Analogue Systems
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Analogue horizons make one of black-hole physics’ most delicate mechanisms experimentally accessible: waves propagating on a moving or time-dependent medium can obey the same near-horizon scattering equations as quantum fields on a black-hole background. The shared structure is kinematic. The medium does not acquire Einstein dynamics, a gravitational event horizon, Bekenstein–Hawking entropy, or semiclassical backreaction merely because its perturbations see an effective metric.
Four laboratory routes to horizon scattering
Section titled “Four laboratory routes to horizon scattering”| Platform | Propagating degree of freedom | Horizon control | Most direct observable |
|---|---|---|---|
| Acoustic black holes | Linear sound perturbation in a moving barotropic fluid | Normal flow crosses the low-frequency sound speed | Scattering coefficients and positive/negative-norm mode conversion |
| Bose–Einstein-condensate horizons | Bogoliubov quasiparticle on a Gross–Pitaevskii background | Flow and interaction profile set a sonic transition | In situ density correlations and reconstructed quasiparticle spectra |
| Optical horizons | Dispersive optical mode interacting with a moving refractive-index perturbation | Pulse velocity meets a phase- or group-velocity branch | Frequency conversion, partner modes, and photon statistics |
| Water-wave horizons | Surface-gravity/capillary wave on a channel flow | Blocking point occurs when laboratory group velocity vanishes | Classical amplitudes, spectra, and correlations of converted waves |
The common stationary calculation labels modes by a conserved laboratory or comoving frequency, defines a norm from the linearized action, and solves a scattering problem. When a positive-norm incoming mode mixes with a negative-norm outgoing mode, the coefficients satisfy a pseudo-unitarity relation rather than ordinary probability conservation. Quantization promotes that mixing to a Bogoliubov transformation; it does not guarantee that the prepared state is vacuum-like or that the measured output is spontaneous.
A hierarchy of claims
Section titled “A hierarchy of claims”Effective causal structure. A derived wave operator has characteristics that tip across a blocking surface. This is the firmest and most general claim. The derivation must state the background equations, linearization, dimensional reduction, and wavelength range.
Stimulated mode conversion. A classical seed produces converted positive- and negative-norm waves with the predicted spectrum. This tests the scattering matrix and has been demonstrated in several media. It is not vacuum particle creation.
Spontaneous quantum emission. The input state is sufficiently close to the relevant quasiparticle vacuum, and correlations, nonclassicality, or state tomography isolate pairs created by the transformation. Thermal-looking one-mode power alone is insufficient because classical noise, amplification, fluorescence, four-wave mixing, and detector response can mimic it.
Gravitational inference. Even a clean spontaneous analogue effect would support the robustness of a kinematic Hawking mechanism under known dispersion. It would not experimentally confirm Hawking radiation from astrophysical black holes, Einstein’s equations, horizon entropy, or the semiclassical Einstein equation. The distinction is central to the current 2026 review of analogue gravity.
What must be controlled
Section titled “What must be controlled”Every page records the medium’s dispersion and dissipation, because the relativistic effective metric is a long-wavelength approximation. It also records state preparation, finite size and lifetime, background stationarity, mode normalization, detector calibration, and the mapping from raw intensity or density data to quasiparticle occupation. These choices determine whether an experiment probes classical scattering, quantum stimulation, or spontaneous production.
Two checks travel across platforms. First, the low-frequency limit must reproduce the nondispersive horizon and its surface-gravity scale. Second, propagating the measured input covariance through the independently calibrated scattering matrix must account for the output within uncertainty. An unexplained residual is interesting only after losses, nonlinearities, technical noise, and additional mode branches have been varied.
The experiment-specific evidence cutoff throughout this section is 20 August 2026. Results are reported at the level established by the preparation and observable, not by the vocabulary used for the apparatus.
Relating laboratory systems to Hawking radiation
Section titled “Relating laboratory systems to Hawking radiation”Each analogue-system page explains the apparatus and what its measurements establish. To understand the underlying Hawking calculation, read Near-Horizon Universality of the Hawking Effect; for mode creation and norms, read Bogoliubov Coefficients and Pair Creation. Surface Gravity and Hawking Temperature explains the gravitational normalization and thermodynamic interpretation. These distinctions matter because a laboratory temperature parameter does not by itself describe black-hole thermodynamics.
A productive comparison begins with the measured mode basis, reconstructs the full norm-sensitive scattering matrix, and only then asks which quantum state entered the apparatus. Reading the platforms in that order makes shared kinematics visible without erasing medium-specific noise, loss, or dispersion.