A sonic black hole, sometimes called a dumb hole or acoustic black hole, is a phenomenon in which

phonon In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, specifically in solids and some liquids. A type of quasiparticle, a phonon is an excited state in the quantum mechanical ...

s (sound perturbations) are unable to escape from a region of a fluid that is flowing more quickly than the local

speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as ...

. They are called sonic, or acoustic, black holes because these trapped phonons are analogous to light in astrophysical (gravitational)

black hole A black hole is a region of spacetime where gravity is so strong that nothing, including light or other electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts that a sufficiently compact mass can defo ...

s. Physicists are interested in them because they have many properties similar to astrophysical black holes and, in particular, emit a phononic version of Hawking radiation. This Hawking radiation can be spontaneously created by quantum vacuum fluctuations, in close analogy with Hawking radiation from a real black hole. On the other hand, the Hawking radiation can be stimulated in a classical process. The boundary of a sonic black hole, at which the flow speed changes from being greater than the speed of sound to less than the speed of sound, is called the event horizon. A rotating sonic black hole was used in 2010 to give the first laboratory testing of

superradiance In physics, superradiance is the radiation enhancement effects in several contexts including quantum mechanics, astrophysics and relativity. Quantum optics In quantum optics, superradiance is a phenomenon that occurs when a group of ''N'' emit ...

, a process whereby energy is extracted from a black hole. Sonic black holes are possible because phonons in perfect fluids exhibit the same properties of motion as fields, such as gravity, in space and time. For this reason, a system in which a sonic black hole can be created is called a gravity analogue. Nearly any fluid can be used to create an acoustic event horizon, but the viscosity of most fluids creates random motion that makes features like Hawking radiation nearly impossible to detect. The complexity of such a system would make it very difficult to gain any knowledge about such features even if they could be detected. Many nearly perfect fluids have been suggested for use in creating sonic black holes, such as

superfluid Superfluidity is the characteristic property of a fluid with zero viscosity which therefore flows without any loss of kinetic energy. When stirred, a superfluid forms vortices that continue to rotate indefinitely. Superfluidity occurs in two ...

helium, one–dimensional degenerate

Fermi gas An ideal Fermi gas is a state of matter which is an ensemble of many non-interacting fermions. Fermions are particles that obey Fermi–Dirac statistics, like electrons, protons, and neutrons, and, in general, particles with half-integer s ...

es, and Bose–Einstein condensate. Gravity analogues other than phonons in a fluid, such as

slow light Slow light is the propagation of an optical pulse or other modulation of an optical carrier at a very low group velocity. Slow light occurs when a propagating pulse is substantially slowed by the interaction with the medium in which the propagatio ...

and a system of ions, have also been proposed for studying black hole analogues. The fact that so many systems mimic gravity is sometimes used as evidence for the theory of emergent gravity, which could help reconcile relativity, and quantum mechanics. Acoustic black holes were first theorized to be useful by

William Unruh William George "Bill" Unruh (; born August 28, 1945) is a Canadian physicist at the University of British Columbia, Vancouver who described the hypothetical Unruh effect in 1976. Early life and education Unruh was born into a Mennonite family in ...

in 1981. However, the first black hole analogue was not created in a laboratory until 2009. It was created in a rubidium Bose–Einstein condensate using a technique called density inversion. This technique creates a flow by repelling the condensate with a potential minimum. The

surface gravity The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experien ...

and temperature of the sonic black hole were measured, but no attempt was made to detect Hawking radiation. However, the scientists who created it predicted that the experiment was suitable for detection and suggested a method by which it might be done by lasing the phonons. In 2014, stimulated Hawking radiation was reported in an analogue black-hole laser by the same researchers. Quantum, spontaneous Hawking radiation was observed later.

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* * * {{black holes Black holes Fluid dynamics

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