Optical cavities are pairs of light-trapping mirrors and underpin the operation of lasers, atomic clocks and many types of precision sensors. If the distance between the mirrors is changed rapidly enough, some theories predict that the waves inside them should bunch together into short, intense pulses. So far, however, this prediction has never been tested directly.
Through new experiments, David Weld and colleagues at the University of California, Santa Barbara, have confirmed it for the first time by swapping the roles of light and matter. Strikingly, they found that the cavity behaves in ways that mimic the strange physics of curved spacetime.
Their results have been published in Physical Review Letters.
Mimicking black and white holes
In an ordinary cavity, light bounces back and forth between the two opposite-facing mirrors. Since they can only confine light with wavelengths perfectly matching their spacing, the cavity is able to pick out highly stable frequencies. However, if one of the mirrors is vibrated at a rate matching the light’s round trip between the mirrors, one theory suggests that it would steadily drain in one direction to form a short, concentrated pulse.
Mathematically, this one-way flow resembles light near the event horizon of a black hole, or near its purely hypothetical counterpart, a “white hole.” To test for this in practice, however, an experiment would require a mirror to repeatedly accelerate to a sizable fraction of the speed of light within billionths of a second—far beyond the abilities of existing technologies.
Swapping light for atoms
To sidestep this problem, Weld’s team replaced cavity-confined light with atoms, which behave just like waves at ultracold temperatures. The researchers started by cooling a gas of lithium atoms to near absolute zero, then held it in a grid of laser light. Similarly to light, the grid forced these “matter waves” to travel at an almost constant speed, only at speeds of less than 1 meter per second.
The atoms were confined using two sheets of laser light which acted as weightless mirrors, and one of them was moved back and forth around 100 times per second. The team then imaged how the atoms were distributed over time and compared the results with theoretical predictions.
Fine-tuned paths
Within a few cycles, the moving light sheet transformed the initially diffuse cloud into a tight packet of atoms following a repeating path. Packets that started in different positions all drifted away from an unstable path and converged on the same stable one: the same one-way motion predicted to mirror the event horizons of black and white holes.
When they doubled the driving frequency, the researchers produced two concentrated packets, each following its own path. By abruptly shifting the timing of the light sheet’s motion, they could swap the stable and unstable paths, reversing the packets’ motion. They also spotted subtle effects beyond the ideal light-like picture, which they traced to the atoms’ speed not being quite as constant as light’s.
Weld’s team now hopes the effect could lead to new ways of compressing and decompressing optical signals, potentially in solid materials whose optical properties can be tuned electrically, with no need for moving mirrors at all. The setup could also help physicists explore other fundamental predictions, such as how pairs of light particles can be conjured from empty space between rapidly moving mirrors.
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Publication details
Jeremy L. Tanlimco et al., Matter-Wave Fabry-Pérot Cavity in the Ultrastrong Driving Regime, Physical Review Letters (2026). DOI: 10.1103/44mq-jfp1. On arXiv: arxiv.org/abs/2601.21122
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Vibrating mirrors of light create analog of a black hole event horizon (2026, November 6)
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