Analogue gravity (2026 edition)

Analogue gravity

Carlos Barceló, Instituto de Astrofísica de Andalucía (IAA-CSIC), Granada. Stefano Liberati, SISSA, INFN and IFPU, Trieste. Matt Visser, School of Mathematics and Statistics, Victoria University of Wellington.

Living Reviews in Relativity (2026) 29:2. Received 30 January 2025; accepted 12 February 2026.

This article is a revised version of the 2011 Living Review; the 2011 edition is on this site at /library/stm-9d9474c4b5. The review runs to 222 pages. Reproduced below are the abstract, the introduction with its overview and motivations, the authors' own summary of what changed, and the conclusions. Sections 2 to 7 — acoustics as the simplest analogue spacetime, the history, the catalogue of models, phenomenological applications, experimental efforts and hints for quantum gravity — are omitted for length; the complete text is at the source.

Abstract

Analogue gravity is a research programme that explores analogues of general relativistic gravitational fields within other physical systems, particularly but not exclusively in condensed matter systems, with the aim of gaining new insights into related problems. Analogue models of gravity boast a long and distinguished history, dating back to the early years of general relativity. This review article delves into the history, aims, results, and future prospects of various analogue models. We begin by presenting a particularly simple example of an analogue model, then traverse the rich history and complex array of models discussed in the literature. The last decade has witnessed significant and sustained advances in analogue gravity, resulting in hundreds of published articles, workshops, and books. The future of the analogue gravity programme looks promising, with rapid technological advances on the experimental front and the potential for analogue models to inspire innovative approaches to the problem of quantum gravity on the theoretical front. Most of all, these recent years have seen the rise of an unprecedented collaboration and interplay between different communities that we believe will set a new standard for interdisciplinary research in the years to come.

Change summary

Major revision, updated and expanded. This Living Review has undergone a major update and upgrade, most critically adding a survey of the last 15 years of research since the previous 2011 version. The number of references has increased from 702 to 1285. There have also been significant updates in pedagogy and presentation.

1. Introduction

"And I cherish more than anything else the Analogies, my most trustworthy masters. They know all the secrets of Nature, and they ought to be least neglected in Geometry." — Johannes Kepler

Analogies have played a very important role in physics and mathematics — they provide new ways of looking at problems that permit cross-fertilization of ideas among different branches of science. A carefully chosen analogy can be extremely useful in focusing attention on a specific problem, and in suggesting unexpected routes to a possible resolution. In this review article we shall focus on a specific analogy popularly known as "analogue gravity", though one might make a case for the alternative phrase "analogue spacetime", which has proved to be an extremely rich research theme. This research line started with the identification of several sharp analogies, typically but not always based on condensed matter physics, to probe aspects of the physics of curved spacetime — and in particular to probe aspects of curved space quantum field theory, and to obtain lessons of potential relevance on the road towards a theory of quantum gravity. Of course, analogy is not identity, and it is clear that the analogue models considered in the literature are not completely equivalent to general relativity — we only need that the analogue gravity model, in order to be interesting from the gravitational perspective, should capture and reflect a sufficient number of relevant features of general relativity, or sometimes special relativity.

Moreover, it is precisely the imperfection of those models what makes them really interesting — in that they can provide a coherent framework for counter-factual alternatives to standard general relativity and so allow us to test the robustness of some theoretical expectations with respect to new physics.

Footnote 1. Sigmund Freud once said "Analogies, it is true, decide nothing, but they can make one feel more at home."

Footnote 2. However, it is important not to take the analogy too far. There have, thankfully rarely, been claims that manipulating an analogue spacetime metric could result in actual physical changes to the gravitational field — an idea that is clearly untenable.

The most well-known of these analogies is based on the realization that sound waves in a moving fluid provide an analogue system for light waves in a curved spacetime. A supersonic fluid flow can then generate a "dumb hole", the acoustic analogue of a "black hole", and the analogy can be extended all the way to theoretically proving the necessity of phononic Hawking radiation from the acoustic horizon, at least in situations where a second quantization of the sound wave field is meaningful. This particular example provides, at least in principle, a concrete, and most paradigmatic, laboratory model for curved-space quantum field theory in a realm that was expected, even 25 years ago, to be technologically accessible to experiment. As we shall see below, this expectation has been, at present, more than borne out.

