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Scientists do not create miniature astrophysical black holes. They engineer a boundary—a horizon—for waves in a controllable medium, then measure how fluctuations scatter, change frequency, and become correlated across that boundary. Depending on the platform, the signal may be a density-correlation pattern, a thermal-looking spectrum, a negative-frequency partner mode, entanglement, or stimulated emission from a supplied probe. These are laboratory analogue effects: they test the wave physics behind Hawking’s prediction without detecting radiation from a real black hole.
What “Hawking radiation” means in an analogue experiment
Hawking radiation is predicted for a gravitational event horizon, where quantum fields near a black hole produce an outward flux with a thermal spectrum. An analogue experiment reproduces selected mathematics and wave-scattering processes in a different system. The horizon is defined by the medium’s motion or by a moving optical structure, not by spacetime curvature strong enough to trap light.
How an effective horizon forms
A horizon appears where the motion of a medium outruns the speed at which a particular type of wave can travel upstream. In a flowing Bose–Einstein condensate (BEC), for example, sound-like disturbances are phonons. If the condensate flow changes from subsonic to supersonic, phonons on one side can no longer propagate back across the transition. That boundary is a sonic horizon.
Water waves, polariton fluids and optical fibres use different physical mechanisms. Their dispersion relations—the relationship between frequency and wave number—determine which waves can move against a flow or moving refractive-index pattern. The analogy is therefore operational: investigators compare mode conversion and correlations at the boundary with the corresponding calculations for a gravitational horizon.
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Why frequency sign and norm matter
In a moving medium, an observer can assign a wave a laboratory frequency while the medium’s motion gives it a different Doppler-shifted frequency. Some solutions have negative frequency or negative norm in the horizon’s preferred frame. Mixing positive- and negative-norm modes is the mathematical feature associated with particle creation in analogue Hawking calculations. The 2024 review “Negative frequencies and negative norms in analogue Hawking radiation systems” explains why analysing the horizon in a static frame makes this relationship especially clear. It surveys water waves, BECs, polariton fluids and optical fibres.
What an analogue result cannot establish
A detector in a tank, condensate or fibre has not observed photons emitted by an astrophysical black hole. It has measured excitations in its own medium. Even a convincing analogue signal does not prove that an event horizon exists in the laboratory, nor does it by itself settle every question about quantum fields in curved spacetime.
The common detection workflow
Although apparatus differs, experiments generally follow the same logic.
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- Characterise the background. They measure the unperturbed flow and dispersion, including sound speed, surface-wave speed or optical group velocity. This establishes where the horizon should be and which frequencies can cross it.
- Record fluctuations on both sides. Cameras, density imaging, optical detectors or wave gauges collect time-resolved data upstream and downstream of the boundary.
- Extract mode information. Fourier analysis separates frequencies and wave numbers. The analysis looks for converted negative-frequency or negative-norm partners, shifted frequencies and the predicted direction of propagation.
- Test correlations and spectra. Cross-correlation functions, coincidence counts or reconstructed occupation numbers reveal whether paired modes appear across the horizon and whether their distribution is thermal. Controls with no horizon, reversed flow or altered timing are essential for removing ordinary scattering and technical noise.
Bose–Einstein condensates: density correlations and quantum signatures
Creating the sonic horizon
In a BEC, ultracold atoms are cooled into a coherent quantum fluid. A shaped potential or moving barrier accelerates part of the condensate until the flow passes the local sound speed. The transition between subsonic and supersonic regions acts as the acoustic horizon.
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Measuring the observable
Experiments repeatedly prepare the condensate and image its atomic density. Each run contains random fluctuations, so a single image is not enough. By averaging many runs, scientists calculate equal-time or unequal-time density correlation functions. A characteristic correlation line linking points on opposite sides of the horizon is expected when excitations are produced in paired modes: one member travels outward while its partner is carried into the supersonic region.
The correlation pattern is more informative than a simple increase in noise. Its position, slope and frequency dependence are compared with a model of the measured flow. Researchers also check whether the feature disappears or changes when the horizon is removed.
