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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Black hole | 8/13 | https://en.wikipedia.org/wiki/Black_hole | reference | science, encyclopedia | 2026-05-05T13:31:52.244381+00:00 | kb-cron |
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{\displaystyle r_{\mathrm {+} }={\frac {GM}{c^{2}}}.}
Since the volume within the Schwarzschild radius increases with the cube of the radius, average density of a black hole inside its Schwarzschild radius is inversely proportional to the square of its mass: supermassive black holes are much less dense than stellar black holes. The average density of a 108 M☉ black hole is comparable to that of water.
=== Event horizon ===
The defining feature of a black hole is the existence of an event horizon, a boundary in spacetime through which matter and light can pass only inward towards the center of the black hole. Nothing, not even light, can escape from inside the event horizon. The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach or affect an outside observer, making it impossible to determine whether such an event occurred. For non-rotating black holes, the geometry of the event horizon is precisely spherical, while for rotating black holes, the event horizon is oblate. To a distant observer, a clock near a black hole would appear to tick more slowly than one further from the black hole. This effect, known as gravitational time dilation, would also cause an object falling into a black hole to appear to slow as it approached the event horizon, never quite reaching the horizon from the perspective of an outside observer. All processes on this object would appear to slow down, and any light emitted by the object to appear redder and dimmer, an effect known as gravitational redshift. An object falling from half of a Schwarzschild radius above the event horizon would fade away until it could no longer be seen, disappearing from view within one hundredth of a second for an 8 M☉ black hole. It would also appear to flatten onto the black hole, joining all other material that had ever fallen into the hole. On the other hand, an observer falling into a black hole would not notice any of these effects as they cross the event horizon. Their own clocks appear to them to tick normally, and they cross the event horizon after a finite time without noting any singular behaviour. In general relativity, it is impossible to determine the location of the event horizon from local observations, due to Einstein's equivalence principle.
=== Internal geometry ===
==== Cauchy horizon ====
Black holes that are rotating and/or charged have an inner horizon, often called the Cauchy horizon, inside of the black hole. The inner horizon is divided up into two segments: an ingoing section and an outgoing section. At the ingoing section of the Cauchy horizon, radiation and matter that fall into the black hole would build up at the horizon, causing the curvature of spacetime to go to infinity. This would cause an observer falling in to experience tidal forces. This phenomenon is often called mass inflation, since it is associated with a parameter dictating the black hole's internal mass growing exponentially, and the buildup of tidal forces is called the mass-inflation singularity or Cauchy horizon singularity. Some physicists have argued that in realistic black holes, accretion and Hawking radiation would stop mass inflation from occurring. At the outgoing section of the inner horizon, infalling radiation would backscatter off of the black hole's spacetime curvature and travel outward, building up at the outgoing Cauchy horizon. This would cause an infalling observer to experience a gravitational shock wave and tidal forces as the spacetime curvature at the horizon grew to infinity. This buildup of tidal forces is called the shock singularity. Both of these singularities are weak, meaning that an object crossing them would only be deformed a finite amount by tidal forces, even though the spacetime curvature would still be infinite at the singularity. This is as opposed to a strong singularity, where an object hitting the singularity would be stretched and squeezed by an infinite amount. They are also null singularities, meaning that a photon could travel parallel to them without ever being intercepted.
==== Singularity ====
Ignoring quantum effects, every black hole has a singularity inside, points where the curvature of spacetime becomes infinite, and geodesics terminate within a finite proper time. For a non-rotating black hole, this region takes the shape of a single point; for a rotating black hole it is smeared out to form a ring singularity that lies in the plane of rotation. In both cases, the singular region has zero volume. All of the mass of the black hole ends up in the singularity. Since the singularity has nonzero mass in an infinitely small space, it can be thought of as having infinite density.
Observers falling into a Schwarzschild black hole (i.e., non-rotating and not charged) cannot avoid being carried into the singularity once they cross the event horizon. As they fall further into the black hole, they will be torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the noodle effect. Eventually, they will reach the singularity and be crushed into an infinitely small point. However, any perturbations, such as those caused by matter or radiation falling in, would cause space to oscillate chaotically near the singularity. Any matter falling in would experience intense tidal forces rapidly changing in direction, all while being compressed into an increasingly small volume. Alternative forms of general relativity, including addition of some quantum effects, can lead to regular, or nonsingular, black holes without singularities. For example, the fuzzball model, based on string theory, states that black holes are actually made up of quantum microstates and need not have a singularity or an event horizon. The theory of loop quantum gravity proposes that the curvature and density at the center of a black hole is large, but not infinite.