Analogue Gravity Laboratories

Speculative If the Hawking effect really is independent of the physics that governs a system's shortest wavelengths — which no experiment has tested by deliberately changing that physics, and 7 says precisely what would — then a table-top fluid is the only instrument anyone will ever build that measures radiation from a horizon. That conditional is the whole proposition, and it is stated first because the field's rhetoric routinely omits it.

Established The baseline is a null of fifty years' standing. No instrument has detected Hawking radiation from an astrophysical source; the 2026 Nature paper that reports the most recent analogue result opens by saying the radiation “has never been observed in astronomy, only in laboratory analogues, and the chances of ever observing it in space are astronomically small.” Against that baseline, a Bose-Einstein condensate at the Technion returned a correlation spectrum that is thermal at 0.35 nK — the temperature its own measured surface gravity predicts — assembled from 7,400 repetitions of a single sequence on a single optical table. The cost comparison is not close: one cold-atom laboratory against an astrophysical detection whose own practitioners decline to price.

Frontier The prize is a specific, bounded transfer: the kinematic half of black-hole quantum field theory becomes an experimental science, at nanokelvin rather than at the Planck scale. Horizon formation, mode conversion, pair correlation, stationarity and ringdown have all been measured in at least one medium — water channels, optical fibre, polariton fluids, superfluid helium at 600 µK, a ten-qubit superconducting chain. What is bought is a check on a derivation that has never otherwise been checked. What is not bought is anything about the Einstein equations, because in every one of these systems the metric is fixed algebraically by the experimenter rather than dynamically by the field it carries.

Frontier The gains accrue to a field of perhaps a few hundred people and to the instrument-makers downstream of them. The UK consortium QSimFP lists four universities and 54 publications between October 2019 and September 2024 (consortium's own listing); the January 2026 Benasque workshop ran with eight invited lecturers. Outside that circle, the near-term return is metrological — imaging at 0.8 µm, heralded single photons with a coincidence-to-accidental ratio of 75, microwave amplifiers that measure two-mode squeezing — not gravitational.

Speculative The condition on all of it: the analogy is kinematic, and the one experiment that would show the kinematics carry the physics has not been run. The bottleneck list in 5 is where that gets expensive.

Established An analogue horizon is a place where a medium flows faster than its own waves travel. William Unruh's 1981 paper put it in one sentence: “It is shown that the same arguments which lead to black-hole evaporation also predict that a thermal spectrum of sound waves should be given out from the sonic horizon in transsonic fluid flow.” Sound in a moving fluid obeys a wave equation that is formally identical to a massless scalar field on a curved spacetime, and the curved spacetime in question — the acoustic metric — is built from the flow velocity and the local sound speed. Where the flow crosses the sound speed, sound emitted downstream cannot get back. That surface is a horizon in the same technical sense the mathematics uses for a black hole.

Established The identity is exact at the level of the wave equation and absent at the level of the field equations, and that split is the entire subject. Barcelo, Liberati and Visser's Living Reviews in Relativity survey states the distinction directly: in general relativity “the spacetime metric is related to the distribution of matter by the non-linear Einstein-Hilbert differential equations. In contrast, in the present context, the acoustic metric is related to the distribution of matter in a simple algebraic fashion.” An experimenter sets the analogue metric by setting the flow. It does not respond to the energy carried by the waves on it, except through effects that are, in every apparatus built so far, either negligible or externally imposed.

Established The Hawking relation the analogues test is a statement about surface gravity, not about mass. In the gravitational case the temperature is fixed by the surface gravity kappa through k_B T_H = hbar kappa / 2pi; in the acoustic case the surface gravity is written in the Technion papers as g = c(dv/dx − dc/dx) evaluated at the sonic point. Everything an analogue experiment claims is a claim about that relation, and nothing it claims is a claim about the mass loss, lifetime or entropy of anything.

Established Six families of apparatus now carry horizons, and they differ in the one respect that matters. Open-channel water flumes use surface waves over an obstacle. Atomic Bose-Einstein condensates use a step in the potential that accelerates the flow past the speed of sound. Optical fibres use an intense pulse whose Kerr-effect index perturbation moves at the group velocity of the probe. Polariton and photon fluids use a driven nonlinear optical medium. Superfluid helium-3 uses spin-precession waves on a magnon condensate. Superconducting circuits and qubit chains synthesise the metric from engineered couplings. The respect in which they differ is short-wavelength dispersion: a condensate's Bogoliubov branch turns superluminal at high wavenumber, water gravity waves and fibre modes turn subluminal. Frontier That difference is the field's best available experimental handle on the trans-Planckian problem, and 7 argues it is the handle nobody has pulled.

Established The dynamical Casimir effect is a cousin, not a horizon. A mirror accelerated hard enough converts vacuum fluctuations into real photon pairs; no horizon is required, and the connection to Hawking and Unruh physics is through the same Bogoliubov machinery rather than through a shared geometry. It belongs in this brief because it is the cleanest demonstration in the whole family that a vacuum can be made to radiate by moving a boundary condition, and because the trick that made it work — modulating a boundary instead of moving a mass — is the trick the rest of the field runs on.

Speculative The word “simulation” is doing contested work throughout the literature. A wind tunnel and an aircraft share their governing equations; a fluid and a black hole share half of them. Handwave Whether the shared half carries the physical content is the question the field mostly assumes rather than argues, and it is asked properly in 4.

Established The measured record is real, nearly forty years deep, and resting almost entirely on a handful of apparatus. What follows is that record, platform by platform, with the disputes attached rather than summarised away.

Established Water channels came first and measured stimulated emission. Weinfurtner, Tedford, Penrice, Unruh and Lawrence placed a streamlined obstacle in an open channel and measured the conversion of long upstream waves into short deep-water waves at a white-hole horizon, reporting in Physical Review Letters in January 2011 that “our measurements of the amplitudes of the converted waves demonstrate the thermal nature of the conversion process for this system.” Five years later Euve, Michel, Parentani, Philbin and Rousseaux went after the noise rather than the driven signal, measuring the power spectrum and two-point correlation function of the free surface downstream of an obstacle at a maximum Froude number of about 0.85 and finding “a clear correlation between pairs of modes of opposite energies.” Frontier Water analogues are classical throughout: they establish that the scattering coefficients at a horizon are what the calculation says, which is the classical skeleton of the Hawking result and not its quantum content.

