Superradiance
Encyclopedia
In quantum mechanics
, superradiance refers to a class of radiation effects (or enhanced radiation effects) typically associated with the acceleration or motion of a nearby body (which supplies the energy and momentum for the effect). It is also sometimes described as the consequence of an "effective" field differential around the body (e.g. the effect of tidal forces).
Superradiance allows a body with concentration of angular or linear momentum to move towards a lower energy state, even when there is no obvious classical mechanism for this to happen. In this sense, the effect has some similarities with quantum tunnelling
(e.g. the tendency of waves and particles to "find a way" to exploit the existence of an energy potential, despite the absence of an obvious classical mechanism for this to happen).
The term also sometimes appears in discussions of laser
technology, where the radiation is again triggered in a "driven" system by quantum effects ("spontaneous" emission), but the energy for the emergent wave is instead provided by an explicit, particulate, gain medium.
Where a classical description of a rotating isolated weightless sphere in a vacuum will tend to say that the sphere will continue to rotate indefinitely, due the lack of friction
al effects or any other form of obvious coupling with its smooth empty environment, under quantum mechanics the surrounding region of vacuum is not entirely smooth, and the sphere's field can couple with quantum fluctuations and accelerate them to produce real radiation. Hypothetical virtual wavefronts with appropriate paths around the body are stimulated and amplified
into real physical wavefronts by the coupling process. Descriptions sometimes refer to these fluctuations "tickling" the field to produce the effect.
In theoretical studies of black holes, the effect is also sometimes described as the consequence of the gravitational tidal forces around a strongly-gravitating body pulling apart virtual particle pair
s that would otherwise quickly mutually annihilate, to produce a population of real particles in the region outside the horizon.
, the most popularly-known example of superradiance is probably Zel'dovich radiation. Yakov Borisovich Zel'dovich
picked on the case under quantum electrodynamics
("QED") where the region around the equator of a spinning metal sphere is expected to throw off electromagnetic radiation
tangent
ially, and suggested that the case of a spinning gravitational mass, such as a Kerr black hole ought to produce similar coupling effects, and ought to radiate in an analogous way.
This was followed by arguments from Stephen Hawking
and others that an accelerated observer near a black hole (e.g. an observer carefully lowered towards the horizon at the end of a rope) ought to see the region inhabited by "real" radiation, whereas for a distant observer this radiation would be said to be "virtual". If the accelerated observer near the event horizon
traps a nearby particle and throws it out to the distant observer for capture and study, then for the distant observer, the appearance of the particle can be explained by saying that the physical acceleration of the particle has turned it from a virtual particle
into a "real" particle (see Hawking radiation
).
Similar arguments apply for the cases of observers in accelerated frames (Unruh radiation). Cherenkov radiation
, electromagnetic radiation emitted by charged particles travelling through a particulate medium at more than the nominal speed of light in that medium, has also been described as being a "superradiant" effect.
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
, superradiance refers to a class of radiation effects (or enhanced radiation effects) typically associated with the acceleration or motion of a nearby body (which supplies the energy and momentum for the effect). It is also sometimes described as the consequence of an "effective" field differential around the body (e.g. the effect of tidal forces).
Superradiance allows a body with concentration of angular or linear momentum to move towards a lower energy state, even when there is no obvious classical mechanism for this to happen. In this sense, the effect has some similarities with quantum tunnelling
Quantum tunnelling
Quantum tunnelling refers to the quantum mechanical phenomenon where a particle tunnels through a barrier that it classically could not surmount. This plays an essential role in several physical phenomena, such as the nuclear fusion that occurs in main sequence stars like the sun, and has important...
(e.g. the tendency of waves and particles to "find a way" to exploit the existence of an energy potential, despite the absence of an obvious classical mechanism for this to happen).
The term also sometimes appears in discussions of laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
technology, where the radiation is again triggered in a "driven" system by quantum effects ("spontaneous" emission), but the energy for the emergent wave is instead provided by an explicit, particulate, gain medium.
