Wednesday, 8 July 2009

Black Holes

Black Holes:
The fact that light is bent by a gravitational field brings up the following thought experiment.
Imagine adding mass to a body. As the mass increases, so does the gravitational pull and objects
require more energy to reach escape velocity. When the mass is sufficiently high enough that the
velocity needed to escape is greater than the speed of light we say that a black hole has been

Another way of defining a black hole is that for a given mass, there is a radius where if all the mass
is compress within this radius the curvature of spacetime becomes infinite and the object is
surrounded by an event horizon. This radius called the Schwarzschild radius and varys with the
mass of the object (large mass objects have large Schwarzschild radii, small mass objects have
small Schwarzschild radii).

The Schwarzschild radius marks the point where the event horizon forms, below this radius no light
escapes. The visual image of a black hole is one of a dark spot in space with no radiation emitted.
Any radiation falling on the black hole is not reflected but rather absorbed, and starlight from
behind the black hole is lensed.

Even though a black hole is invisible, it has properties and structure. The boundary surrounding the
black hole at the Schwarzschild radius is called the event horizon, events below this limit are not
observed. Since the forces of matter can not overcome the force of gravity, all the mass of a black
hole compresses to infinity at the very center, called the singularity.

A black hole can come in any size. Stellar mass black holes are thought to form from supernova
events, and have radii of 5 km. Galactic black hole in the cores of some galaxies, millions of solar
masses and the radius of a solar system, are built up over time by cannibalizing stars. Mini black
holes formed in the early Universe (due to tremendous pressures) down to masses of asteroids with
radii the size of a grain of sand.

Note that a black hole is the ultimate entropy sink since all information or objects that enter a black
hole never return. If an observer entered a black hole to look for the missing information, he/she
would be unable to communicate their findings outside the event horizon.

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