Established The Bose-Einstein condensate line is where the quantum claims live, and it is one group. Lahav and colleagues realised a sonic black-hole analogue in a condensate in 2010. Steinhauer reported a self-amplifying “black-hole laser” in Nature Physics in 2014, then in 2016 reported spontaneous Hawking radiation with correlations between escaping particles and their trapped partners, writing that “the high-energy pairs are entangled, while the low-energy pairs are not, within the reasonable assumption that excitations with different frequencies are not correlated.” Frontier Both results were contested on publication and one of them did not survive the contest intact.

Frontier The black-hole laser claim was re-explained as a classical stimulated process. Wang, Jacobson, Edwards and Clark modelled the 2014 apparatus and concluded that Bogoliubov-Cerenkov radiation generated at the white-hole horizon and Doppler-shifted at the black-hole horizon reproduces the observation, stating that “mean field behavior similar to that in the experiment can thus be fully explained without the presence of self-amplifying Hawking radiation” and that “we find that the black hole laser mechanism plays no role.” Established A laboratory result that a careful simulation can reproduce without the headline mechanism is not a refutation of the mechanism, but it removes the result from the evidence column, and this brief does not count it.

Frontier Leonhardt's 2016 critique of the entanglement claim is the sharpest published attack on any result in this field, and it has never been fully answered in print. It raises five objections: that “the observed spectrum of Hawking radiation is not fully Planckian, although the article states it to be”; that “particle correlations are observed without particles being present” beyond the peak wavenumber; that with wavenumber uncertainties included “the data of the correlations is only distinguishable from the data of the populations squared for one standard deviation”; that no measurement of the partner population was reported; and that no independent measurement of the velocity gradient was reported, so “a quantitative comparison of the inferred temperature with the theoretical Hawking temperature … is not possible.” The 5.7 standard-deviation entanglement confidence, Leonhardt argues, “reduces to the order of 1 sigma after an analysis of the uncertainties involved.” Frontier Steinhauer published a reply in the same journal. A reader should treat the 2016 entanglement claim as contested, and should notice that the objection about the missing independent surface-gravity measurement is the one this brief's decisive experiment is designed around.

Established The 2019 measurement is the anchor, and it was built to answer the 2016 objections. Munoz de Nova, Golubkov, Kolobov and Steinhauer rebuilt the apparatus with reduced magnetic-field noise, improved mechanical and thermal stability and redesigned optics, and reported in Nature in May 2019 that “the correlation spectrum of Hawking radiation agrees well with a thermal spectrum, and its temperature is given by the surface gravity, confirming the predictions of Hawking's theory,” adding that the radiation sits “in the regime of linear dispersion, in analogy with a real black hole.” The preprint states the number: “The average over the black curve gives a predicted Hawking temperature of 0.35 nK, or 0.12 mc_out squared.” The experiment was repeated 7,400 times. Frontier The commissioning brief for this record quoted the anchor as 0.35 +/- 0.03 nK. The plus-or-minus could not be verified: neither the arXiv preprint text, its abstract page, nor the Nature abstract attaches an explicit uncertainty to that temperature, and this brief therefore cites 0.35 nK without one and flags the uncertainty as unpublished in the sources it could open.

Established Stationarity was then measured separately, which matters because a thermal spectrum from a transient is not a Hawking spectrum. Kolobov, Golubkov, Munoz de Nova and Steinhauer observed the same system at six different times and reported in Nature Physics in January 2021 that the spontaneous emission is stationary, following a ramp-up, and that the formation of an inner horizon ends the spontaneous emission and starts a stimulated one that evolves from multi-mode to monochromatic. Fabbri and Balbinot's companion theory paper in Physical Review Letters traced the ramp-up analytically and located the emission in a “quantum atmosphere” region “significantly displaced from the horizon.” Frontier The quantum-atmosphere result is worth holding onto: it says the radiation does not originate at the horizon even in the analogue, which weakens the intuitive picture that both the popular and some of the technical literature still uses.

Established Optical fibre horizons are the best-controlled and the least quantum. Philbin and colleagues established the fibre-optical analogue in 2008; Rubino and colleagues measured negative-frequency resonant radiation in 2012; and in January 2019 Drori, Rosenberg, Bermudez, Silberberg and Leonhardt reported that “the probe stimulates Hawking radiation, which occurs in a regime of extreme nonlinear fiber optics where positive and negative frequencies mix,” with a linear relation between probe power and negative-Hawking signal up to saturation. Frontier The same work carries the field's most useful unexplained discrepancy. Leonhardt told Physics World at the time: “Our numerical calculations predict a much stronger Hawking light than we have seen.” The gap is still open, and a theory that over-predicts its own experiment by an unexplained factor is a theory under test rather than a theory confirmed.

Frontier Two 2026 fibre results moved the field's quantum credentials forward and are the most recent measurements this brief could open. In May 2026, Nature Communications published a measurement of analogue Hawking radiation stimulated by a single photon, using heralded photons from spontaneous four-wave mixing: a horizon wavelength of 644.5 nm, pump wavelengths swept from 802 to 838 nm across 36 spectral measurements at 0.16 to 4 mW coupled power, a second-order correlation of g2(0) = 0.106 +/- 0.018 for the Hawking output at 0.72 mW, and a coincidence-to-accidental ratio of 75. That is single-photon-level control of an analogue horizon, which nothing in the condensate line has. Frontier In July 2026 Procopio, Aguero-Santacruz, Bermudez and Leonhardt reported in Nature a direct generation process for fibre Hawking radiation and its backreaction on the field. The quantitative content could not be extracted from the version this brief could read — the page states that data are available on request and the numbers sit in supplementary material — so the result is recorded here as a claim of measured backreaction with its magnitude not in evidence.