Figure 1. Artistic impression of cascading sound cones, in the geometrical acoustics limit, forming an acoustic black hole when supersonic flow tips the sound cones past the vertical.

Figure 2. Artistic impression of trapped waves, in the physical acoustics limit, forming an acoustic black hole when supersonic flow forces the waves to move downstream.

After an initial period, essentially driven by theoretical analysis and speculation, experimentalists started to take on the challenge of testing these ideas around 2008. As of the writing of these lines, in early 2025, there are a significant number of experimental groups which regularly perform experiments with analogue gravity motivation. Numerous master's and doctoral theses, well over 50, have already been defended within these groups. The initial motivation of analogue gravity has been enlarged and the main emphasis somewhat shifted.

Currently, the interest in analogue gravity is not only to provide indirect experimental validation of gravitational effects difficult to observe in their native form, but analogue gravity has also been used:

1. to study and experimentally demonstrate the existence of more general phenomena — such as, for instance, super-radiance and quasi-normal modes — which, like Hawking radiation, appear to be more general than the process discovered in the gravitational realm; this can also be seen as analysis of the robustness of particular effects by showing how they survive the specifics of the different model systems;
2. to open-ended exploration of different but specific model systems for their own sake, but also with the idea of generating cross-fertilization between their physics and that which might lie underneath gravitational systems in regimes beyond the general relativistic.

From this perspective, the plethora of analogue models and systems, besides the already mentioned acoustic analogue, are useful for these or many other reasons — some of the analogue models are interesting for experimental reasons, others are useful for the way they provide new light on perplexing theoretical questions. The information flow is bi-directional, and sometimes insights developed within the context of general relativity can be used to understand aspects of the analogue system.

The list of analogue models is extensive, and in this review we will seek to do justice both to the key models, and to the key features of those models. More importantly we will provide an introduction and an overview of the amazing body of developments performed in the 43 year period since the analogue gravity revolution started.

1.1 Overview

In the following sections we shall:

• Discuss the flowing fluid analogy in some detail.
• Summarise the history and motivation for various analogue models.
• Discuss the many physics issues various researchers have addressed.
• Provide a representative catalogue of extant models.
• Discuss the main physics results obtained to date, both on the theoretical and experimental sides.
• Outline some of the many possible directions for future research.
• Summarise the current state of affairs.

By that stage the interested reader will have had a quite thorough introduction to the ideas, techniques, and hopes of the analogue gravity programme.

1.2 Motivations

The motivation for these investigations, both historical and current, is rather mixed. In modern language, the reasons to investigate analogue models are:

• Partly to develop an observational window on curved-space quantum fields.
• Partly to use condensed matter to gain insight into classical general relativity.
• Partly to use condensed matter to gain insight into curved-space quantum field theory.
• Partly to use classical general relativity to gain insight into condensed matter physics.
• Partly to gain insight into new and radically-different ways of dealing with quantum and emergent gravity.
• Partly to understand the nature and robustness of phenomena appearing in many systems including the gravitational realm.
• Partly to perform open-ended explorations of new phenomenology in systems with analogue gravity behaviours.

1.3 Going further

Apart from this present review article, and the references contained herein, there are several key items that stand out as starting points for any deeper investigation: the book "Artificial Black Holes" edited by Novello and colleagues; the archival website for the turn-of-the-century "Analogue models" workshop, which made analogue spacetimes mainstream; the book "The Universe in a Helium Droplet" by Volovik; the Physics Reports article "Superfluid analogies of cosmological phenomena", by Volovik; the review by Balbinot and colleagues focussing largely on back-reaction and short-distance effects; the Lecture Notes in Physics volume on "Quantum Analogies" edited by Unruh and Schützhold; the book based on the 2011 Como summer school on analogue gravity, presenting an overview of the ideas and problems underlying research in analogue gravity together with a description of several viable experimental settings; the book based on the Belem conference marking the 30-year anniversary of Unruh's 1981 article; the review by Kellay, which provides an overview of what can be done with soap bubbles and soap films; the 2020 article on "The next generation of analogue gravity experiments" by Jacquet and colleagues, focusing on experimental prospects; the more philosophical 2021 survey "The latest frontier in analogue gravity: New roles for analogue experiments" by Field; and the Nature Reviews Physics perspective by Braunstein and colleagues, which gives an up to date overview.

