation [Int. J. Mod. Phys. D. 12, 649 (2003)] – an apparent horizon, non-zero surface gravity, and slow evolution – to essential features in the realisation of its optical analogue – in terms of light sources, nonlinear media, and detectors. The crucial role of dispersion and the roles of phase velocity and group velocity horizons will be made clear. It is also remarkable how robust the Hawking effect turns out to be: there is no gravity, no Schwarzschild geometry, no singularity, no weak dispersion, no static metric, no single modes, and yet we see Hawking radiation.

Discussing these recent answers and the remaining open questions is hoped to guide us to the next generation of analogue gravity experiments.

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10:15-10:30 Discussion

10:30-11:00 Coffee

11:00-11:30 Superconducting circuit analogues of Hawking radiation and the dynamical Casimir and Unruh effects

Professor Miles Blencowe, Dartmouth College, USA

Abstract

Professor Blencowe begins by giving an introduction to superconducting microwave circuits and the all-important Josephson tunnel junction element. The latter serves as a low noise, flux tunable inductor, thus enabling the realisation of microwave transmission lines and cavities for which the effective index of refraction can be spatially and/or temporally modulated. He then describes how such circuits can furnish analogues of the dynamical Casimir effect (photon pairs produced from a cavity or semi-infinite transmission line microwave vacuum through an effective moving mirror boundary), and Hawking radiation (emission of photon pairs from an effective propagating event horizon in a microwave transmission line). In the final part, Professor Blencowe describes current work that considers GHz mechanically oscillating (ie accelerating) photodetectors that couple to the microwave cavity modes, and argue that measurable photon production rates from vacuum should result.

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11:30-11:45 Discussion

11:45-12:15 Optical experiments for analogue gravity

Jack Petty, University of St Andrews, UK

Abstract

Since the creation of the field of analogue gravity, many different experimental systems have been proposed in which versions of the Hawking effect can be made possible, each with their own advantages. 

The Photonic Crystal Fiber is a unique medium giving us the opportunity to control the parameters of the interaction with great precision and to perform experiments in carefully chosen regimes. In particular, our understanding of similar processes in optical fibers give a solid starting point from which to investigate the Hawking process. Although much progress has been made in the field, many questions remain unanswered. In particular, spectral correlations of the emitted quanta, a telltale sign of Hawking radiation, as well as the entanglement of the photon pairs have yet to be observed in fibre optical analogues. In this talk, Jack Petty will describe the advantages of the fibre optical system and the challenges which need to be tackled in order to progress towards the next generation of analogue gravity experiments.

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12:15-12:30 Discussion

12:30-13:30 Lunch

Session 2 13:30-17:00

Atomic analogues

4 talks Show detail Hide detail

Chairs

Dr David Bermudez, Cinvestav, Mexico

13:30-14:00 Simulating false vacuum decay in cold atom BECs

Dr Jonathan Braden, Canadian Institute for Theoretical Astrophysics, Canada

Abstract

Local energy density minima (so called false vacua) are a ubiquitous feature of quantum field theories with complex interaction potentials. Large regions of space trapped in one of these local minima are rendered metastable by quantum effects. The decay is expected to occur via the nucleation and subsequent coalescence of bubbles of new phase, as in a first order phase transition. This process of false vacuum decay has wide-ranging cosmological applications, including describing the origin of our Universe, providing a source of primordial gravitational waves, and determining the ultimate fate of our current Higgs vacuum. The relevant dynamics occur in a regime where we have limited theoretical control, and which is inaccessible via direct experimentation.

Dr Braden will show how the key dynamics of relativistic false vacuum decay can be reproduced by an interacting two-component dilute gas cold atom Bose-Einstein condensate (BEC), providing a possible experimental window into some of the most fundamental questions about our cosmos. In the appropriate low energy limit, the evolution of the relative phase is approximately governed by a relativistic wave equation. By periodically modulating the coupling between the two species, a series of true and false vacuum minima can be induced, allowing for the exploration of relativistic false vacuum decay. Dr Braden presents results for false vacuum decay in the analogue cold atom system, using numerical simulations of the Gross-Pitaevskii equation (GPE) describing the condensates. These simulations demonstrate that for a range of parameters the relativistic field description remains valid and the decay proceeds through bubble nucleation. Meanwhile, for other parameter ranges the decay occurs through either spinodal instability or parametric resonance of short wavelength fluctuations. Finally, Dr Braden provides an analytic characterisation of the resonant instability, which may provide a way to experimentally test the validity of the GPE description of the condensates.