Established The cosmological analogues are quantitatively the strongest results in the whole field, and they are not about black holes. Viermann and colleagues built a two-dimensional potassium condensate whose scattering length was ramped from 350 to 50 Bohr radii in 1.5 to 3.0 ms with a deceleration index of 0.5, producing a controllable expanding spacetime with tunable positive and negative spatial curvature, at 40 nK, with a final sound speed of 1.2 µm per ms and 0.8 µm optical resolution; they measured particle-pair production and extracted Sakharov oscillations, reporting quantitative agreement with analytical predictions. Steinhauer's group measured the same class of physics in an ultracold quantum fluid of light in 2022, with density-density correlations of order 10⁻⁶, a background population of about 1.2 quasiparticles per mode and an effective temperature below roughly 30 mK. Frontier Expanding-universe analogues are cleaner than horizon analogues for a structural reason: they need no horizon, so they do not need the trans-Planckian argument at all.

Established Rotating analogues gave the field its first ringdown. Svancara and colleagues at Nottingham confined a vortex carrying of order 10⁴ circulation quanta in superfluid helium — four orders of magnitude above the elementary quantum — and reported in Nature in March 2024 bound states with co-rotating striped patterns extending to 40 Hz and analogue black-hole ringdown signatures in counter-rotating waves, with the solid-body rotation ratio held below 2.5% and a core radius of roughly 4 to 6 mm. Frontier Rotational superradiance in these same apparatus is covered in Rotational Superradiance and is not re-argued here; what belongs here is that the Nottingham vortex is the only analogue that has produced a quasinormal-mode spectrum, which is the observable gravitational-wave astronomy actually uses.

Frontier The exotic platforms are proofs of principle with poor signal budgets. Czlovecko, Gazo, Kupka and Skyba built magnonic black- and white-hole horizons on spin supercurrents between two magnon condensates in superfluid helium-3B at about 600 µK, observed white-hole formation and an amplification effect, and estimated the spontaneous Hawking temperature at roughly four orders of magnitude below the ambient temperature. Shi and colleagues synthesised a curved spacetime on a chain of ten superconducting transmon qubits with nine tunable couplers, put the horizon analogue at the third site, performed tomography on the seven qubits outside it and recovered a blackbody form, with a simulated Hawking temperature of about 1.7 × 10⁻⁵ K in theory against about 7.7 × 10⁻⁵ K in experiment. Frontier That is a factor of roughly 4.5 between the designed and the measured temperature in a system whose Hamiltonian the experimenters wrote themselves, which is a useful calibration of how much agreement this field's “agreement” usually means.

Established The dynamical Casimir effect is the cleanest vacuum-radiation measurement in the family. Wilson, Johansson, Pourkabirian, Simoen, Johansson, Duty, Nori and Delsing published in Nature on 16 November 2011, modulating the inductance of a SQUID terminating a coplanar waveguide at about 11 GHz so that the electrical length changed at a few percent of the speed of light, and observing two-mode squeezing of the emitted radiation, which is the signature that distinguishes vacuum conversion from thermal noise. The proposal paper by Johansson and colleagues had set the design target: an effective amplitude of 10⁻⁴ m at 10¹⁰ Hz and a maximum effective velocity near 10⁶ m/s. Frontier The effective mirror speed is the number most often misquoted. The paper's own abstract says a few percent of the speed of light; MIT Technology Review reported speeds “approaching 5 per cent light speed”; the Swedish Research Council's press release said “up to 25 percent of the speed of light” (funder's press release). This brief uses the paper's figure and records the spread because it is a clean example of how a number inflates on the way out of a laboratory.

Established What all of this adds up to is narrower than the headlines and more solid than the sceptics allow. Horizons form where the mathematics says they form. Waves scatter off them with the coefficients the mathematics predicts. Pairs appear correlated across them. In one apparatus, in one laboratory, the correlation spectrum is thermal at the temperature the local surface gravity implies. Speculative What none of it establishes is that an astrophysical black hole radiates, and the reason is structural rather than technical: the gravitational part of the problem has been replaced by a fluid, and the fluid obeys no field equation that resembles Einstein's. Laboratory Astrophysics declines to treat analogue gravity as evidence about astrophysics for exactly this reason, and Quantum Gravity treats it the same way; this brief is where the argument itself is made rather than deferred.

Frontier The first open question is whether the Hawking temperature survives a change in the short-wavelength physics, and it is the question the whole field exists to answer. Hawking's derivation traces the emitted modes back through unbounded blueshift to arbitrarily short wavelengths — the trans-Planckian problem — which means the result appears to depend on physics nobody knows. Analogue systems have a known short-wavelength physics, so they can be asked directly, and the theoretical answer since the 1990s has been that the effect is robust. Frontier Del Porro, Liberati and Schneider restated it in 2025 and again in April 2026, writing that the persistence of the effect “despite the modified dispersion relations characterising analogue models, has been crucial in advancing our understanding of the robustness of this phenomenon against ultraviolet modifications of our spacetime description,” and locating the emission at “an effective horizon slightly shifted” from the metric's Killing horizon, with controlled deviations from exact thermality.

Frontier Robustness is a theoretical result that has not been given an experimental test of the form it deserves. Robertson's detailed treatment of dispersive Hawking radiation finds the low-frequency temperature holds in the adiabatic limit but that “thermality is not so robust, for only in the dispersionless case is the Hawking spectrum strictly thermal,” and identifies a regime with no group-velocity horizon at all where spectra are non-zero but “highly non-thermal” and vary with the square of the maximum deviation. Speculative Nobody has measured the temperature of two horizons with deliberately different dispersion at matched surface gravity and compared them. That is the experiment 7 argues for, and its absence is the largest single gap in the field's evidence.

