Sonic black hole
Encyclopedia
A sonic black hole
(sometimes called a dumb hole) is a phenomenon in which phonons (sound perturbations) are unable to escape from a fluid that is flowing more quickly than the local speed of sound
. They are called sonic, or acoustic, black holes because these trapped phonons are analogous to light in astrophysical (gravitational) black holes. Physicists are interested in them because they have many properties similar to astrophysical black holes and, in particular, are predicted to emit a phononic version of Hawking radiation
. The border 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
. At this point the wavelength of phonons approaches zero.
Sonic black holes are possible because phonons in perfect fluid
s 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
ic helium, a degenerate Fermi gas
, and Bose-Einstein condensate. Gravity analogues other than phonons in a fluid, such as slow light 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. 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
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.
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
(sometimes called a dumb hole) is a phenomenon in which phonons (sound perturbations) are unable to escape from a fluid that is flowing more quickly than the local speed of sound
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
. They are called sonic, or acoustic, black holes because these trapped phonons are analogous to light in astrophysical (gravitational) black holes. Physicists are interested in them because they have many properties similar to astrophysical black holes and, in particular, are predicted to emit a phononic version of Hawking radiation
Hawking radiation
Hawking radiation is a thermal radiation with a black body spectrum predicted to be emitted by black holes due to quantum effects. It is named after the physicist Stephen Hawking, who provided a theoretical argument for its existence in 1974, and sometimes also after the physicist Jacob Bekenstein...
. The border 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
Event horizon
In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman's terms it is defined as "the point of no return" i.e. the point at which the gravitational pull becomes so great as to make escape impossible. The most common case...
. At this point the wavelength of phonons approaches zero.
Sonic black holes are possible because phonons in perfect fluid
Perfect fluid
In physics, a perfect fluid is a fluid that can be completely characterized by its rest frame energy density ρ and isotropic pressure p....
s 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
Analog models of gravity
Analogue models of gravity is a direction of research in physics, where various phenomena of general relativity are modeled by other physical systems, such as acoustics in a moving fluid, superfluid helium, or Bose-Einstein condensate; gravity waves in water; and propagation of electromagnetic...
. 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
Superfluid
Superfluidity is a state of matter in which the matter behaves like a fluid without viscosity and with extremely high thermal conductivity. The substance, which appears to be a normal liquid, will flow without friction past any surface, which allows it to continue to circulate over obstructions and...
ic helium, a degenerate Fermi gas
Fermi gas
A Fermi gas is an ensemble of a large number of fermions. Fermions, named after Enrico Fermi, are particles that obey Fermi–Dirac statistics. These statistics determine the energy distribution of fermions in a Fermi gas in thermal equilibrium, and is characterized by their number density,...
, and Bose-Einstein condensate. Gravity analogues other than phonons in a fluid, such as slow light 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
Induced gravity
Induced gravity is an idea in quantum gravity that space-time background emerges asa mean field approximation of underlying microscopic degrees of freedom, similar to the fluid mechanics approximation of Bose–Einstein condensates...
, which could help reconcile relativity and quantum mechanics.
Acoustic black holes were first theorized to be useful by William Unruh. 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
Surface gravity
The surface gravity, g, of an astronomical or other object is the gravitational acceleration experienced at its surface. The surface gravity may be thought of as the acceleration due to gravity experienced by a hypothetical test particle which is very close to the object's surface and which, in...
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.
See also
- Quantum GravityQuantum gravityQuantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...
- Analog models of gravityAnalog models of gravityAnalogue models of gravity is a direction of research in physics, where various phenomena of general relativity are modeled by other physical systems, such as acoustics in a moving fluid, superfluid helium, or Bose-Einstein condensate; gravity waves in water; and propagation of electromagnetic...
- Black holeBlack holeA black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
- Acoustic metricAcoustic metricIn mathematical physics, a metric describes the arrangement of relative distances within a surface or volume, usually measured by signals passing through the region – essentially describing the intrinsic geometry of the region. An acoustic metric will describe the signal-carrying properties...
- Optical Black HoleOptical black holeAn optical black hole is a phenomenon in which slow light is passed through a Bose-Einstein condensate that is itself spinning faster than the local speed of light within to create a vortex capable of trapping the light behind an event horizon just as a gravitational black hole would.Unlike other...
External links
- Top 100 Stories of 2009 #79: Sonic Black Hole Created in Lab, Discover magazine, from the January-February 2010 special issue; published online December 22, 2009
- Ars Technica: A potential solution to the black hole information loss paradox
- "Analogue Gravity", a detailed mathematical analysis with diagrams