Thermal spectra and analogue temperature
Some BEC studies reconstruct the occupation of outgoing modes and report a spectrum compatible with a thermal distribution. A fitted temperature is an analogue Hawking temperature determined by the condensate’s gradients and sound speed; it is not the temperature of a gravitational black hole. The 2025 review by Paul Raymond Anderson, Roberto Balbinot, Richard Allen Dudley and Alessandro Fabbri discusses correlation-function signatures and cites experimental reports on thermal radiation and its temperature published in 2019.
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Entanglement claims
Quantum theory predicts that the two members of a Hawking pair can be entangled. Some BEC experiments reported entanglement across the acoustic horizon, including work listed in the 2025 review from 2016. Such a claim requires more than a classical correlation: the measured covariance must violate an appropriate separability bound after accounting for detection loss, finite temperature and imaging noise. It should be attributed to the particular experiment rather than presented as an uncontested property of every analogue horizon.
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Time evolution and stationary emission
The same review lists a 2021 report examining stationary spontaneous emission and the time evolution of a black-hole analogue. These measurements ask whether the signal persists after the flow settles and whether its development follows the changing horizon parameters. Stability matters because a transient disturbance can imitate a radiation pulse without representing steady Hawking-like emission.
Water waves: measuring classical horizon scattering
What the apparatus records
Water-tank experiments drive a controlled current and send surface waves toward a region where the flow changes their ability to travel upstream. Wave gauges or cameras record amplitudes, phases and frequencies before and after the interaction. Scattering matrices reveal conversion between modes, including components whose laboratory frequency is shifted in the way predicted for a horizon.
What these results demonstrate
Water is a classical medium. Its experiments can reproduce mode conversion, frequency mixing and other classical features of horizon physics, including stimulated Hawking-like emission when an incident wave is supplied. A 2024 Communications Physics paper reported observation of a phase-space horizon with surface-gravity water waves. Such a result is evidence for the analogue wave dynamics, not direct evidence that quantum particles were created.
Earlier water-wave work was discussed as showing classical thermal aspects or stimulated emission in analogue white-hole configurations. “Thermal” here describes a measured distribution of classical wave energy; it does not automatically imply a quantum thermal state.
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Optical fibres: moving pulses and stimulated photons
How a fibre horizon is engineered
An intense laser pulse travels through an optical fibre and changes the refractive properties experienced by a weaker probe. In the probe’s effective frame, the pulse is a moving boundary. A probe wave meeting that boundary can be converted into another frequency and paired with a partner mode, analogous to scattering at a horizon.
What detectors measure
Fast photodetectors and spectrometers resolve the outgoing light. Researchers look for frequency shifts, the predicted direction of mode conversion and correlations between mode pairs. Because optical systems permit precise timing and filtering, they can distinguish a supplied probe from background light.
The 2026 single-photon result
A 2026 Nature Communications report described analogue Hawking radiation stimulated by a single photon. A heralded signal photon met the moving pulse and was converted into an analogue Hawking photon. This is a deliberately stimulated process: the input photon seeds the conversion. It demonstrates controlled analogue mode conversion, but it must not be described as conclusive detection of spontaneous Hawking radiation. Spontaneous emission would require the outgoing signal to arise without an intentionally injected probe and would demand especially stringent exclusion of detector dark counts, pump noise and other sources.
Polariton fluids and spin horizons
Polariton fluids—hybrid light–matter excitations in semiconductors—are another platform covered by the 2024 review. Their flow and interactions can create horizons for collective excitations, while optical readout gives access to fluctuations in space and time.
