The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is a physical parameter that shows up in the Schwarzschild solution to Einstein's field equations, corresponding to the radius defining the event horizon of a Schwarzschild black hole. 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.
The Schwarzschild radius is given as
where G is the gravitational constant, M is the object mass, and c is the speed of light.^{[1]}
In 1916, Karl Schwarzschild obtained the exact solution^{[2]}^{[3]} to Einstein's field equations for the gravitational field outside a non-rotating, spherically symmetric body with mass (see Schwarzschild metric). The solution contained terms of the form and , which become singular at and respectively. The has come to be known as the Schwarzschild radius. The physical significance of these singularities was debated for decades. It was found that the one at is a coordinate singularity, meaning that it can be removed by a change of coordinates, while the one at is physical, and cannot be removed.^{[4]} The Schwarzschild radius is nonetheless a physically relevant quantity, as noted above and below.
This expression had previously been calculated, using Newtonian mechanics, as the radius of a spherically symmetric body at which the escape velocity was equal to the speed of light. It had been identified in the 18th century by John Michell^{[5]} and by 19th century astronomers such as Pierre-Simon Laplace.^{[6]}
The Schwarzschild radius of an object is proportional to the mass. Accordingly, the Sun has a Schwarzschild radius of approximately 3.0 km (1.9 mi), whereas Earth's is only about 9 mm (0.35 in) and the Moon's is about 0.1 mm (0.0039 in). The observable universe's mass has a Schwarzschild radius of approximately 13.7 billion light-years.^{[7]}^{[8]}
Object | Mass: | Schwarzschild radius: | Schwarzschild density: or |
---|---|---|---|
Observable universe^{[7]} | 8.8×10^{52} kg | 1.3×10^{26} m (13.7 billion ly) | 9.5×10^{-27} kg/m^{3} |
Milky Way | 1.6×10^{42} kg | 2.4×10^{15} m (~0.25 ly) | 0.000029 kg/m^{3} |
SMBH in NGC 4889 | 4.2×10^{40} kg | 6.2×10^{13} m | 0.042 kg/m^{3} |
SMBH in Messier 87^{[9]} | 1.3×10^{40} kg | 1.9×10^{13} m | 0.44 kg/m^{3} |
SMBH in Andromeda Galaxy^{[10]} | 3.4×10^{38} kg | 5.0×10^{11} m | 640 kg/m^{3} |
Sagittarius A* (SMBH) | 8.2×10^{36} kg | 1.2×10^{10} m | 1.1×10^{6} kg/m^{3} |
Sun | 1.99×10^{30} kg | 2.95×10^{3} m | 1.84×10^{19} kg/m^{3} |
Jupiter | 1.90×10^{27} kg | 2.82 meters | 2.02×10^{25} kg/m^{3} |
Earth | 5.97×10^{24} kg | 8.87×10^{-3} m | 2.04×10^{30} kg/m^{3} |
Moon | 7.35×10^{22} kg | 1.09×10^{-4} m | 1.35×10^{34} kg/m^{3} |
Human | 70 kilograms | 1.04×10^{-25} m | 1.49×10^{76} kg/m^{3} |
Big Mac | 0.215 kilograms | 3.19×10^{-28} m | 1.58×10^{81} kg/m^{3} |
Planck mass | 2.18×10^{-8} kg | 3.23×10^{-35} m | 1.54×10^{95} kg/m^{3} |
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. As the radius is linearly related to mass, while the enclosed volume corresponds to the third power of the radius, small black holes are therefore much more dense than large ones. The volume enclosed in the event horizon of the most massive black holes has an average density lower than main sequence stars.
A supermassive black hole (SMBH) is the largest type of black hole, though there are few official criteria on how such an object is considered so, on the order of hundreds of thousands to billions of solar masses. (Supermassive black holes up to 21 billion (2.1 × 10^{10}) M_{☉} have been detected, such as NGC 4889.)^{[11]} Unlike stellar mass black holes, supermassive black holes have comparatively low average densities. (Note that a black hole is a spherical region in space that surrounds the singularity at its center; it is not the singularity itself.) With that in mind, 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.^{[12]} In contrast, the physical radius of the body is proportional to the cube root of its volume. Therefore, as the body accumulates matter at a given fixed density (in this example, 997 kg/m^{3}, 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 solar masses (1.36 × 10^{8}) M_{☉}, its physical radius would be overtaken by 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.^{[]}
The Schwarzschild radius of the supermassive black hole at the Galactic Center is approximately 12 million kilometres.^{[13]}
Stellar black holes have much greater average densities than supermassive black holes. If one accumulates matter at nuclear density (the density of the nucleus of an atom, about 10^{18}kg/m^{3}; neutron stars also reach this density), such an accumulation would fall within its own Schwarzschild radius at about 3 M_{☉} and thus would be a stellar black hole.
A small mass has an extremely small Schwarzschild radius. A mass similar to Mount Everest^{[14]}^{[note 1]} has a Schwarzschild radius much smaller than a nanometre.^{[note 2]} 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.
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:
where:
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.
The Newtonian gravitational field near a large, slowly rotating, nearly spherical body can be reasonably approximated using the Schwarzschild radius as follows:
and
Therefore, on dividing above by below:
where:
On the surface of the Earth:
For all circular orbits around a given central body:
Therefore,
but
Therefore,
where:
This equality can be generalized to elliptic orbits as follows:
where:
For the Earth, as a planet orbiting the Sun:
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:
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.
For the Planck mass , the Schwarzschild radius and the Compton wavelength are of the same order as the Planck length .
Classification of black holes by type:
A classification of black holes by mass:
If Mount Everest is assumed* to be a cone of height 8850 m and radius 5000 m, then its volume can be calculated using the following equation:
volume = ?r^{2}h/3 [...] Mount Everest is composed of granite, which has a density of 2750 kg m^{-3}.