Building Black Holes in a Lab

Black holes may be impossible to build directly, but physicists can still test key black-hole ideas in the lab using “analog black holes”—non-black-hole systems that mimic the same horizon physics. The payoff is practical: these setups let researchers probe phenomena tied to event horizons, including Hawking-like radiation and effects related to rotation, without needing a real astrophysical black hole.

Studying real black holes is notoriously indirect. Event horizons hide what happens inside, the nearest supermassive black holes are tens of millions of light-years away, and the objects are too small on cosmic scales to manipulate experimentally. Yet the evidence for their existence is strong—stars orbiting a dark center in the Milky Way, superheated accretion disks in quasars and X-ray binaries, and gravitational-wave signals matching predictions for black hole mergers. The remaining challenge is physics: turning theoretical expectations into experimentally grounded understanding.

Analog black holes begin with a conceptual bridge. In 1972, Bill Unruh used a thought experiment: a blind fish in a river can’t hear sounds upstream if a powerful waterfall drags sound faster than it can propagate. Replace sound with light and the river with spacetime, and the “waterfall” becomes an event horizon where nothing can escape. By 1982, Unruh recognized that fluid dynamics equations can be written in forms closely analogous to general relativity, making certain fluid flows behave like black holes—including the emergence of Hawking radiation in the mathematical description.

The lab implementations lean on water. In one experiment, water flows over a sloped obstacle: as the depth decreases, the current accelerates while surface-wave speed drops. Where the flow outruns the waves, an analog event horizon forms; reversing the direction yields an analog “white hole.” In another, a shaped tank produces a vortex—often called a “bathtub vortex”—where downward flow reaches the speed of surface ripples, again creating a horizon. Researchers then look for Hawking-like signatures. Hawking’s 1974 prediction says real black holes lose mass via radiation, often described as quantum particle pairs near the horizon, with one escaping and the other effectively reducing the black hole’s energy. In analog systems, the “escaping particles” correspond to how quantum-field vibrations map onto measurable ripple modes on the water surface. Observations have reported Hawking-like behavior, at least in the frequency perturbations of surface ripples.

Analog setups also target what happens to the black hole’s energy and angular momentum—an area where the details are contentious in the standard Hawking picture. In vortex analogs, researchers report sapping of the gravitational field analog, including energy and angular momentum extraction. Rotating analog black holes are especially valuable because they connect to the Penrose process and superradiance: waves passing through an ergosphere can be amplified. Experiments with large tanks of fluorescent water have demonstrated superradiance-like increases in wave height when waves skim a whirlpool’s horizon region.

To push beyond classical analogs, researchers use quantum matter. Bose-Einstein condensates—ultracold rubidium gas cooled near absolute zero—can be driven by lasers so that the laser edge acts as an event horizon for the atomic flow. In these experiments, Hawking radiation is not just detected; its effective temperature is measured, offering some of the strongest direct experimental evidence for Hawking-like behavior. Quantum-optical analogs using slowed light can create apparent horizons too, though they remain approximations.

The central debate is philosophical as much as physical: do analog horizons genuinely inform real black holes, or are they imperfect proxies? Supporters point to repeated Hawking-like observations across different systems as meaningful corroboration. Skeptics argue analogs lack the full uniqueness of real black holes and may not provide complementary evidence strong enough to claim direct relevance. Either way, until real black holes become controllable in a lab, analog systems—especially bathtub vortices and Bose-Einstein condensates—remain the most direct experimental route to probing horizon physics and the quantum nature of spacetime.