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14:00-14:15 Discussion

14:15-14:45 Trapped atoms and cosmic analogues

Dr Ian B Spielman, JQI: NIST and University of Maryland, USA

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14:45-15:00 Discussion

15:00-15:30 Tea

15:30-16:00 Phonon pair creation by inflating quantum fluctuations in an ion trap

Professor Tobias Schaetz, University of Freiburg, Germany

Abstract

Quantum theory predicts intriguing dynamics during drastic changes of external conditions. An extremal example is a rapid cosmic expansion, tearing apart quantum fluctuations and, thereby, turning them into pairs of 'real' particles. This is accompanied by the generation of quantum entanglement at large distances. Similar mechanisms cause the Sauter-Schwinger effect and Hawking radiation. A closely related process of expanding space-time during cosmic inflation explains the creation of the seeds for structure formation. Even though signatures of this effect can still be observed today in the cosmic microwave background radiation, direct tests remain out of reach. As an alternative, analogue features have been observed in several experimental platforms.

However, preserving the fragile quantum dynamics requires fast control of the system on the level of single quanta, as well as close to ideal isolation from the environment. Trapped atomic ions are well suited to study fundamental quantum dynamics as they feature unique fidelities in preparation, control, and detection of quantum states.

Professor Schaetz reports results on switching the trapping field of two ions sufficiently fast to tear apart quantum vacuum fluctuations, ie, create pairs of phonons and, thereby, squeeze the ions’ motional state. He discusses this process as an experimental analogue to cosmological particle creation in the early universe, accompanied by the formation of spatial entanglement.

This platform might allow studying the causal connections of squeezing, pair creation, and entanglement and might permit to cross-fertilise between concepts in cosmology and applications of quantum information processing.

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16:00-16:15 Discussion

16:15-17:00 Panel discussion led by Professor Ian Walmsley FRS

Professor Ian Walmsley FRS, Imperial College London, UK

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17:00-18:00

Poster session

10 December

Session 3 09:00-13:30

Rotating geometries

5 talks Show detail Hide detail

Chairs

Dr Maria Chiara Braidotti, University of Glasgow, UK

09:00-09:30 Spontaneous Hawking radiation and beyond: observing the time evolution of an analogue black hole

Professor Jeff Steinhauer, Technion – Israel Institute of Technology, Israel

Abstract

We observe the time dependence of the Hawking radiation in an analogue black hole. Soon after the formation of the horizon, there is little or no Hawking radiation. The Hawking radiation then ramps up during approximately one period of oscillation, until it reaches the quantity expected for spontaneous emission. This is similar to a black hole created from gravitational collapse. The spectrum remains approximately constant at the spontaneous level for some time, similar to a stationary black hole. An inner horizon then forms, in analogy with a charged black hole. The inner horizon causes stimulated Hawking radiation. Both types of stimulation predicted by Ted Jacobson and coworkers likely contribute, but the monochromatic stimulation probably contributes more than does the black-hole lasing.

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09:30-09:45 Discussion

09:45-10:15 Quantum fluids of light for simulating rotating black holes

Professor Elisabeth Giacobino, Laboratoire Kastler Brossel, France

Abstract

Quantum fluids of light can be created in an optical cavity or in paraxial propagation. In such conditions propagating photons behave like massive particles, because they acquire a small effective mass. Moreover they interact with each other when the medium in which they propagate contains a nonlinear material. In the past few years two such systems, exciton polaritons in a semiconductor microcavity and photons in a hot Rubidium vapour have proved to be interesting platforms for the study of quantum hydrodynamics, including Bose-Einstein condensation, superfluidity, Cerenkov waves, or quantum turbulence. Topological excitations like vortices can be created in a polariton fluid with a defect or injected with engineered orbital angular using a specific laser excitation. 