Comparison with classical physics
- In classical physics, the motion or rotation of a body in a particulate medium will normally be expected to result in momentumMomentumIn classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
and energyEnergyIn physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
being transferred to the surrounding particles, and there is then an increased statistical likelihood of particles being discovered following trajectories that imply removal of momentum from the body.
- In quantum mechanics, this principle is extended to the case of bodies moving, accelerating or rotating in a vacuumVacuumIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
– in the quantum case, quantum fluctuationQuantum fluctuationIn quantum physics, a quantum fluctuation is the temporary change in the amount of energy in a point in space, arising from Werner Heisenberg's uncertainty principle.According to one formulation of the principle,energy and time can be related by the relation...
s with appropriate vectors are said to be stretched and distorted and provided with energy and momentum by the nearby body's motion, with this selective amplificationAmplifierGenerally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
generating real physical radiation around the body.
Where a classical description of a rotating isolated weightless sphere in a vacuum will tend to say that the sphere will continue to rotate indefinitely, due the lack of friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
al effects or any other form of obvious coupling with its smooth empty environment, under quantum mechanics the surrounding region of vacuum is not entirely smooth, and the sphere's field can couple with quantum fluctuations and accelerate them to produce real radiation. Hypothetical virtual wavefronts with appropriate paths around the body are stimulated and amplified
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
into real physical wavefronts by the coupling process. Descriptions sometimes refer to these fluctuations "tickling" the field to produce the effect.
In theoretical studies of black holes, the effect is also sometimes described as the consequence of the gravitational tidal forces around a strongly-gravitating body pulling apart virtual particle pair
Pair production
Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...
s that would otherwise quickly mutually annihilate, to produce a population of real particles in the region outside the horizon.
Cases of superradiance
In astrophysicsAstrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
, the most popularly-known example of superradiance is probably Zel'dovich radiation. Yakov Borisovich Zel'dovich
Yakov Borisovich Zel'dovich
Yakov Borisovich Zel'dovich was a prolific Soviet physicist born in Belarus. He played an important role in the development of Soviet nuclear and thermonuclear weapons, and made important contributions to the fields of adsorption and catalysis, shock waves, nuclear physics, particle physics,...
picked on the case under quantum electrodynamics
Quantum electrodynamics
Quantum electrodynamics is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved...
("QED") where the region around the equator of a spinning metal sphere is expected to throw off electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
tangent
Tangent
In geometry, the tangent line to a plane curve at a given point is the straight line that "just touches" the curve at that point. More precisely, a straight line is said to be a tangent of a curve at a point on the curve if the line passes through the point on the curve and has slope where f...
ially, and suggested that the case of a spinning gravitational mass, such as a Kerr black hole ought to produce similar coupling effects, and ought to radiate in an analogous way.
This was followed by arguments from Stephen Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...
and others that an accelerated observer near a black hole (e.g. an observer carefully lowered towards the horizon at the end of a rope) ought to see the region inhabited by "real" radiation, whereas for a distant observer this radiation would be said to be "virtual". If the accelerated observer near 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...
traps a nearby particle and throws it out to the distant observer for capture and study, then for the distant observer, the appearance of the particle can be explained by saying that the physical acceleration of the particle has turned it from a virtual particle
Virtual particle
In physics, a virtual particle is a particle that exists for a limited time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle...
into a "real" particle (see 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...
).
Similar arguments apply for the cases of observers in accelerated frames (Unruh radiation). Cherenkov radiation
Cherenkov radiation
Cherenkov radiation is electromagnetic radiation emitted when a charged particle passes through a dielectric medium at a speed greater than the phase velocity of light in that medium...
, electromagnetic radiation emitted by charged particles travelling through a particulate medium at more than the nominal speed of light in that medium, has also been described as being a "superradiant" effect.