UC Santa Barbara researchers mimic black hole horizon with atom waves

David Weld and colleagues at the University of California, Santa Barbara, confirmed a cavity prediction for the first time on Nov. 6, 2026, by replacing light with ultracold atoms in a new laboratory experiment.

An ordinary optical cavity traps light between two facing mirrors and keeps only wavelengths that match the mirror spacing exactly, selecting highly stable frequencies. Theory predicts that vibrating one mirror at the light’s round-trip rate makes the light drain steadily in one direction and bunch into a short, intense pulse. That prediction had never been tested directly. The same mathematics apply to light approaching an event horizon and light approaching a white hole horizon.

Testing the idea with light is not yet possible: a mirror would have to reach a large part of light speed within billionths of a second, beyond current technology. Weld’s team sidestepped the barrier by swapping light for matter. At temperatures near absolute zero, lithium atoms behave like waves, so they can stand in for light while moving slowly enough to manipulate.

The researchers began by chilling lithium gas to near absolute zero and trapping it in a grid of laser light. They confined the atoms using two sheets of laser light acting as weightless mirrors, then moved one sheet back and forth about 100 times per second. The matter waves traveled at almost constant speed, at less than 1 meter per second. The team imaged the atoms’ distribution over time and compared the results with theoretical predictions.

Within a few cycles, the moving light sheet transformed an initially diffuse cloud of atoms into a tight packet following a repeating path. Packets beginning in different positions moved away from an unstable route and gathered on the same stable route. That convergence mirrors the one-way motion predicted for black and white hole horizons.

When the researchers doubled the driving frequency, they produced two concentrated packets, each following its own path. A sudden change in the timing of the moving light sheet let them exchange the stable and unstable routes and flip the packets’ direction. The shift demonstrated control over the simulated horizon-like behavior.

The team also spotted subtle effects beyond the ideal light-like picture. They traced those effects to the atoms’ speed not being as constant as light’s, a deviation that emerged because the atoms replaced photons as the waves confined in the cavity. Weld’s team hopes the effect could lead to new ways of compressing and decompressing optical signals.

One possibility involves solid materials whose optical properties change with electrical tuning, with no moving mirrors. The arrangement might additionally let researchers investigate the creation of pairs of light particles from empty space by mirrors moving quickly. The study is Jeremy L. Tanlimco et al., Matter-Wave Fabry-Pérot Cavity in the Ultrastrong Driving Regime, published in Physical Review Letters in 2026, with DOI 10.1103/44mq-jfp1 and arXiv identifier 2601.21122.

Lifeboat News reported the same experiment on Nov. 5, 2026, describing sheets of light corralling waves of ultracold atoms in an experimental platform for studying black hole event horizons. Theory holds that rapidly vibrating a pair of facing mirrors makes the optical field between them display black-hole-like traits, including an event horizon equivalent, though these conditions have not yet been realized in the lab with light. Researchers have nonetheless managed something similar by swapping light and matter. Researchers call the apparatus a matter-wave cavity.

Vibrating one mirror in an optical cavity makes the cavity length oscillate, which can alter the light’s properties. Oscillating a mirror at the cavity’s resonant frequency would require motion at close to the speed of light, compressing all light into a short, intense pulse bouncing between the mirrors. That speed is far beyond existing technology, which made the matter-wave substitution necessary. Vibrating the light sheets simulates light propagating close to a black hole while potentially enabling the study of many other exotic phenomena.

Researchers are unlikely to measure Hawking radiation from a real black hole, but researchers can test it in laboratory analogs, according to Yuval Rosenberg of the Weizmann Institute of Science, writing on April 14, 2020. Hawking radiation is the thermal radiation Hawking predicted black holes emit from the quantum vacuum — the quantum vacuum being empty space as described by quantum mechanics. Hawking predicted that black holes thermally radiate from the quantum vacuum. Hypothetical microscopic primordial black holes might emit significant Hawking radiation, though the lack of observation bounds the rates.

Wave kinematics control the Hawking process, which appears in the presence of an effective horizon. In the analog picture, the event horizon is the surface separating subsonic and supersonic flows. Fluid flow moving at the speed of sound matches the geometry around a black hole and should produce sound waves like Hawking radiation. Researchers have proposed and realized many analogs using water waves, Bose-Einstein condensates — clouds of atoms so cold that they act as a single quantum object — and optics.

Rosenberg’s paper examines optical analogs that create an artificial black-hole horizon using a disturbance in a material’s refractive index — the refractive index being a measure of how much a material slows and bends light. One approach, described by Leonhardt and König, sends a light pulse through a dielectric — an insulating material that an electric field can polarize — so the medium appears to flow in the opposite direction in the pulse’s co-moving frame, the frame moving with the pulse.

The nonlinear Kerr effect slows probe light when it interacts with a pulse, expressed as n equals n0 plus n2 I, with n2 typically 10^-16 centimeter squared per watt. In this frame, probe light of a different frequency travels at a different velocity because of dispersion, setting up the relative motion needed to mimic a horizon. Philbin et al. showed that a light pulse could slow, blue-shift, and reflect probe light, conserving frequency in the co-moving frame but not in the laboratory frame.

Philbin et al. used 70-femtosecond pulses at 800 nanometers with about 50 watts of peak power inside a 1.5-meter photonic crystal fiber — a fiber with a periodic internal structure that guides light — whose core measured under 2 micrometers across. Their setup reached only 10^-3 percent efficiency for shifting the probe. Researchers expected just 10^-4 of the probe power to interact over 1.5 meters, plus a tenfold tunneling loss. No detectable negative-frequency partners — wave components whose frequency runs opposite to the direction of motion — resulted.

Rubino et al. later measured negative-frequency resonant radiation after researchers had long neglected and disregarded it. Rosenberg says they created it using rapid non-adiabatic temporal changes of the pulse envelope — non-adiabatic meaning changes too fast for the system to adjust smoothly. One setup used 7-femtosecond solitons — self-reinforcing waves that keep their shape — of about 300 picojoules at 800 nanometers in 5-millimeter photonic crystal fibers. Another used 60-femtosecond Bessel beams of about 20 microjoules at 800 nanometers through 2-centimeter bulk calcium fluoride.

Belgiorno et al. detected radiation around a phase velocity horizon in super-luminal fused-silica filaments — glass filaments where light’s phase velocity exceeds its usual speed in the material — at peak intensity up to 10^13 watts per square centimetre, centred at 1055 nanometres. The team could not measure along the propagation direction because the intense pulses produced overwhelming radiation there. The work highlighted that a blocking group velocity horizon may be necessary for the Hawking effect.

The extreme faintness of Hawking radiation from astrophysical objects is the main reason researchers consider direct detection unlikely. Rosenberg states that stellar and heavier black holes produce Hawking radiation much weaker than cosmic microwave background fluctuations — the cosmic microwave background being the faint afterglow of the early universe. Rosenberg also flags the trans-Planckian problem — Hawking’s original derivation relies on infinitely high frequencies and neglects unknown high-energy physics.

Because analog systems have well-understood microscopic physics, researchers can use them to test how high frequencies influence Hawking predictions, sidestepping issues with the original derivation. The new cavity experiment simulates light propagating close to a black hole while potentially enabling the study of many other exotic phenomena. Weld’s group also points to particle creation from the quantum vacuum as a related fundamental prediction. The experiment offers a laboratory route to probing horizon physics because researchers consider Hawking radiation from real black holes too faint to measure.