Black Hole
Article by: Jay M. Pasachoff, Hopkins Observatory, Williams College, Williamstown, Massachusetts.
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A region of spacetime exerting a gravitational field so
strong that neither matter nor radiation can escape. Black
holes are extreme cosmic objects predicted by German-born
U.S. physicist Albert Einstein's theory of general relativity. With-
in a boundary known as the event horizon, the escape velocity
needed to overcome the gravitational attraction of the black
hole would exceed the speed of light, meaning that noth-
ing that crosses over the event horizon can ever leave. Black
holes are therefore by definition invisible, but because of their
powerful gravitational fields, they can be indirectly observed
through the highly conspicuous effects they have on their cos-
mic environment. These effects include the gravitational intake
of matter through accretion disks, a process which generates
tremendous heat and light and is well-observed at scales from
binary star systems to the cores of galaxies. In the absence of
ongoing accretion, black holes should also theoretically cause
severe localized warping of spacetime, gravitationally lensing
light from luminous sources and distorting their appearance
(Fig. 1). The merging of two black holes each of about 30 times
the Sun's mass, detected in 2015 with the Laser Interferometer
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Content
• Black hole classes
• Stellar black holes
• Supermassive black holes
• Observation
• Black holes and gravitational
waves
• Fate of black holes
Key Concepts
• A black hole is a region of spacetime exerting a gravitational field so strong that
neither matter nor radiation can escape.
• Within a boundary known as the event horizon, the escape velocity needed to
overcome the gravitational attraction of a black hole would exceed the speed of light,
meaning that nothing that crosses over the event horizon can ever leave.
• Most black holes fall into two general classes, depending on their mass: stellar black
holes and supermassive black holes; intermediate-mass black holes have been found
in recent years and mini-black holes are theoretically possible.
• Observations of black holes rely on indirect methods using the gravitational
interaction of the black hole with its surroundings.
• At the center of a black hole, a finite mass can theoretically be compressed into a point
of zero volume, creating an infinitely dense state of matter known as a singularity.
Fig. 1 An artist's impression of the gravitational lensing caused by a black hole's warping of
localized spacetime.
(Credit: Ute Kraus, Physics education group Kraus, Universität Hildesheim)
