Template:Properties of mass The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is the radius of a sphere such that, if all the mass of an object were to be compressed within that sphere, the escape speed from the surface of the sphere would equal the speed of light. An example of an object where the mass is within its Schwarzschild radius is a black hole. Once a stellar remnant collapses below this radius, light cannot escape and the object is no longer directly visible.[1] It is a characteristic radius associated with every quantity of mass. The Schwarzschild radius was named after the German astronomer Karl Schwarzschild who calculated this exact solution for the theory of general relativity in 1916.

## History

In 1916, Karl Schwarzschild obtained an exact solution[2][3] to Einstein's field equations for the gravitational field outside a non-rotating, spherically symmetric body (see Schwarzschild metric). Using the definition ${\displaystyle M={\frac {Gm}{c^{2}}}}$, the solution contained a term of the form ${\displaystyle {\frac {1}{2M-r}}}$; where the value of ${\displaystyle r}$ 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.

## Parameters

The Schwarzschild radius of an object is proportional to the mass. Accordingly, the Sun has a Schwarzschild radius of approximately Template:Convert while the Earth's is only about 9.0 mm, the size of a peanut. The observable universe's mass has a Schwarzschild radius of approximately 13.7 billion light years.[4][5]

${\displaystyle {\text{radius}}_{s}}$ (m) ${\displaystyle {\text{density}}_{s}}$ (g/cm3)
Milky Way 2.08Template:E (~0.2 ly) 3.72Template:E
Sun 2.95Template:E 1.84Template:E
Earth 8.87Template:E 2.04Template:E

## Formula

The Schwarzschild radius is proportional to the mass with a proportionality constant involving the gravitational constant and the speed of light:

${\displaystyle r_{\mathrm {s} }={\frac {2Gm}{c^{2}}},}$

where:

rs is the Schwarzschild radius;
Template:Var is the gravitational constant;
Template:Var is the mass of the object;
Template:Var is the speed of light in vacuum.

The proportionality constant, 2G/c2, is approximately Template:Val, or Template:Val.

An object of any density can be large enough to fall within its own Schwarzschild radius,

${\displaystyle V_{\mathrm {s} }\propto \rho ^{-3/2},}$

where:

${\displaystyle V_{\mathrm {s} }\!={\frac {4\pi }{3}}r_{\mathrm {s} }^{3}}$ is the volume of the object;
${\displaystyle \rho \!={\frac {m}{V_{\mathrm {s} }}}}$ is its density.

## Black hole classification by Schwarzschild radius

Any object whose radius is 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".

Black holes can be classified based on their Schwarzschild radius, or equivalently, by their density. Black holes with very high density have a very small Schwarzschild radius, while larger black holes can have much lower density.

### Supermassive black hole

A supermassive black hole (SMBH) is the largest type of black hole, on the order of hundreds of thousands to billions of solar masses. (Supermassive black holes up to 21 billion Template:Solar mass have been detected, such as NGC 4889.)[6] Unlike stellar mass black holes, supermassive black holes have low densities if you assume that the Schwarzschild radius is the outer edge of the black hole (note that this assumption is in contrast to the typical assumption that a black hole is a singularity, and therefore has zero radial extent). Under this assumption, the average density of a supermassive black hole can be less than the density of water.

The Schwarzschild radius of a body is proportional to its mass and therefore to its volume, assuming that the body has a constant mass-density.[7] In contrast, the physical radius of the body is proportional to the cube root of its volume. Therefore, as the body accumulates matter at normal density (in this example, 103 kg/m3, the density of water), its Schwarzschild radius will increase more quickly than its physical radius. When a body of this density has grown to around 136 million Template:Solar mass, its physical radius would be smaller than its Schwarzschild radius, and thus it would form a supermassive black hole.

It is thought that supermassive black holes like these do not form immediately from the singular collapse of a cluster of stars. Instead they may begin life as smaller, stellar-sized black holes and grow larger by the accretion of matter, or even of other black holes.Template:Cn

The Schwarzschild radius of the supermassive black hole at our Galactic Center would be approximately 13.3 million kilometres.[8]

### Stellar black hole

Stellar black holes have much greater densities than supermassive black holes. 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 Template:Solar mass and thus would be a stellar black hole.

### Primordial black hole

A small mass has an extremely small Schwarzschild radius. A mass similar to Mount Everest[9]Template:Refn has a Schwarzschild radius much smaller than a nanometre.Template:Refn 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 miniature black holes are called primordial black holes.