Based on these properties, Professor Giacobino will show that these systems are very promising for the study of analogue gravity physics in a fluid of light. As proposed in several theoretical works, cavity polaritons can be considered for the emulation of Hawking physics. A closed 2D event horizon, appearing in the polariton superfluid where the fluid velocity becomes larger than the speed of sound in the fluid can be used to simulate the physics of black holes. Moreover, by injecting orbital angular momentum in the flow, the physics of rotating black holes can be studied, in particular Penrose process such as extracting rotation energy from the black hole or acceleration of excitations passing close to a rotating black hole.

 

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10:15-10:30 Discussion

10:30-11:00 Coffee

11:00-11:30 Optical superradiant wave amplification in rotating space-times

Professor Daniele Faccio, University of Glasgow, UK
Dr Maria Chiara Braidotti, University of Glasgow, UK

Abstract

Superradiance is the amplification of waves scattered by a rapidly rotating object, first introduced by Roger Penrose in 1969 for rotating black holes. This phenomenon is not peculiar of the field of astrophysics: a few years after Penrose's proposal, in 1971, Zel’dovich showed that electromagnetic waves scattered by a metallic rotating cylinder get amplified whenever their angular frequency ω satisfies the condition ω<mΩ, where m is the waves angular frequency and Ω is the cylinder angular velocity. Theoretical predictions of rotating superradiance amplification have been presented for many other physical systems as in hydrodynamics, nonlinear optics and Bose-Einstein condensates. However, the only measurement of superradiance has been recently reported in water waves, in a draining bathtub experiment.

In this talk, Professor Faccio elucidates how the process of Penrose-like superradiance can be observed in superfluids of light, and shows how it can be realised in a true optics experiment. Thanks to a novel analysis based on Noether currents, they show that superradiant scattering occurs in a 2D rotating nonlinear superfluid of light. This analysis addresses the full Nonlinear Schrödinger (NLS) equation without making use of the analogy with curved space-time and accounting for quantum pressure, always present in Bose-Einstein condensates and photon fluid analogues. Results of numerical simulations demonstrate the observability of the phenomenon is a nonlinear optics experiment.

This work deepens the understanding of the superradiance scattering in superfluids, reporting an experimental proposal to observe superradiated light, and unveils novel phenomena in the field of nonlinear optics.

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11:30-11:45 Discussion

11:45-12:00 Superradiance, geodesics and quasi-normal modes of rotating black holes in vortex flows

Theo Torres, University of Nottingham, UK

Abstract

While the original analogy between condensed matter systems and curved space-time geometries was originally derived under strict conditions, recent analogue gravity experiments suggest that vortex flows outside the analogue regime and rotating black holes still share similar fundamental effects.

In this talk, Theo Torres will focus on light-bending, superradiant scattering and quasi-normal modes emission in the presence of dispersive effects.

Using tools originally developed in black hole physics, Theo Torres will characterise theoretically the quasi-normal mode spectrum emitted by a perturbed vortex flow. This is done by extending and reinterpreting the notion of light-rings. An experiment will be presented where the relaxation spectrum of a vortex flow was measured and found to be in agreement with the light-ring prediction. 
Finally, relying on the fact that the quasi-normal modes are fully characterised by the properties of the background flow, Theo will introduce a new and non-invasive flow measurement technique applicable to fluids and superfluids alike. This method, that is called analogue black hole spectroscopy, will be presented theoretically and tested experimentally.

These theoretical and experimental results exhibit a new facet of the fluid-gravity analogy and shine new light on fundamental processes of rotational systems.

 

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12:00-12:15 Back-reaction in rotating black hole vortex flows

Dr Cisco Gooding, University of Nottingham, UK

Abstract

The current focus of analogue gravity research is on the behaviour of classical and quantum fields on fixed background spacetimes. However, in many cases of interest, including the scattering of high-amplitude waves from small compact objects or the extreme case of late-stage Hawking evaporation, the spacetime geometry is altered significantly by interactions with a field. Dr Gooding will discuss a recent analogue rotating black hole experiment in a vortex flow that exhibits back-reaction, and also admits a relatively simple theoretical description. Dr Gooding will then comment on future back-reaction experiments, and the possibility of using analogue systems to provide experimental guidance for quantum gravity.  

