Black Hole Entropy is Thermodynamic Entropy

Schematic illustration of a black hole Carnot cycle. The system consists of a black hole and a photon gas, enclosed in a box. The size of the black hole is proportional to the temperature of the system, i.e. small is hot and large is cold.

Carina E. A. Prunkl, Christopher G. Timpson
The comparison of geometrical properties of black holes with classical thermodynamic variables reveals surprising parallels between the laws of black hole mechanics and the laws of thermodynamics. Since Hawking’s discovery that black holes when coupled to quantum matter fields emit radiation at a temperature proportional to their surface gravity, the idea that black holes are genuine thermodynamic objects with a well-defined thermodynamic entropy has become more and more popular. Surprisingly, arguments that justify this assumption are both sparse and rarely convincing. Most of them rely on an information-theoretic interpretation of entropy, which in itself is a highly debated topic in the philosophy of physics. We discuss some of the pertinent arguments that aim at establishing the identity of black hole surface area (times a constant) and thermodynamic entropy and show why these arguments are not satisfactory. We then present a simple model of a Black Hole Carnot cycle to establish that black hole entropy is genuine thermodynamic entropy which does not require an information-theoretic interpretation.
Read more at https://arxiv.org/pdf/1903.06276.pdf

Black Hole as Extreme Particle Accelerator

Life of the jet set. This simulation follows along in a “co-moving” reference frame with a fixed set of particles as they are blasted out of an active galactic nucleus (AGN). The magnetic field lines they experience change as they move from a smoother region (left) to a region with a kink instability (right).  [Credit: E. P. Alves et al., Phys. Rev. Lett. (2018)]

Efficient Nonthermal Particle Acceleration by the Kink Instability in Relativistic Jets

E. Paulo Alves, Jonathan Zrake, Frederico Fiuza
Relativistic magnetized jets from active galaxies are among the most powerful cosmic accelerators, but their particle acceleration mechanisms remain a mystery. We present a new acceleration mechanism associated with the development of the helical kink instability in relativistic jets, which leads to the efficient conversion of the jet’s magnetic energy into nonthermal particles. Large-scale three-dimensional ab initio simulations reveal that the formation of highly tangled magnetic fields and a large-scale inductive electric field throughout the kink-unstable region promotes rapid energization of the particles. The energy distribution of the accelerated particles develops a well-defined power-law tail extending to the radiation-reaction limited energy in the case of leptons, and to the confinement energy of the jet in the case of ions. When applied to the conditions of well-studied bright knots in jets from active galaxies, this mechanism can account for the spectrum of synchrotron and inverse Compton radiating particles, and offers a viable means of accelerating ultra-high-energy cosmic rays to 1020 eV.

Read more at https://physics.aps.org/articles/v11/130 and https://arxiv.org/pdf/1810.05154.pdf

The black hole fifty years after: Genesis of the name

Ann Ewing’s article in 1964 where the term Black Hole is published for the first time

Carlos A. R. Herdeiro, José P. S. Lemos
Black holes are extreme spacetime deformations where even light is imprisoned. There is an extensive astrophysical evidence for the real and abundant existence of these prisons of matter and light in the Universe. Mathematically, black holes are described by solutions of the field equations of the theory of general relativity, the first of which was published in 1916 by Karl Schwarzschild.
Another highly relevant solution, representing a rotating black hole, was found by Roy Kerr in 1963. It was only much after the publication of the Schwarzschild solution, however, that the term black hole was employed to describe these objects. Who invented it?
Conventional wisdom attributes the origin of the term to the prominent North American physicist John Wheeler who first adopted it in a general audience article published in 1968. This, however, is just one side of a story that begins two hundred years before in an Indian prison colloquially known as the Black Hole of Calcutta.
Robert Dicke, also a distinguished physicist and colleague of Wheeler at Princeton University, aware of the prison’s tragedy began, around 1960, to compare gravitationally completely collapsed stars to the black hole of Calcutta. The whole account thus suggests reconsidering who indeed coined the name black hole and commends acknowledging its definitive birth to a partnership between Wheeler and Dicke.
Read more at https://arxiv.org/pdf/1811.06587.pdf

What Is a Black Hole?

Erik Curiel
Although black holes are objects of central importance across many fields of physics, there is no agreed upon definition for them, a fact that does not seem to be widely recognized. Physicists in different fields conceive of and reason about them in radically different, and often conflicting, ways. All those ways, however, seem sound in the relevant contexts. After examining and comparing many of the definitions used in practice, I consider the problems that the lack of a universally accepted definition leads to, and discuss whether one is in fact needed for progress in the physics of black holes. I conclude that, within reasonable bounds, the profusion of different definitions is in fact a virtue, making the investigation of black holes possible and fruitful in all the many different kinds of problems about them that physicists consider, although one must take care in trying to translate results between fields.
Read more at https://arxiv.org/pdf/1808.01507.pdf

The Hawking temperature, the uncertainty principle and quantum black holes

blackhole1

A static black hole. The horizon (H ) is at a distance RS from the singularity (S).

Jorge Pinochet
In 1974, Stephen Hawking theoretically discovered that black holes emit thermal radiation and have a characteristic temperature, known as the Hawking temperature. The aim of this paper is to present a simple heuristic derivation of the Hawking temperature, based on the Heisenberg uncertainty principle. The result obtained coincides exactly with Hawking’s original finding. In parallel, this work seeks to clarify the physical meaning of Hawking’s discovery. This article may be useful as pedagogical material in a high school physics course or in an introductory undergraduate physics course.
Read more at https://arxiv.org/pdf/1808.05121.pdf