The next generation of analogue gravity experiments

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.

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

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

Jeff Steinhauer is an associate professor in the Physics Department at the Technion – Israel Institute of Technology. He studies superfluid aspects of Bose-Einstein condensation. Since 2009, his research has focused exclusively on Hawking radiation in a sonic black hole, resulting in the only observations of spontaneous Hawking radiation to date. He pioneered the combination of mesoscopic physics and ultracold atoms by creating a system for imaging and manipulating the atoms with high optical resolution. This allowed him to make the first observation of the AC Josephson effect in a Bose-Einstein condensate, the first observation of the DC Josephson effect in any system other than superconductors, and the first in situ study of an optical lattice with single-site resolution and tunnelling between sites.  During his postdoc at the Weizmann Institute of Science, he performed the textbook measurement of the dispersion relation of a Bose-Einstein condensate. His interest in superfluids began during his graduate work at UCLA and UC Berkeley, where he studied vortex nucleation mechanisms in superfluid ⁴He.

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.

Professor Elisabeth Giacobino, Laboratoire Kastler Brossel, France

Professor Elisabeth Giacobino, Laboratoire Kastler Brossel, France

Elisabeth Giacobino is one of the pioneers of quantum optics and quantum information, with the first experimental demonstration of two-mode squeezing with an optical parametric oscillator in 1987. She showed the generation of squeezed and correlated light in a semiconductor microcavity. She achieved squeezed and entangled light generation and quantum state storage with cold atomic ensembles, with the first demonstration of a quantum memory for photons carrying orbital angular momentum. Recently she investigated quantum fluid properties of light in semiconductor microcavities, with the first demonstration of superfluidity and of quantum turbulence effects in microcavity polaritons. An important direction now concerns the use of polariton fluids for quantum simulation of systems of various nature, ranging from condensed matter to astrophysics.

Elisabeth Giacobino is the author of 230 publications, and she is a member of Academia Leopoldina, a fellow of EPS, EOS and OSA.

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.

Professor Daniele Faccio, University of Glasgow, UK

Professor Daniele Faccio, University of Glasgow, UK

Daniele Faccio is a Royal Academy Chair in Emerging Technologies and Fellow of the Royal Society of Edinburgh. He joined the University of Glasgow in 2017 as Professor in Quantum Technologies and is adjunct professor at the University of Arizona, Tucson, USA. From 2013-2017, he was professor at Heriot-Watt University where he was also deputy director of the Institute of Photonics and Quantum Sciences. He has been visiting scientist at MIT, USA, Marie-Curie fellow at ICFO, Barcelona, Spain and EU-ERC fellow 2012-2017. He was awarded the Philip Leverhulme Prize in Physics in 2015, the Royal Society of Edinburgh Senior Public Engagement medal and the Royal Society Wolfson Merit Award in 2017. He worked in the optical telecommunications industry for four years before obtaining his PhD in Physics in 2007 at the University of Nice-Sophia Antipolis. His research, funded by the UK research council EPSRC, DSTL, The Leverhulme Trust, and the EU Quantum Flagship program, focuses on the physics of light, on how we harness light to answer fundamental questions and on how we harness light to improve society.

Dr Maria Chiara Braidotti, University of Glasgow, UK

Dr Maria Chiara Braidotti, University of Glasgow, UK

Maria Chiara Braidotti graduated in Physics in 2014 from Sapienza University of Rome, Italy and received a PhD in Physics from the University of L’Aquila, Italy, in 2018 for the study of light in extreme conditions to address open questions in physics, such as time irreversibility and gravitational analogues. Since March 2018 she has been a Research Associate at the School of Physics and Astronomy at the University of Glasgow, UK, where her research activity focuses on the study of fundamental physics through optics and photonics applications and the physics of light in complex systems. In particular, she carries out both theoretical and experimental investigations of quantum and nonlinear optical systems aiming at studying basic concepts of gravitational physics, such as artificial curved space-time geometries and physics at the Planck scale. She has authored and co-authored 15 publications and given numerous talks in international conferences.

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.

