Black Holes
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Contents
Introduction To Black Holes
A black hole is a theoretical region of space in which the gravitational field is so powerful that nothing, not even electromagnetic radiation (e.g. visible light), can escape its pull after having fallen past its event horizon. The term derives from the fact that the absorption of visible light renders the hole's interior invisible, and indistinguishable from the black space around it.
Despite its interior being invisible, a black hole may reveal its presence through an interaction with matter that lies in orbit outside its event horizon. For example, a black hole may be perceived by tracking the movement of a group of stars that orbit its center. Alternatively, one may observe gas (from a nearby star, for instance) that has been drawn into the black hole. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and earth-orbiting telescopes.. Such observations have resulted in the general scientific consensus that?barring a breakdown in our understanding of nature?black holes do exist in our universe..
The idea of an object with gravity strong enough to prevent light from escaping was proposed in 1783 by John Michell,. an amateur British astronomer. In 1795, Pierre-Simon Laplace, a French physicist independently came to the same conclusion.Laplace; see Israel, Werner (1987), "Dark stars: the evolution of an idea", in Hawking, Stephen W. & Israel, Werner, 300 Years of Gravitation, Cambridge University Press, Sec. 7.4 Black holes, as currently understood, are described by the general theory of relativity. This theory predicts that when a large enough amount of mass is present in a sufficiently small region of space, all paths through space are warped inwards towards the center of the volume, preventing all matter and radiation within it from escaping.
While general relativity describes a black hole as a region of empty space with a point-like singularity at the center and an event horizon at the outer edge, the description changes when the effects of quantum mechanics are taken into account. Research on this subject indicates that, rather than holding captured matter forever, black holes may slowly leak a form of thermal energy called Hawking radiation and may well have a finite life.. However, the final, correct description of black holes, requiring a theory of quantum gravity, is unknown.
Introduction To Event Horizon
In general relativity, an event horizon is a boundary in spacetime, an area surrounding a black hole or a wormhole, inside which events cannot affect an outside observer. Light emitted from inside the horizon can never reach the observer, and anything that passes through the horizon from the observer's side disappears.
More specific types of horizon include the related but distinct absolute and apparent horizons found around a black hole. Still other distinct notions include the Cauchy and Killing horizon; the photon spheres and ergospheres of the Reissner-Nordström solution; particle and cosmological horizons relevant to cosmology; and isolated and dynamical horizons important in current black hole research.
Introduction To Ergosphere
The ergosphere is a region located outside a rotating black hole. Its name is derived from the Greek word ergon, which means ?work?. It received this name because it is theoretically possible to extract energy and mass from the black hole in this region.
The ergosphere is ellipsoidal in shape and is situated so that at the poles of rotating black hole it touches the event horizon and stretches out to a distance that is equal to the radius of the event horizon. Within the ergosphere spacetime is dragged along in the direction of the rotation of the black hole at a speed greater than the speed of light in relation to the rest of the universe. This process is known as the Lense-Thirring effect or frame-dragging. Because of this dragging effect objects within the ergosphere are not stationary with respect to the rest of the universe, unless they travel faster than the speed of light, which is impossible based on the laws of physics. Another result of this dragging of space is the existence of negative energies within the ergosphere.
The outer limit of the ergosphere is the stationary limit. At the stationary limit objects moving at the speed of light are stationary with respect to the rest of the universe. This is because the space here is being dragged at exactly the speed of light relative to the rest of space. Outside this limit space is still dragged, but at a rate less than the speed of light.
Since the ergosphere is outside the event horizon, it is still possible for objects to escape from the gravitational pull of the black hole. An object can gain energy by entering the black hole's rotation and then escaping from it, thus taking some of the black hole's energy with it. This process of removing energy from a rotating black hole was proposed by the mathematician Roger Penrose in 1969, and is called the Penrose process. The theoretical maximum of possible energy extraction is 29% of the total energy of a rotating black hole. Once this energy is removed the black hole loses its spin and the ergosphere no longer exists. This process is considered a possible explanation for a source of energy of such energetic phenomena as gamma ray bursts. Results from computer models show that the Penrose Process is capable of producing the high energy particles that are observed being emitted from quasars and other active galactic nuclei.
Introduction To Photon Sphere
A photon sphere is a spherical region of space where gravity is strong enough that photons of light are forced to travel in orbits. The formula to find the radius for a circular photon orbit is: r=3GM/C2. Because of this equation photon spheres can only exist in the space surrounding an extremely compact object, such as a black hole.
As photons travel near the event horizon of a black hole they can escape being pulled in by the gravity of a black hole by traveling at a nearly vertical direction known as an exit cone. A photon on the boundary of this cone will not completely escape the gravity of the black hole. Instead it orbits the black hole. These orbits are not stable.
The photon sphere is located further from the center of a black hole than the event horizon and ergosphere. Within a photon sphere it is possible to imagine a photon that starts at the back of your head and orbits around a black hole only then be seen by your eyes.
For non-rotating black holes, the photon sphere is a sphere of radius 3/2 Rs, where Rs denotes the Schwarzschild radius (the radius of the event horizon) - see below for a derivation of this result. No unaccelerated orbit with a semi-major axis less than this distance is possible, but within the photon sphere, a constant acceleration will allow a spacecraft or probe to hover above the event horizon.
A rotating black hole has two photon spheres. As a black hole rotates it drags space with it. The photon sphere that is closer to the black hole is moving in the same direction as the rotation, whereas the photon sphere further away is moving against it. The greater the angular velocity of the rotation of a black hole the greater distance between the two photon spheres. Because the black hole has an axis of rotation this only holds true if approaching the black hole in the direction of the equator. If approaching at a different angle, such as one from the poles of the black hole, to the equator there is only one photon sphere. This is because approaching at this angle the possibility of traveling with or against the rotation does not exist.
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Black Holes: Creation & Consumption of Galaxies. Or: What CERN & the Hadron Collider Can Do For You!
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Galaxies in Turmoil: The Active and Starburst Galaxies and the Black Holes That Drive Them (Astronomers' Universe)
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Black Holes and Baby Universes and Other Essays
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The Collapsing Universe: The Story of the Black Holes
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Black Holes: An Introduction
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Quantum Field Theory in Curved Spacetime and Black Hole Thermodynamics (Chicago Lectures in Physics)
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Hawking on the Big Bang and Black Holes (Advanced Series in Astrophysics and Cosmology, Vol 8)
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