## Other uses

### 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:

${\displaystyle {\frac {t_{r}}{t}}={\sqrt {1-{\frac {r_{\mathrm {s} }}{r}}}}}$

where:

Template:Var is the elapsed time for an observer at radial coordinate "r" within the gravitational field;
Template:Var is the elapsed time for an observer distant from the massive object (and therefore outside of the gravitational field);
Template:Var is the radial coordinate of the observer (which is analogous to the classical distance from the center of the object);
rs is the Schwarzschild radius.

The results of the Pound–Rebka experiment in 1959 were found to be consistent with predictions made by general relativity. By measuring Earth’s gravitational time dilation, this experiment indirectly measured Earth’s 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:

${\displaystyle {\frac {g}{r_{\mathrm {s} }}}\left({\frac {r}{c}}\right)^{2}={\frac {1}{2}}}$

where:

Template:Var is the gravitational acceleration at radial coordinate "r";
rs is the Schwarzschild radius of the gravitating central body;
Template:Var is the radial coordinate;
Template:Var is the speed of light in vacuum.

On the surface of the Earth:

${\displaystyle {\frac {9.80665\ \mathrm {m} /\mathrm {s} ^{2}}{8.870056\ \mathrm {mm} }}\left({\frac {6375416\ \mathrm {m} }{299792458\ \mathrm {m} /\mathrm {s} }}\right)^{2}=\left(1105.59\ \mathrm {s} ^{-2}\right)\left(0.0212661\ \mathrm {s} \right)^{2}={\frac {1}{2}}.}$

### In Keplerian orbits

For all circular orbits around a given central body:

${\displaystyle {\frac {r}{r_{\mathrm {s} }}}\left({\frac {v}{c}}\right)^{2}={\frac {1}{2}}}$

where:

Template:Var is the orbit radius;
rs is the Schwarzschild radius of the gravitating central body;
Template:Var is the orbital speed;
Template:Var is the speed of light in vacuum.

This equality can be generalized to elliptic orbits as follows:

${\displaystyle {\frac {a}{r_{\mathrm {s} }}}\left({\frac {2\pi a}{cT}}\right)^{2}={\frac {1}{2}}}$

where:

Template:Var is the semi-major axis;
Template:Var is the orbital period.

For the Earth orbiting the Sun:

${\displaystyle {\frac {1\,\mathrm {AU} }{2953.25\,\mathrm {m} }}\left({\frac {2\pi \,\mathrm {AU} }{\mathrm {light\,year} }}\right)^{2}=\left(50655379.7\right)\left(9.8714403\times 10^{-9}\right)={\frac {1}{2}}.}$

### Relativistic circular orbits and the photon sphere

The Keplerian equation for circular orbits can be generalized to the relativistic equation for circular orbits by accounting for time dilation in the velocity term:

${\displaystyle {\frac {r}{r_{s}}}\left({\frac {v}{c}}{\sqrt {1-{\frac {r_{s}}{r}}}}\right)^{2}={\frac {1}{2}}}$
${\displaystyle {\frac {r}{r_{s}}}\left({\frac {v}{c}}\right)^{2}\left(1-{\frac {r_{s}}{r}}\right)={\frac {1}{2}}}$
${\displaystyle \left({\frac {v}{c}}\right)^{2}\left({\frac {r}{r_{s}}}-1\right)={\frac {1}{2}}.}$

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.

Classification of black holes by type:

A classification of black holes by mass:

## References

1. Chaisson, Eric, and S. McMillan. Astronomy Today. San Francisco, CA: Pearson / Addison Wesley, 2008. Print.
2. K. Schwarzschild, "Über das Gravitationsfeld eines Massenpunktes nach der Einsteinschen Theorie", Sitzungsberichte der Deutschen Akademie der Wissenschaften zu Berlin, Klasse fur Mathematik, Physik, und Technik (1916) pp 189.
3. K. Schwarzschild, "Über das Gravitationsfeld einer Kugel aus inkompressibler Flussigkeit nach der Einsteinschen Theorie", Sitzungsberichte der Deutschen Akademie der Wissenschaften zu Berlin, Klasse fur Mathematik, Physik, und Technik (1916) pp 424.
4. Template:Cite arXiv
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8. http://www.thetimes.co.uk/tto/news/world/article1967154.ece
9. Template:Cite web