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A 2025 theoretical study by Anna Berti and colleagues proposed stimulated and spontaneous emission from spin modes in a two-component BEC. It emphasises that measuring spin-excitation quadratures could provide an experimental advantage. This is a proposal, not a reported detection: no laboratory observation should be inferred from the theory paper.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare the evidence
| Evidence type | What is measured | What it supports | Important qualification |
|---|---|---|---|
| Mode conversion | Negative-frequency or negative-norm partner modes, frequency shifts and scattering amplitudes | The horizon-scattering model | Classical and quantum systems can both show mode conversion. |
| Cross-horizon correlation | Density, intensity or wave-height correlations between opposite sides | Paired excitations associated with an analogue horizon | Technical noise and ordinary waves must be modelled and removed. |
| Entanglement | Quantum covariance or an entanglement witness | A specifically quantum relation between the pair members | Only some BEC experiments report this result; losses and finite temperature affect the test. |
| Thermal spectrum or temperature | Mode occupation versus frequency fitted to a thermal law | Hawking-like statistical behaviour | The temperature belongs to the analogue medium and is platform-specific. |
| Stimulated emission | Response to an injected wave or photon | Controlled conversion at the effective horizon | It is not spontaneous emission. |
| Classical wave signature | Surface-wave or optical scattering in a classical medium | Analogue kinematics and dispersion | It does not by itself establish quantum particle creation. |
Spontaneous versus stimulated emission
This distinction is central when reading headlines. In a stimulated experiment, an incoming wave, photon or density perturbation is intentionally supplied. The horizon converts that input into measurable output, making the signal easier to detect and the transfer function easier to map. In a spontaneous experiment, the outgoing excitation is attributed to vacuum or intrinsic fluctuations, so the experiment must show that no classical seed accounts for it.
Stimulated measurements are valuable because they test the same mode-conversion coefficients used in spontaneous calculations. They answer a narrower question, however. A single-photon probe in an optical fibre remains a stimulated analogue experiment even though the probe itself is quantum.
How scientists guard against false positives
- Map the horizon independently. Flow velocity, density and dispersion are measured rather than inferred only from the radiation signal.
- Use no-horizon controls. Runs with a subcritical flow, a stationary pulse or a changed barrier test whether the feature requires the horizon.
- Check phase and direction. Genuine partner modes have predicted phase relationships and propagate on the correct sides of the boundary.
- Separate frequencies. Spectral filtering and calibrated detectors prevent pump leakage, reflections and harmonics from masquerading as converted modes.
- Compare with full simulations. Numerical models include the measured, nonuniform flow and the platform’s dispersion instead of assuming an ideal relativistic law.
- Repeat across conditions. A signal that tracks horizon gradient, flow speed or pulse intensity as theory predicts is stronger evidence than a feature seen at one setting.
- Report interpretation limits. Authors should state whether the result is classical, quantum, stimulated, spontaneous, thermal or entanglement-based, and whether independent replication exists.
What is established—and what remains debated
The platforms now provide several complementary signatures. BEC experiments concentrate on density correlations and have reported thermal spectra, analogue temperatures and entanglement. Water tanks reproduce classical horizon scattering and stimulated effects. Optical fibres offer precise frequency conversion and, in the 2026 report, a stimulated single-photon demonstration. Reviews published in 2024 and 2025 place these results within a broader framework of negative-frequency and negative-norm modes.
The strength of a claim still depends on the observable and the interpretation. A 2018 Bayesian analysis described the evidence available then as an unsettled combination of classical thermal claims in water and a quantum-entanglement claim in BECs, concluding that an uncontested, repeated demonstration of the full analogue phenomenon remained outstanding at that time. That was a historical assessment, not a verdict on every later experiment. The sources available through 2026 do not establish a single universally accepted consensus covering all platforms, especially the newest optical-fibre result.
What a convincing future test would include
The clearest demonstration would combine an independently calibrated horizon, spontaneous rather than seeded emission, a spectrum with the predicted temperature, cross-horizon partner correlations and a quantum entanglement test in the same platform. Repetition by an independent group under varied flow conditions would address concerns about apparatus-specific noise. No one measurement is required to satisfy every criterion, but readers should identify exactly which criterion a paper has met before calling it “the detection of Hawking radiation.”
Further reading
For the underlying quantum-field theory, N. D. Birrell and P. C. W. Davies’s Quantum Fields in Curved Space (Cambridge University Press, 1982) is the advanced reference cited by the 2025 BEC correlation-function review. It is a theoretical text rather than an experimental manual.
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