Theo Torres, University of Nottingham, UK

Theo Torres, University of Nottingham, UK

Theo Torres is currently a PhD student working with Dr Weinfurtner on hydrodynamic simulation of rotating black holes. Before starting his PhD he was a student at the University Paris VII in Paris, France. Theo Torres' research interests are mainly focused around black holes, modified gravity and quantum gravity. Initially trained as a theoretical physicist, he has however followed the idea of logical positivism by investigating these subjects from an experimental point of view by conducting various experiments to mimic black hole physics in a laboratory environment.

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.  

Dr Cisco Gooding, University of Nottingham, UK

Dr Cisco Gooding, University of Nottingham, UK

Cisco Gooding is a Canadian theoretical physicist. He received his Bachelor of Science in Mathematical Physics from Simon Fraser University, and then went on to complete his PhD in Physics at the University of British Columbia, under the supervision of Bill Unruh. His thesis topic was gravitational decoherence. Cisco then spent a year as a postdoc with Bill Unruh researching quantum gravity and analogue gravity experiments, after which he started his current position in the UK as a postdoc with Silke Weinfurtner at the University of Nottingham, where he continues to study quantum gravity and analogue gravity experiments. Cisco's other research interests include quantum information theory, cosmology, optomechanics, and fluid dynamics.

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.

Dr Germain Rousseaux, CNRS, Institut Pprime, France

Dr Germain Rousseaux, CNRS, Institut Pprime, France

Germain Rousseaux is a CNRS research scientist with initial training in both Condensed Matter Physics and Fluid Mechanics at INPG, Grenoble Institute of Technology. After completing a PhD on the formation of sand ripples at ESPCI (Paris) under the supervision of José-Eduardo Wesfreid, he studied Nonlinear Physics with Pierre Coullet at Nice University, where he started his work on Analogue Gravity by performing seminal experiments on wave-current interaction in a coastal engineering company. Since 2012, he has worked in Poitiers on interdisciplinary subjects with a fluid mechanics initial impulse like Analogue Gravity, the Ancient Naval Battle of Actium, Tidal Bore Physics, Wave Resistance and the Physics of Ship Wakes with collaborators in Physics, Fluid Mechanics, Mathematics and Human Sciences.

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.

Professor Ulf Leonhardt, Weizmann Institute of Science, Israel

Professor Ulf Leonhardt, Weizmann Institute of Science, Israel

Ulf Leonhardt joined the Weizmann Institute of Science as Professor of Physics in November 2012 after an international career. Ulf Leonhardt was born in Schlema, in former East Germany, on October 9th 1965. He studied physics at Friedrich-Schiller University Jena, at Moscow State University and at Humboldt University Berlin where he received his PhD in 1993. He was a Visiting Scholar at the Oregon Center for Optics in the US, a Habilitation Fellow at the University of Ulm in Germany and a Göran-Gustafson and Feodor-Lynen fellow at the Royal Institute of Technology in Stockholm, Sweden. From 2000 until his appointment at the Weizmann Institute in 2012, he was the Chair in Theoretical Physics at the University of St Andrews, UK. Ulf Leonhardt also had various visiting positions: in 2008 he was a Visiting Professor at the National University of Singapore and in 2011 at the University of Vienna; in 2012/13 he was an adjunct professor at South China Normal University. Ulf Leonhardt is the first from former East Germany to receive the Otto Hahn Award of the Max Planck Society. For his PhD thesis he received the Tiburtius Prize of the Senate of Berlin. In 2006 Scientific American listed him among the top 50 policy, business and research leaders of the world of that year. In 2008 he received a Royal Society Wolfson Research Merit Award and in 2009 a Theo Murphy Blue Skies Award of the Royal Society. In 2012 he received a thousand-talent award of China. He is a Fellow of the Institute of Physics and of the Royal Society of Edinburgh.

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.

Dr Karim Thébault, University of Bristol, UK

Dr Karim Thébault, University of Bristol, UK

Karim Thébault is a Senior Lecturer in the Department of Philosophy at the University of Bristol. His research interests are principally within the philosophy of physics, with an emphasis on classical and quantum theories of gravity, and within the philosophy of science, with an emphasis on scientific methodology. He studied Physics and Philosophy at the University of Oxford and Theoretical Physics at Imperial College London before completing his doctoral studies in Philosophy at the University of Sydney. Before moving to Bristol he was an Assistant Professor at the Munich Center for Mathematical Philosophy.