In 1981, physicist William Unruh realized that the mathematics describing light near a gravitational black hole's event horizon are nearly identical to sound waves traveling through a fluid accelerating past the speed of sound. When a fluid drain accelerates from sub-sonic to super-sonic speeds, it creates an acoustic event horizon: a point of no return where sound waves moving upstream cannot escape the swift fluid flow. These synthetic systems, known as dumb holes or acoustic black holes, provide an unprecedented experimental playground for testing quantum gravitational concepts on Earth.
Most notably, trans-sonic acoustic fluid drains produce dynamically induced Hawking radiation analogs. Just as quantum fluctuations near a real black hole create entangling particle-antiparticle pairs that split at the event horizon, quantum phonons (quantized sound vibrations) or thermal fluctuations split at the sonic horizon. One phonon falls into the acoustic drain while the other escapes downstream as analog Hawking radiation.
Recent laboratory experiments using Bose-Einstein condensates and ultra-cold quantum fluids have successfully observed these spontaneous acoustic phonon pairs, showing a thermal spectrum matching theoretical predictions. By manipulating the flow velocity gradient across the trans-sonic transition, researchers can dynamically trigger and amplify this acoustic radiation. This groundbreaking analogue gravity research allows scientists to empirically test black hole thermodynamics, information loss paradoxes, and quantum field theory in curved spacetimes without ever leaving the laboratory.