Superradiance

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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.

Research on the effect was originally more on the amplification of existing EM waves in a particulate medium than the amplification of "virtual" waves in a vacuum. The terms was originated by Charles Misner.

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[edit] 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 momentum and energy 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 vacuum – in the quantum case, quantum fluctuations 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 amplification 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 frictional 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 "picked on" 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 pairs that would otherwise quickly mutually annihilate, to produce a population of "real" particles in the region outside the horizon.

[edit] Cases of superradiance

In astrophysics, 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 tangentially, 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.

[edit] See also

[edit] References

  • Kip S. Thorne Black holes and timewarps: Einstein's outrageous legacy (1994) – includes a diagram of the Zel'dovich radiation mechanism on pp.432
  • Thorne. Price and Macdonald (eds) Black holes: the membrane paradigm (1986) – includes a description of "mining" a black hole's "virtual" atmosphere by lowering a bucket into the region and pulling out real particles. The bucket's acceleration turns the virtual atmosphere into tangible light and matter.