Frontier The second open question is replication, and the honest statement is uncomfortable. Every quantitative claim about a spontaneous thermal Hawking spectrum in a quantum fluid traces to one group, one apparatus and one laboratory. No independent group has published a spontaneous analogue Hawking temperature in a Bose-Einstein condensate. Water channels, fibres and polariton fluids have independent groups, but they measure stimulated or classical quantities, or they are numerical. Established Single-source results are not thereby wrong, but a field whose anchor is unreplicated should say so on the first page rather than the last.

Frontier The third is backreaction: whether the radiation changes the geometry that produced it. In a real black hole this is the whole of the evaporation story. In analogues it has been approached three ways, all partial. Patrick, Goodhew, Gooding and Weinfurtner showed in 2021 that a finite draining vortex responds to waves on timescales much longer than the wave dynamics, producing “a significant global change in the total mass of our system,” while stating plainly that “hydrodynamic black holes are externally driven systems whose effective mass and angular momentum are set by experimental parameters, and, as such, no appreciable internal backreaction is expected to take place.” Butera and Carusotto proposed a condensate analogue of preheating backreaction in 2023 and found effective friction and a rapid loss of longitudinal spatial coherence, theoretically. The 2026 Nature fibre paper claims measured backreaction. Speculative Whether any of these is backreaction in the sense the gravitational problem needs — the field's own stress-energy sourcing its own metric — is unresolved, and the algebraic relation between fluid and metric is a reason for doubt.

Frontier The fourth is definitional, and it is not a quibble. Asked about the 2019 fibre result, Sergio Cacciatori observed that stimulated emission is a classical effect that could amplify sources other than spontaneous Hawking radiation, and that the community needs to “agree on what exactly the definition of Hawking radiation should be.” Frontier Seven years later there is still no operational criterion that all groups accept for when a measured spectrum counts. Different platforms present different observables — scattering coefficients, density-density correlations, photon antibunching, tomographic populations — and the absence of a shared criterion is why the field's results are hard to combine into a single line of evidence.

Speculative The fifth is the transfer question itself: under what conditions does evidence from a system with the right kinematics license belief about a system with different dynamics? The philosophical literature has taken this seriously — Dardashti, Hartmann, Thebault and Winsberg's Bayesian treatment of analogue Hawking evidence is the standard reference, and could not be opened for this brief — and the physics literature has mostly taken it as settled by universality arguments. Handwave The universality argument in its usual form runs: the Hawking effect appears in every system with a horizon and any reasonable dispersion, therefore it does not depend on the details, therefore it holds for real black holes. The step from “every system we have checked” to “systems with dynamical metrics we cannot check” is the step that works by assertion, and it is the assertion this brief flags hardest.

Frontier The sixth is where the radiation actually comes from. The quantum-atmosphere picture — emission from a region displaced from the horizon rather than at it — was extracted from the Technion data by Fabbri and Balbinot. Speculative If the same displacement held gravitationally, information-paradox arguments that place the relevant physics precisely at the horizon would be working with the wrong geometry. That inference crosses the kinematic-dynamical line and this brief does not endorse it.

Established The binding constraint in every platform is that the analogue Hawking temperature is at or below the apparatus temperature. The Technion condensate's 0.35 nK sits in a system prepared at nanokelvin scales; the helium-3 magnonic horizon's estimated spontaneous temperature is about four orders of magnitude below its 600 µK ambient; the fluid-of-light analogue runs against an incoherent background of about 1.2 quasiparticles per mode. Established The consequence is that nobody detects analogue Hawking radiation directly. They detect a correlation between two regions, accumulated over thousands of repetitions, after subtracting everything else.

Established That statistical route is what makes the results disputable, and the dispute is about statistics rather than physics. 7,400 repetitions produced the 2019 spectrum. Leonhardt's objection to the 2016 result was that with the wavenumber uncertainties properly propagated, the correlation data and the squared population data “melt into each other for 2 sigma.” Frontier A field whose central observable is a two-point function extracted from an ensemble average inherits every pathology of that method: selection of Fourier components, background estimation, and the difficulty of stating an uncertainty that a sceptic will accept. This is a solvable problem and it is largely unsolved.

Established Spontaneous emission in the optical platforms is below the noise floor, so the optical line measures stimulated processes. The review of optical analogues records that spontaneous rates were estimated as too low for direct measurement in the 2019 fibre setup, and that the original 2008 fibre experiment expected only 10⁻⁴ of the continuous-wave probe power to interact with the pump over 1.5 m of fibre. Frontier The 2026 single-photon result changes the character of this constraint without removing it: heralding one photon at a time gives quantum statistics on stimulated emission, which is a different thing from detecting the spontaneous process.

Established The horizon does not stay put. In the condensate, the inner horizon forms and ends the spontaneous emission, replacing it with a stimulated one; the whole stationary window had to be established by observing the system at six separate times. Frontier Every quantum-fluid analogue is a transient, and the emission has a ramp-up, a stationary phase and a termination, none of which has an obvious gravitational counterpart. Comparing a transient's spectrum to an eternal black hole's is an approximation whose error nobody has quantified.

Established The metric is imposed, which is not a technical limit but a structural one. The flow profile is set by trap potentials, channel geometry, pump pulses or coupler settings. Nothing the waves do feeds back into it at leading order. Speculative This is the bottleneck that cannot be engineered away by better hardware, because removing it would require a medium whose metric obeyed a dynamical equation sourced by its own excitations — which is to say, a medium that was gravity. No proposal in the literature does this, and 14 records it as a scientific result nobody is producing rather than a capability somebody could build.

Frontier Cross-platform comparison is blocked by calibration rather than by physics. To compare the Hawking temperature of a water horizon and a condensate horizon you must measure the surface gravity of each independently and to comparable precision. Leonhardt's fifth objection was precisely that the 2016 condensate work did not report an independent velocity-gradient measurement. Frontier Surface gravity in a condensate requires resolving a velocity and sound-speed gradient across a horizon a few microns wide, against imaging resolution of order 0.8 µm in the best cold-atom setups. That is the hard measurement in the decisive experiment, and it is an instrumentation problem, not a conceptual one.

