Black Hole

A Black Hole is a region in space into which matter has collapsed and from which no material object, light or signal of any kind can escape. In other words, within this spatial region, the gravitational field is too powerful to let anything escape.

The concept of Black Holes date back to the suggestion made in 1783 by John Michell and in 1798 by Pierre Laplace that there might exist in the universe massive bodies which would be quite invisible because the force of gravity at their surfaces would be too great to allow light to escape. At the surface of such a body the escape velocity would be greater than the speed of light.

Black holes were probably formed as a result of the collapse of the most massive stars as they run out of fuel at the end of their life cycles. Most stars collapse to form dense compact objects, but the most massive stars will continue to collapse until, in principle, all their matter is compressed into a point of infinite density known as a singularity. Before this happens, the shrinking star will have passed inside its Schwarzschild radius, giving rise to a black hole, and disappearing forever from view.

A black hole would arise if a quantity of matter were compressed inside its Schwarzschild radius, and the boundary of the black hole (known as the event horizon) would be a sphere having this radius. Nothing that falls inside can ever escape again. It is important to note that a black hole is not a solid body. It is a region of space whose radius depends only on the amount of matter that has fallen inside. What happens to that matter thereafter has no bearing on the hole’s size, and any matter which subsequently falls in only serves to increase the radius of the hole.

The Schwarzschild radius is normally very small. For the Sun, it is only 3 km, and if, in some way, the Earth could be compressed within its Schwarzschild radius, the resulting black hole would be only the size of a marble.

The most hopeful way in which black holes may be detected is by looking for their effects on matter in their vicinity. For example, if a black hole is part of a binary system, then the visible star will be seen to follow an orbit round an invisible object. Again, if material is falling into a black hole, it will become excessively heated and will emit x-rays.

Black holes are important energy sources, as matter falling in towards a black hole, before crossing the event horizon, may release considerable amounts of gravitational potential energy. A rotating black hole is even more efficient in this respect. Not surprisingly, black holes have been canvassed as possible ‘explanations’ of hitherto unexplained energy sources in the universe.

Questions to Ponder

• What is the density of a black hole?
• Could the Universe's dark matter be made up of black holes?
• If light has no mass, then what draws it into a black hole?