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12:15-12:30 Discussion

12:30-13:30 Lunch

Session 4 13:30-17:00

Beyond the horizon

4 talks Show detail Hide detail

Chairs

Professor Miles Blencowe, Dartmouth College, USA

13:30-14:00 Classical hydrodynamics for analogue spacetimes: hydraulic channels and thin films

Dr Germain Rousseaux, CNRS, Institut Pprime, France

Abstract

In this talk, Dr Rousseaux will first review the way to build analogue space-times in fluid mechanics by looking at the flow phase diagram and the corresponding analogue experiments performed during the last 13 years in the associated flow regimes (Nice 2008–2010, Vancouver 2011, Poitiers 2013–2017). Then, they will discuss the effect of non-linearities and capillarity on analogue Hawking radiation in hydraulic channels and their impact on the interpretation of the above experiments with some guidelines for future work. Thin films like the circular jump with different dispersive properties will be discussed in a second part with the presentation of a brand new system for the next generation of analogue gravity experiments in hydrodynamics.

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14:00-14:15 Discussion

14:15-14:45 Cosmology in the laboratory

Professor Ulf Leonhardt, Weizmann Institute of Science, Israel

Abstract

Experiments on analogues of gravity have extended the theory of quantum phenomena in gravitational fields to other areas of physics, confirming them there, but their primary goal is to learn something new for astrophysics. This lecture explains how analogues of gravity may shed light on one of the greatest challenges of theoretical cosmology: the enigma of dark energy. 

In 1968 Zeldovich published the conjecture that the cosmological constant – now known as dark energy – is a consequence of vacuum fluctuations. While he got the correct structure of Einstein’s cosmological term, he got a vastly incorrect quantitative value. In its modern version, the theory is off by 120 orders of magnitude from the measured value. 

What is the problem? While all real experiments on forces of the quantum vacuum are explicable by renormalised vacuum fluctuations, gravity was thought to perceive the total, bare vacuum energy. One might regard the measured value of the cosmological constant as the result of an experiment disproving this idea. In fact, as the lecture will explain, the most successful theory of vacuum forces in real materials, Lifshitz theory, may be adapted to account for the correct order of magnitude of the measured cosmological constant [https://doi.org/10.1016/j.aop.2019.167973]. This adaptation is possible due to the analogy between gravitational fields and electromagnetic media, and heavily draws on insights gained in the physics of horizons.

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14:45-15:00 Discussion

15:00-15:30 Tea

15:30-16:00 On the limits of experimental knowledge

Dr Karim Thébault, University of Bristol, UK

Abstract

To demarcate the limits of experimental knowledge Dr Thébault probes the limits of what might be called an experiment. By appeal to examples of scientific practice from astrophysics and analogue gravity, they demonstrate that the reliability of knowledge regarding certain phenomena gained from an experiment is not circumscribed by the manipulability or accessibility of the target phenomena. Rather, the limits of experimental knowledge are set by the extent to which strategies for what we call ‘inductive triangulation’ are available: that is, the validation of the mode of inductive reasoning involved in the source-target inference via appeal to one or more distinct and independent modes of inductive reasoning. When such strategies are able to partially mitigate reasonable doubt, we can take a theory regarding the phenomena to be well supported by experiment. When such strategies are able to fully mitigate reasonable doubt, we can take a theory regarding the phenomena to be established by experiment. There are good reasons to expect the next generation of analogue experiments to provide genuine knowledge of unmanipulable and inaccessible phenomena such that the relevant theories can be understood as well supported.

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16:00-16:15 Discussion

16:15-17:00 Panel discussion led by Professor William G Unruh FRS

Professor William G Unruh FRS, University of British Columbia, Canada and Texas A&M University, USA

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The next generation of analogue gravity experiments

9 - 10 December 2019

The Royal Society, London 6-9 Carlton House Terrace London SW1Y 5AG UK
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