Frontier The rotating platforms are limited by how fast they can spin. The Nottingham giant vortex held the solid-body rotation ratio below 2.5% and reported that direct observation of the ergoregion requires further increasing the azimuthal velocity. Speculative A directly observed superfluid ergoregion would be the first laboratory realisation of the region where no observer can remain static, and the gap between 2.5% and what is needed has not been published as a number.

Established This field runs almost entirely on other people's instruments. Every platform is borrowed apparatus: a cold-atom machine, a fibre laser bench, a dilution refrigerator, a circuit-QED chip, an open channel flume. The dependency list is therefore a list of other fields' roadmaps, which is the main reason it is cheap.

Established Cold-atom control is the first dependency and it is mature. The 2022 curved-spacetime simulator needed a scattering length ramped from 350 to 50 Bohr radii on a millisecond timescale, 0.8 µm imaging and 40 nK preparation; the 2019 Hawking measurement needed reduced magnetic-field noise, mechanical and thermal stability and redesigned optics relative to its 2016 predecessor. Frontier Each of those is an increment on standard cold-atom technique rather than a new capability, which is why the field's progress tracks the cold-atom community's rather than leading it.

Established Quantum optics supplies the second, and it has recently supplied something new. The 2026 single-photon fibre measurement required heralded photons from spontaneous four-wave mixing, a coincidence-to-accidental ratio of 75 and a single-to-multi-photon ratio near 100 to 1. Frontier That is photon-counting technology built for quantum communication, arriving in analogue gravity roughly a decade after it matured elsewhere. Precision Quantum Sensing covers the measurement chain this field consumes.

Established Cryogenics and circuit engineering supply the third. The dynamical Casimir measurement needed a SQUID-terminated coplanar waveguide modulated near 11 GHz and an amplifier chain capable of resolving two-mode squeezing; the helium-3 magnonic horizon needed about 600 µK; the ten-qubit Hawking simulation needed nine tunable couplers and tomography on seven qubits. Frontier The qubit route is the one with an independent industrial roadmap behind it, described in Fault Tolerant Quantum Computing, and it is plausible that the largest analogue spacetime of 2035 will be a processor rather than a fluid.

Frontier Theory is a dependency in a specific and unusual way: the field needs a quantitative prediction for how the temperature should shift, not just an assurance that it is robust. Del Porro, Liberati and Schneider's tunnelling treatment is the closest thing available, giving an effective surface gravity at a shifted effective horizon with calculable deviations from thermality. Speculative Without a number to compare against, a cross-platform temperature comparison can only confirm or deny equality; with one, it becomes a quantitative test of the dispersion correction, which is a much stronger result for the same experimental effort.

Frontier Numerical simulation is load-bearing and has already overturned one experimental claim. Truncated-Wigner and Bogoliubov codes decide what a measured correlation function means; the re-analysis that removed the black-hole-laser reading from the 2014 result was a simulation. Established A field in which simulations adjudicate experiments needs its codes benchmarked against each other, and no published cross-code comparison for analogue Hawking observables was found for this brief.

Frontier The decisive experiment is a trans-Planckian robustness test: measure the analogue Hawking temperature in two platforms whose short-wavelength dispersion differs in sign, at matched surface gravity, and see whether the temperature is unchanged. A Bose-Einstein condensate's Bogoliubov branch curves superluminal at high wavenumber; surface gravity waves on water and guided modes in optical fibre curve subluminal. If k_B T_H = hbar kappa / 2pi holds to the precision of the surface-gravity calibration in both, the universality claim stops being a theoretical expectation and becomes a measured fact about how little the ultraviolet matters. If it fails, the entire transfer argument from analogue to astrophysical horizons fails with it, and that would be the most informative negative result available anywhere in this corpus.

Established It needs no new hardware. The condensate apparatus exists at the Technion, water flumes exist at Nottingham, Poitiers and British Columbia, fibre benches exist at the Weizmann Institute and CINVESTAV, and each has already produced a horizon and a spectrum. Frontier What does not exist is a common calibration: each group reports surface gravity in its own units, with its own systematic treatment, and no cross-platform comparison of the Hawking relation with propagated uncertainties has been published. The experiment is therefore an instrumentation and metrology project rather than a physics project, and it has no home because no funder buys cross-platform calibration.

Frontier The strong version is better and harder: vary the dispersion inside a single apparatus. A condensate's dispersion scale is set by the healing length, which is set by the scattering length, which is tunable by a Feshbach resonance — the 2022 curved-spacetime simulator swept it from 350 to 50 Bohr radii. Holding the surface gravity fixed while sweeping the healing length would trace the temperature against the dispersion scale in one laboratory with one set of systematics. Speculative Whether the two can be decoupled cleanly enough to hold kappa constant across the sweep has not been demonstrated and is the experiment's main technical risk.

Frontier The second experiment is independent replication of the 2019 thermal spectrum, and it is overdue. A second group, with a different condensate species, a different horizon-forming potential and a blind analysis pipeline, reproducing a thermal correlation spectrum at the temperature implied by an independently measured surface gravity, would answer Leonhardt's objections at once. Established The objection about the missing independent velocity-gradient measurement is answerable with existing imaging; it was a choice of analysis, not a limit of the apparatus.

Frontier The third is a quantum backreaction measurement that is not externally driven. Butera and Carusotto's preheating proposal and the number-conserving condensate treatments set out what would count: an effective friction on the driving mode and a loss of coherence traceable to the emitted pairs rather than to the drive. Speculative A measured energy balance in which the analogue horizon demonstrably loses what the radiation carries would be the first result in this field with any claim on the evaporation problem, and no apparatus has yet reported one.

Frontier The fourth is single-shot detection of spontaneous analogue Hawking pairs. Everything quantitative in the condensate line is an ensemble average over thousands of runs. A single-shot coincidence between a Hawking quantum outside and a partner inside would remove the entire statistical dispute. Speculative The detection efficiency required has not been published as a target, and the fibre platform's 2026 heralded-photon result is the nearest anyone has come in any medium.

