John Michell (1783) and Pierre-Simon Laplace (1796) were the first people to propose the concept of “dark stars” or object which, if compressed into a sufficiently small size, would have an escape velocity which exceeded even the speed of light. Later, the term “frozen star” was used to describe the last phase of a star’s gravitational collapse, when light unable to escape from its surface would make the star appear frozen in time to an observer. In the 20th century, John Wheeler eventually coined the phrase “black hole” as the object would absorbs all the light that hits it while reflecting nothing back.
It is important to realize that a black hole’s gravitational field is the same as that of any other object in space of the same mass. In other words, it won’t “suck” objects in any more than any other normal star. If our Sun were replaced with a black hole of equal mass, for example, the Earth would continue experiencing the same gravitational force as before. Only when objects get too close to the black hole’s event horizon, would the stronger gravitational force become apparent (simply because at any given radius, the black hole has significantly larger mass than any other object).
According to Einstein’s General Theory of Relativity, any massive object actually distorts the space-time around it, including our Sun, Earth, or even us. A black hole is an extreme case in the sense that at its singularity the curvature of space-time becomes infinite, preventing even light to escape. The boundary beyond which light cannot escape the black hole’s gravity well is known as the event horizon, while its radius is called the Schwarzschild radius. Once particles and light-rays go past the event horizon their light cones “tip over” and point to the singularity, which now represents all future-directed paths with no escape possible.
A black hole is formed when a massive star starts running out of nuclear fuel at its interior (mainly hydrogen and helium) and begins to collapse under its own gravity. Such a star may become a white dwarf or a neutron star, but if the star is sufficiently massive then it may continue shrinking eventually to the size of a tiny atom: this is the so-called “gravitational singularity”. A black hole refers to the region in space around the singularity in which the gravitational force is so strong that not even light can escape its pull.
Department of Astrophysics,
IMAPP, Radboud University,
P.O. Box 9010,
6500 GL Nijmegen, The Netherlands
Institute for Theoretical Physics (ITP),
Max-Planck-Institut für Radioastronomie (MPIfR),
Auf dem Hügel 69, D-53121