Black hole


May 17, 2022

A black hole is an object whose mass is concentrated in an extremely small volume and, as a result of this compactness, generates such strong gravity in its immediate vicinity that even light cannot leave or pass through this area. The outer limit of this area is called the event horizon. Nothing can cross an event horizon from the inside out - no information, no radiation and certainly no matter. The general theory of relativity conclusively describes the fact that a “way to the outside” is no longer even conceivable by means of an extreme curvature of space-time. There are different classes of black holes with their respective formation mechanisms. The easiest to understand are stellar black holes, which are formed when a star of a certain size has used up all of its nuclear fuel and collapses. While the outer shells are then expelled in a supernova, the core collapses into an extremely compact body due to its gravitational pressure. For a hypothetical black hole the mass of the Sun, the event horizon would be only about six kilometers in diameter, which is 230,000 times the current diameter of the Sun. At the other end of the spectrum, there are supermassive black holes, millions to billions of times the mass of our Sun, that sit at the core of galaxies and play important roles in their evolution. Outside the event horizon, a black hole behaves like a normal mass body and can be orbited by other celestial bodies on stable orbits. From the outside, the event horizon appears visually as a completely black and opaque object, near which the space behind it is imaged as if distorted by an optical lens (gravitational lens). However, the black hole is often covered by gas clouds, so that it is only visible at certain wavelengths (radio waves), and because of the distortion one does not "see" the event horizon but a so-called shadow. The term black hole was coined by John Archibald Wheeler in 1967. At that time, the existence of the black holes, which had only been described theoretically, was considered very probable, but had not yet been confirmed by observations. Later, numerous examples of black hole effects were observed, e.g. B. from 1992 the investigations of the supermassive black hole Sagittarius A* in the center of the Milky Way in the infrared range. In 2016, the merger of two black holes was observed by LIGO via the gravitational waves generated and in 2019 a radiotelescopic image of the supermassive black hole M87* at the center of the galaxy M87 was obtained with the Event Horizon Telescope. In 2022, the Event Horizon Telescope was also able to image the black hole Sagittarius A* in the center of the Milky Way. In 2020, the scientists Roger Penrose, Reinhard Genzel and Andrea Ghez were awarded the Nobel Prize in Physics for their research on black holes.

Research History

18th Century

As early as 1783, the British naturalist John Michell speculated about dark stars with sufficient gravity to trap light. In a letter published by the Royal Society he wrote: The idea of ​​heavy stars from which corpuscular light cannot escape was also described by Pierre Simon Laplace in his 1796 Exposition du Système du Monde. He coined the term “dark body” (corps obscur) for this. These ideas moved within Newtonian physics.

First half of the 20th century: Contribution of the general theory of relativity

After Albert Einstein established the field equations of the general theory of relativity in 1915, the German astronomer Karl Schwarzschil gave