Frontier The fifth is an entanglement test with a criterion agreed in advance. The 2016 claim rested on an assumption that excitations of different frequencies are uncorrelated, and the assumption is where the dispute lives. A pre-registered Cauchy-Schwarz or positive-partial-transpose violation, with uncertainties propagated through the wavenumber calibration, would be a cleaner statement than any published so far. Established This costs analysis discipline, not apparatus.

Frontier The sixth is the ergoregion. Pushing the Nottingham vortex past the 2.5% solid-body rotation ratio to where an ergoregion is directly observable would give the field a second geometric structure to measure, distinct from the horizon, with its own quasinormal modes. Speculative The engineering margin required is unpublished.

Speculative And the experiment that would settle nothing, because the field keeps proposing it: another platform showing another thermal-looking spectrum. A seventh medium with a horizon and a Planckian-looking correlation function adds almost nothing to the evidence, because the calculation already predicts it and because each new medium arrives with its own uncalibrated surface gravity. Handwave The claim that breadth of platforms is itself evidence for universality is the field's most common unearned inference: breadth across systems that share the same kinematic derivation tests the derivation once, not seven times.

Established None of these requires a facility, a treaty or a capital budget. The decisive test is a metrology campaign across laboratories that already exist, costed in person-years of calibration.

Established There is no device at the end of this brief. An analogue horizon converts one wave into another with coefficients set by a flow profile. It generates no usable power, stores nothing, and computes nothing a conventional simulation does not compute more cheaply. The engineering content of the field is instrumentation for its own experiments.

Established The one genuine engineering achievement is the modulated boundary condition. Replacing a mechanically moving mirror with a SQUID whose inductance is modulated at 11 GHz bought roughly fourteen orders of magnitude in photon production rate — about 10⁵ photons per second against 10⁻⁹ for the nanomechanical route. Frontier That is a real and transferable technique: parametric amplifiers and non-reciprocal microwave elements descend from the same time-varying-boundary physics, and the balance-sheet discipline that applies to them is the one Vacuum Energy Engineering and Rotational Superradiance both insist on.

Frontier The instrumentation requirement that would most change the field is a surface-gravity meter. Every dispute in 3 reduces to the same missing capability: an independent, cross-platform-comparable measurement of the velocity and sound-speed gradient at a horizon a few microns across, with a stated systematic budget. Speculative Nothing in the literature reads as an attempt to build one, and the metrology culture that would produce it lives in Metrology Infrastructure rather than in analogue gravity.

Frontier The qubit platform is the one with an engineering trajectory that is not this field's to drive. Ten transmons with nine couplers produced a blackbody-form emission spectrum in 2023; the same hardware family is scaling for reasons that have nothing to do with gravity. Speculative A synthetic spacetime on a few hundred qubits would allow geometries no fluid can realise — including, in principle, a metric updated between timesteps from the measured field state, which is the nearest anyone has proposed to a dynamical analogue. Nobody has published that as a design.

Established The cost profile is the field's most underrated feature. The results in 3 came from optical tables, flumes and dilution refrigerators, not from facilities. Frontier Compared with the shot-rate-limited machines of Laboratory Astrophysics, which fire a few hundred times a year, an analogue experiment repeating 7,400 times in a campaign is statistically rich and institutionally cheap.

Established The nearest neighbour is rotational superradiance, which shares the apparatus and differs in the effect. The draining-vortex and photon-superfluid experiments that measured Penrose amplification are analogue-gravity rigs first; Braidotti and colleagues measured superradiant amplification in a photon superfluid in 2022 by focusing a weak orbital-angular-momentum beam onto a pump vortex core. Frontier Rotational Superradiance owns the energy-balance question those experiments raise; this brief owns the horizon question, and the two share hardware, groups and funding.

Established Quantum simulation of field theory is the adjacent field with the larger budget. The 2022 expanding-universe simulator and the 2023 ten-qubit spacetime belong as much to that community as to this one. Frontier Quantum Materials and Fault Tolerant Quantum Computing both describe capability curves this field rides.

Frontier On the theory side the adjacency is to quantum gravity, and it is the adjacency most often misused. Quantum Gravity records that analogue results confirm a derivation rather than a quantisation of gravity, and the present brief agrees. Speculative The useful traffic runs the other way: analogue experiments discipline theory by forcing quantitative predictions for observables that can actually be measured, which is a rarer service than it sounds in a field where most predictions are unfalsifiable at present energies.

Frontier Gravitational-wave observation is the adjacency with a real shared observable. The Nottingham vortex produced quasinormal-mode ringdown, the observable that Gravitational Wave Engineering and the merger catalogues in Black Hole Physics Applications work with. Speculative A laboratory ringdown does not calibrate an astrophysical one, but it tests the mode-extraction analysis on a system where the answer is known, which no astrophysical source can provide.

Established Metrology is the adjacency most likely to pay a dividend outside physics. Heralded single photons with a coincidence-to-accidental ratio of 75, two-mode squeezing measured in a microwave line, and correlation imaging at 0.8 µm are all transferable capabilities. Frontier Precision Quantum Sensing and Metrology Infrastructure are where those land, and the transfer is real even though it is small.

Established This is a small field organised around a handful of groups, and its institutional weakness is legible in its bibliography. The UK consortium QSimFP — Quantum Simulators for Fundamental Physics — lists four universities: St Andrews, Nottingham, Cambridge and Royal Holloway, with 54 publications between October 2019 and September 2024 (consortium's own listing). The Technion, the Weizmann Institute, Heidelberg, Trento, Paris Sorbonne, CINVESTAV and Valencia carry most of the rest. Frontier A field this size cannot self-replicate: there is no second condensate group positioned to repeat the anchor measurement, and the incentive to build one is weak because repeating somebody else's result is not how a cold-atom laboratory gets funded.