(Sections 2 to 7 — acoustics as the simplest example of an analogue spacetime; the history, from the Gordon metric to the contemporary period; the catalogue of models, including the new material on quantum fluids of light; phenomenological applications; experimental efforts, including the wave tanks at Nice, Vancouver and Poitiers and the Bose-Einstein condensate programme; and hints for quantum gravity — are omitted for length. The complete text is at the source.)

8. Conclusions

20 years have now passed since the first version of this Living Review appeared, and these authors feel it important to remark how prolific and energetic this field of research has become over this lapse of time. Developments have amounted to a virtuoso performance by the research community.

Indeed, by looking back at the previous version of this work, it is clear not only that the field has been growing in both scope and reach, but has also vastly changed in the composition of its community, attracting a wide range of practitioners with competencies well beyond those of the original researchers. What used to be mainly a community of researchers with a gravitation theory background, has now become a multi-faceted community in which experimentalists and theoreticians from both the general relativity and condensed matter communities meet.

We feel this is now the defining trait of Analogue Gravity, as it represents one of the few fields of modern physics where the general trend towards specialization and narrowing of communities has been successfully reverted. Analogue gravity has indeed grown from a largely theoretical endeavour into a field where table-top experiments are nowadays just as relevant, if not dominant, in settling the agenda and in driving very fruitful interdisciplinary collaborations.

This synergy has proven extremely successful: We nowadays have a much better understanding of the nature of Hawking radiation, and how and why it can be robust against the ultraviolet structure of spacetime. Furthermore, we have been able to reproduce it experimentally, together with several other phenomena, from cosmological particle creation to superradiance, which are pillars of modern gravitation theory. At the same time analogue gravity has been able to stimulate new work and investigations in condensed matter theory by providing a new framework within which methods and intuition provided from gravitational phenomena can be used. It is a pity that so many insights have sometimes been neglected by researchers in theoretical physics working on the same issues, merely on the basis that analogue gravity is not "real" gravity.

Indeed, the fact that analogue models are not perfect mimics of the standard phenomena, for example because of the breakdown of Lorentz invariance in the ultraviolet, has proven to be a strength, rather than a weakness: These new features allowed one to break degeneracies, and exposed subtleties of these phenomena, so greatly enhancing our understanding of them.

Also, analogue gravity has provided many toy models for the possible interplay between large scale, classical and continuum structure of spacetime and the microphysics from which it might emerge. All useful lessons that are there to be used by the quantum gravity community, as they have been in rare cases.

In conclusion, following the research trajectory we hope we have properly conveyed by this work, these authors expect that interest in analogue models will continue unabated in the years to come, and suspect that several key, unexpected, issues will come up in the future. Analogue gravity can by now be viewed as a mature field, but being supported by a large, interdisciplinary and vibrant community, we expect that advances will continue to be very significant in the years to come.

Change details

This Living Review has undergone a major update and upgrade, most critically adding a survey of the last 15 years of research since the previous 2011 version. There have also been significant updates in pedagogy and presentation. Specifically:

• We have updated the presentation of the section on acoustic analogues to be a little clearer and more notationally consistent as to the separation between the background flow and the linearized fluctuations. We are also somewhat more careful concerning the distinction between the speed of sound and the speed of light.
• We have totally rewritten and reorganized the historical section — we have split the discussion into Historical, Classical, Early Modern, Late Modern, and Contemporary periods. Much of the Late Modern and all of the Contemporary periods post-date the previous 2011 version of this Living Review.
• We have added a brief sub-section on analogue-related doctoral and master's theses — by now well over 50 analogue-related doctoral theses have been generated.
• We have updated and streamlined the Catalogue of models, and in particular have added a new subsection on quantum fluids of light.
• The section on phenomenological applications has been updated, specifically with regard to the utility of quantum correlations as probes for the Hawking effect.
• The section on experimental efforts has been massively updated, specifically to reflect advances in both Bose-Einstein condensate experiments and in water tank experiments.
• The section on hints for quantum gravity now has an extra section on the Marolf theorem, and has otherwise been generally updated.
• We have revised many of the figures to make them more visually bold, and to improve legibility, clarity, and readability.

(Acknowledgements and the 1285-item reference list are omitted here; the complete text is at the source.)