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Schwarzschild radius is to do with gravitational collapse.
The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is a characteristic radius associated with every mass. It is the radius for a given mass where, if that mass could be compressed to fit within that radius, no known force or degeneracy pressure could stop it from continuing to collapse into a gravitational singularity. The term is used in physics and astronomy, especially in the theory of gravitation, and general relativity.
In 1916, Karl Schwarzschild obtained an exact solution to Einstein's field equations for the gravitational field outside a non-rotating, spherically symmetric body (see Schwarzschild metric). The solution contained a term of the form 1 / (2M - r); the value of r making this term singular has come to be known as the Schwarzschild radius. The physical significance of this singularity, and whether this singularity could ever occur in nature, was debated for many decades; a general acceptance of the possibility of a black hole did not occur until the second half of the 20th century.
An object smaller than its Schwarzschild radius is called a black hole. The surface at the Schwarzschild radius acts as an event horizon in a non-rotating body. (A rotating black hole operates slightly differently.) Neither light nor particles can escape through this surface from the region inside, hence the name "black hole". The Schwarzschild radius of the (currently hypothesized) Supermassive black hole at our Galactic Center would be approximately 7.8 million km.
History of Schwarzschild radius.
The significance of the singularity at r = 2M (in natural units) was first raised by Jacques Hadamard, who, during a conference in Paris in 1922, asked what might happen if a physical system could ever obtain this singularity. Albert Einstein insisted that it could not, pointing out the dire consequences for the universe, and jokingly referred to the singularity as the "Hadamard disaster".
Schwarzschild's original model of a star assumed an incompressible fluid; Einstein pointed out that this was an unreasonable assumption, as sound waves would propagate at infinite speed. In his own work, Einstein reconsidered a model of a star where the components of the star were orbiting masses, and showed that the orbital velocities would exceed the speed of light at the Schwarzschild radius. In 1939, he used this to argue that no such thing can happen, and so the singularity could not occur in nature. The same year, Robert Oppenheimer and Hartland Snyder considered a model of a dust cloud, where the dust particles of the cloud were moving radially, towards a single point, and showed that the dust particles could reach the singularity in finite proper time. After passing the limit, Oppenheimer and Snyder noted that light cones were directed inwards, and that no signal could escape outside
Formula for the Schwarzschild radius.
The Schwarzschild radius is proportional to the mass with a proportionality constant involving the gravitational constant and the speed of light:
The proportionality constant,2G / c2, is approximately 1.48×10-27 m/kg, or 2.95 km/solar mass.
The formula for the Schwarzschild radius can be found by setting the escape velocity to the speed of light. Note that although the result is correct, general relativity must be used to properly derive the Schwarzschild radius. It is only a coincidence that Newtonian physics produces the same result.
An object of any density can be large enough to fall within its own Schwarzschild radius,
Classification by Schwarzschild radius: Supermassive black hole.
If one accumulates matter at normal density (1000 kg/m³, for example, the density of water) up to about 150,000,000 times the mass of the Sun, such an accumulation will fall inside its own Schwarzschild radius and thus it would be a supermassive black hole of 150,000,000 solar masses. (Supermassive black holes up to 18 billion solar masses have been observed.) The supermassive black hole in the center of our galaxy (3.7 million solar masses) constitutes observationally the most convincing evidence for the existence of black holes in general. It is thought that large black holes like these don't form directly in one collapse of a cluster of stars. Instead they may start as a stellar-sized black hole and grow larger by the accretion of matter and other black holes. The larger the mass of a galaxy, the larger is the mass of the supermassive black hole in its center.
Schwarzschild radius: Stellar black hole.
If one accumulates matter at nuclear density (the density of the nucleus of an atom, about 1018 kg/m³; neutron stars also reach this density), such an accumulation would fall within its own Schwarzschild radius at about 3 solar masses and thus would be a stellar black hole.
Schwarzschild radius: Primordial black hole.
Conversely, a small mass has an extremely small Schwarzschild radius. A mass similar to Mount Everest has a Schwarzschild radius smaller than a nanometre. Its average density at that size would be so high that no known mechanism could form such extremely compact objects. Such black holes might possibly be formed in an early stage of the evolution of the universe, just after the Big Bang, when densities were extremely high. Therefore these hypothetical baby black holes are called primordial black holes.
Other uses for the Schwarzschild radius.
The Schwarzschild radius in gravitational time dilation
Gravitational time dilation near a large, slowly rotating, nearly spherical body, such as the earth or sun can be reasonably approximated using the Schwarzschild radius as follows:
The Schwarzschild radius in Newtonian gravitational fields
The Newtonian gravitational field near a large, slowly rotating, nearly spherical body can be reasonably approximated using the Schwarzschild radius as follows:
The Schwarzschild radius in Keplerian orbits
For all circular orbits around a given central body:
Relativistic circular orbits and the photon sphere.
This final equation indicates that an object orbiting at the speed of light would have an orbital radius of 1.5 times the Schwarzschild radius. This is a special orbit known as the photon sphere.
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