Established It convenes, which is more than many small fields manage. The Analogue Gravity 2026 workshop ran in Benasque from 12 to 16 January 2026, with a school on 8 to 10 January, organised from Louisiana State University, Valencia and Paris Sorbonne, covering quantum field theory in curved spacetime, relativistic quantum information, fluids of light, nonlinear optics, quantum Hall systems and condensates, with eight invited lecturers. Frontier The programme's own framing is that experimental advances are now driving the subject, which is a change from the two decades when theory drove it and is the most credible institutional signal available.

Frontier The funding is national-agency science at ordinary grant scale, and the brief could not verify the numbers. No funder statement or grant value appears on the QSimFP pages this brief could read. Established What can be said without a figure is structural: no facility, no consortium agreement of the kind big physics runs on, no standing instrument, and therefore no budget line that survives a change of institutional priority.

Frontier The institutional gap that matters is the absence of a replication mechanism. The field's anchor result is unreplicated; its most prominent earlier result was re-explained away by a simulation; its sharpest critique was published in the same journal as a comment and answered by a reply. Speculative A modest instrument would fix a lot of this: a pre-registered, cross-laboratory comparison of the Hawking relation with a common calibration protocol, of the kind particle physics and metrology run routinely and condensed-matter physics rarely does. Nobody has proposed one.

Frontier Publication incentives push toward new platforms and away from calibration. A seventh medium with a horizon is publishable in a high-impact journal; re-measuring somebody else's surface gravity is not. Established The record shows it: six platform families, one thermal-spectrum measurement, no cross-platform temperature comparison at all.

Established The safety questions here are nil, and the headlines suggest otherwise. Nothing in this field creates a gravitational object, concentrates energy, or has a dual-use pathway; the apparatus are colder and less energetic than an undergraduate laser lab. The ethical content is entirely epistemic.

Frontier The epistemic problem is real and the field is partly responsible for it. “Physicists create a black hole in the lab” is the standard treatment, and the papers themselves are often written in a register that invites it: titles claim observation of Hawking radiation without the qualifier that the horizon is sonic and the gravity absent. Established The Technion work is a conspicuous exception in one respect — Steinhauer has said directly that the information paradox belongs to the physics of a real black hole, not an analogue — and a conspicuous example in another, since the paper titles do not carry that caution.

Frontier Press amplification is measurable and this brief measured one instance. The dynamical Casimir effect's effective mirror speed is “a few percent” of light speed in the paper, about 5% in a trade report, and “up to 25 percent” in the funder's release (funder's press release). Established A factor of five on the headline number, between paper and institution, is the kind of drift that makes a field's claims hard to audit later.

Speculative The subtler cost is to the credibility of analogue reasoning elsewhere in science. Surrogate evidence is everywhere — animal models, wind tunnels, scaled hydrodynamics, climate emulators — and the transfer rules are the same in each case. Frontier A high-profile field that asserts transfer without stating its conditions makes the general argument harder to defend where the stakes are practical rather than cosmological.

Frontier There is also a small equity point about where the results come from. Six of the platform families require a dilution refrigerator, a cold-atom machine or a femtosecond laser bench, which restricts the field to well-funded institutions in about a dozen countries. Established Water channels are the exception: an open flume is affordable almost anywhere, and it is the one platform on which an independent replication could be mounted cheaply.

Frontier The civilizational content of this subject is methodological, not technological, and it is larger than the subject. Analogue gravity is the clearest live test case for a question every empirical discipline faces: when a system you can manipulate shares part of its structure with one you cannot, how much does an experiment on the first tell you about the second? The question is unusually crisp here, because the shared part — the wave equation on a metric — and the unshared part — the field equation for the metric — are separable and both written down.

Speculative If the robustness test in 7 comes back positive, the gain is a licensed inference rather than a technology. It would establish that a prediction derived from physics at scales nobody can reach survives a controlled change in that physics, which is the strongest form of argument available for any trans-Planckian claim. Handwave Extending that licence from “the ultraviolet does not matter for this effect” to “the ultraviolet does not matter for black-hole thermodynamics, entropy and information” is a further step the experiment does not authorise, and the literature takes it routinely.

Frontier If it comes back negative, the consequence is sharper and lands on other fields. A measured dependence of the Hawking temperature on short-wavelength dispersion would mean that a derivation trusted for fifty years imports assumptions about unknown physics, which would put pressure on every argument downstream of black-hole thermality — holography, the entropy-area relation, the information paradox industry. Speculative That is a large consequence obtained from a small experiment, which is the pattern this corpus looks for.

Established The near-term material consequence is close to zero and should be stated as such. No energy, no propulsion, no computation, no materials. Black Hole Physics Applications documents what happens to the applications literature when the mechanism it assumes has never been observed; this brief is about the only line of work that has produced any measurement of that mechanism at all, and it still produces nothing usable.

Speculative The durable asset is the habit, not the result. A culture that can build a horizon on an optical table, measure its temperature, and then say in print that the measurement does not settle the astrophysical question is demonstrating something scarce. Frontier Whether this field is demonstrating it is contested: the measurements are careful and the titles are not.

These horizons track the evidential status of the analogue programme — whether the kinematic results acquire a calibration and a replication — rather than any device, because this brief's argument is that there is no device.

• 10 yr: Frontier Independent replication of a spontaneous thermal analogue Hawking spectrum, a published cross-platform surface-gravity calibration, and quantum-statistical measurements on horizons in more than one medium following the 2026 single-photon fibre result. Each is achievable with existing apparatus; none is currently funded as such. Synthetic spacetimes on superconducting processors grow with the hardware described in Fault Tolerant Quantum Computing whether or not this field asks for it.
• 25 yr: Speculative The trans-Planckian robustness test either returns a result or the field concedes it never intends to run it. A measured quantum backreaction, in which an analogue horizon loses what its radiation carries, is the other plausible milestone. If both land, analogue gravity becomes a settled chapter of quantum field theory in curved spacetime; if neither does, it stabilises as an unusually elegant branch of condensed-matter physics, which is what Laboratory Astrophysics already calls it.
• 50 yr: Speculative Any claim on the gravitational side requires an astrophysical detection of Hawking radiation, and there is no programme positioned to deliver one; the 2026 Nature assessment that the chances of observing it in space are astronomically small is the field's own estimate. Absent that, the analogue programme's ceiling is a very well characterised statement about wave equations.
• 100 / 250+ yr: Handwave Proposals for analogue systems with dynamical metrics — media whose geometry is sourced by their own excitations — are not forecasts and this brief declines to date them. What can be said is that no published proposal describes such a medium, and that without one the kinematic ceiling in 5 is permanent rather than technical.

