Superradiance -- the 2020 Edition (1501.06570v8)

Abstract: Superradiance is a radiation enhancement process that involves dissipative systems. With a 60 year-old history, superradiance has played a prominent role in optics, quantum mechanics and especially in relativity and astrophysics. In General Relativity, black-hole superradiance is permitted by the ergoregion, that allows for energy, charge and angular momentum extraction from the vacuum, even at the classical level. Stability of the spacetime is enforced by the event horizon, where negative energy-states are dumped. Black-hole superradiance is intimately connected to the black-hole area theorem, Penrose process, tidal forces, and even Hawking radiation, which can be interpreted as a quantum version of black-hole superradiance. Various mechanisms (as diverse as massive fields, magnetic fields, anti-de Sitter boundaries, nonlinear interactions, etc...) can confine the amplified radiation and give rise to strong instabilities. These "black-hole bombs" have applications in searches of dark matter and of physics beyond the Standard Model, are associated to the threshold of formation of new black hole solutions that evade the no-hair theorems, can be studied in the laboratory by devising analog models of gravity, and might even provide a holographic description of spontaneous symmetry breaking and superfluidity through the gauge-gravity duality. This work is meant to provide a unified picture of this multifaceted subject. We focus on the recent developments in the field, and work out a number of novel examples and applications, ranging from fundamental physics to astrophysics.

• The paper demonstrates that superradiant scattering enables the extraction of energy from rotating black holes, based on precise wave frequency conditions.
• The paper explores the formation of black-hole bombs by showing how reflective barriers trap amplified waves and trigger instabilities.
• The paper examines the role of massive fields in Kerr systems, highlighting their impact on instabilities and potential implications for astrophysics and beyond Standard Model physics.

An Overview of Superradiance in Black Hole Physics

Superradiance, a process that transforms dissipative systems into radiation amplifiers, constitutes a fascinating instance of energy extraction mechanisms in classical and relativistic physics. Collectively introduced by Dicke in 1954 and investigated by Zel'dovich in 1971, superradiance allows for waves to receive energy from rotating bodies. It stands prominently in a multitude of areas including optics, quantum mechanics, and general relativity. The phenomenon manifests in various guises across these domains: for black holes (BHs), superradiance arises from the ergoregion, allowing them to channel angular momentum and energy via the Penrose Process.

This essay reviews the vital inclusion of superradiant amplification in the framework of BH physics, examining mechanisms and implications of this unique amplified radiation instigation. Through the inclusion of charged and rotating BHs, we examine how superradiance is not merely a subject of academic consideration but also a topic with practical applications in high-energy astrophysics, exotic matter searches and perhaps in experimental analog systems designed to mimic BH phenomena.

Key Theoretical Insights

Energy Extraction and Amplification: Superradiant scattering processes enable waves incident on a rotating BH to extract energy, which correlates directly with the specific angular momentum of the BH. Derived from the field equations, the superradiance condition formalizes as $\omega < m \Omega_{\rm H}$, where $\omega$ is the wave frequency, $m$ the azimuthal wave number, and $\Omega_{\rm H}$ the BH’s horizon angular velocity. This scenario offers a classical analogy to wave amplification across rotating media such as celestial bodies.

Black-hole Bombs: When bounded by a reflective barrier, a rotating BH can form a so-called BH bomb. This configuration enables superradiantly amplified waves to be confined and enhanced, ultimately destabilizing the rotating BH. This leads to the increased occurrence of superradiant instabilities, with critical implications for compact objects in astrophysics.

Massive Fields and the Kerr System: A complete treatment of superradiance in Kerr backgrounds reveals rich results: scalar, vector, and gravitational fields (massless and massive) can induce instabilities, given favorable conditions dictated by BH spin and wave frequency. Massive fields impose a potential barrier, thus predicting instabilities within this bounded framework, common in alternative theories like massive gravity or scalar-tensor theories.

Experimental Analogues

While direct observation within astrophysical contexts remains a challenge, laboratory analogues offer lines of testing superradiant principles. Fascinatingly, recent experiments modeled superradiant effects in fluid dynamics, using vortices within rotating fluids under controlled conditions. Such setups simulate the conditions of BH horizons, hence reinforcing theoretical models and offering empirical scrutiny prospects.

Practical Implications

Astrophysically, superradiant effects have implications in BH accretion disks and electromagnetic processes influencing compact stars. Additionally, the potential to constrain physics beyond the Standard Model via instabilities induced by massive fields supports a pursuit parallel to direct particle physics searches. In cosmology, superradiance betokens processes that might elucidate dark matter signatures through axionic fields triggered by BH interactions.

Conclusion

Superradiance holds relevance not only as a phenomenon inherent to BH physics, but it also drives forward theoretical constructs and offers potential observational footholds within astrophysical and cosmological landscapes. The direct consequences, theoretical pursuits, and credible simulations extend an invitation to further investigate this pivotal aspect of relativistic physics, situating superradiance as an anchor for interdisciplinary paper that bridges theory with practice across theoretical physics.