• Depends on Nothing on this map blocks the experiments: every result in 3 came from apparatus that already existed for other reasons. Where it leans on the map is for interpretation. Quantum Gravity supplies the standard against which analogue claims about quantisation are judged, and already records that these results confirm a derivation rather than a theory; Black Hole Physics Applications supplies the astrophysical null — Hawking radiation undetected by any instrument — that makes the analogue programme the only empirical access there is; and Laboratory Astrophysics supplies the scaling-analogy criteria this field has never written down for itself.
• Requires (not on this map) The decisive experiment waits on a cross-platform calibration of analogue surface gravity, without which two measured temperatures cannot be compared at all. The anchor result waits on independent replication of a spontaneous analogue Hawking spectrum, which no second group has published. Any claim about evaporation rather than emission waits on a medium whose metric is sourced by its own excitations, which nobody has proposed and which may not exist. The replication waits in turn on a pre-registered multi-laboratory replication protocol for analogue horizons, of the kind metrology runs routinely and this community does not. And all of the calibration work waits on sub-micron velocity and sound-speed gradient imaging at a horizon, which is an instrumentation project nobody has funded.
• Enables A positive robustness result would license the universality argument that Quantum Gravity currently treats as unproven, and would give Black Hole Physics Applications its first empirical support for the emission formula every energy figure in that literature assumes. A negative one would be more consequential still, and would land on the same two briefs.
• Adjacent Rotational Superradiance shares the apparatus and the groups; Precision Quantum Sensing and Metrology Infrastructure supply the measurement chain and would have to supply the calibration; Fault Tolerant Quantum Computing supplies the synthetic-spacetime platform with the fastest independent capability curve; and Vacuum Energy Engineering shares the dynamical Casimir result and the balance-sheet discipline that keeps it from being read as free energy.

Handwave “Scientists have created a black hole in the laboratory.” No object with an event horizon in the gravitational sense has been created. Established What exists is a region where a fluid outruns its own sound, so that sound cannot propagate upstream past a surface. The mathematics of wave propagation across that surface is identical to the mathematics of a field on a black-hole background; the mass, the gravity and the spacetime curvature are absent.

Handwave “Hawking radiation has been observed.” Analogue Hawking radiation has been observed. Established No instrument has detected Hawking radiation from an astrophysical object, and the 2026 Nature paper reporting the most recent analogue result says so in its own first paragraph. The distinction is not pedantry: the entire applications literature catalogued in Black Hole Physics Applications is computed from a formula that has no astrophysical confirmation.

Speculative “The analogue experiments confirm Hawking's prediction.” They confirm the derivation's kinematic core in systems where it applies exactly. Frontier Whether that confirmation transfers to a system whose metric obeys the Einstein equations is the open question, and the honest statement is that the transfer rests on a universality argument that has been calculated but never tested by deliberately varying the thing it claims not to depend on.

Handwave “Analogue black holes solve the information paradox.” They cannot, and the field's own leading experimentalist has said so. Established The paradox is about unitarity in a spacetime that evaporates. An analogue horizon does not evaporate: its metric is set externally, it loses no mass to its radiation, and the flow can be switched off.

Speculative “Many different platforms all see it, so it must be universal.” Breadth is weaker evidence than it looks. Frontier All six platform families are described by the same effective-metric formalism, so agreement among them tests that formalism's internal consistency, not its applicability to a case outside it. The comparison that would carry weight is between platforms chosen to differ in dispersion, at matched surface gravity, which is the experiment nobody has run.

Frontier “The 2016 entanglement result settled the quantum nature of the radiation.” It is contested in print. Established Leonhardt argued that the claimed 5.7 standard-deviation entanglement confidence “reduces to the order of 1 sigma after an analysis of the uncertainties involved,” and Steinhauer replied in the same journal. The 2019 measurement is a different and better-controlled experiment, and is the one this brief treats as the anchor.

Frontier “The black-hole laser was observed in 2014.” A subsequent simulation reproduced the observation without it. Established Wang, Jacobson, Edwards and Clark concluded that “the black hole laser mechanism plays no role” in explaining the mean-field behaviour reported. That does not prove the mechanism absent, but it removes the experiment from the evidence column.

Handwave “The dynamical Casimir effect extracts energy from the vacuum.” It converts work done on a boundary into photons. Established The SQUID is driven; the microwave pairs come out of the drive, mediated by the vacuum, and the two-mode squeezing is what shows the mediation is quantum rather than thermal. Frontier Vacuum Energy Engineering holds the general version of this accounting error.

Speculative “Analogue gravity is not real physics.” The opposite error, and it is also wrong. Established Horizon formation, mode conversion with predicted scattering coefficients, stationary spontaneous emission, quasinormal ringdown from a vortex carrying of order 10⁴ circulation quanta, and vacuum photon pairs with two-mode squeezing are all measured results in condensed-matter and quantum-optical systems, and they would be publishable with no reference to gravity at all. Frontier The dispute is about what they license, not about whether they happened.

Handwave “A bigger or colder analogue would settle it.” Scale is not the binding constraint. Established The constraint is structural: the metric is algebraically fixed by the medium rather than dynamically by a field equation, and no amount of refrigeration changes that. Speculative The experiments that would move the assessment are the calibration and dispersion tests in 7, and they are cheap.