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Union University Department of Physics

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Science Guys > August 2001

August 2001

Where does the matter go when it is pulled into a black hole?

Isacc Newton determined in the late 1600’s that the force of gravity was due to matter or more specifically mass. The force of gravity is created by mass and gravity always attracts other mass. Newton’s theories first appeared in his famous book, "The Principia." In the 1700’s scientists speculated on the existence of massive objects with tremendous gravitational forces which today we call black holes. However it was not until Einstein published his theory of general relativity in 1915 that the black hole theory really received wide attention.

Relativity showed that gravity is related to the curvature of space and that a black hole is a place where the curvature becomes so extreme that a "hole" forms. Imagine a waterbed with a flat surface. Now place a bowling ball on it, the surface is distorted or bent. With a lot of mass, say a lead bowling ball, a "hole" forms on the surface of the bed. Similarly, a mass distorts or bends space. To form a black hole, a lot of matter is needed and must be concentrated in a small amount of space.

With sufficient mass, gravitational attraction within the matter itself overcomes all other forces and matter begins to collapse. The matter continues to collapse to a point that is known as a singularity. This point has infinite mass and density and is infinitely small. The effect of this point on space-time is to distort it so that nothing can escape from the immediate region, not even light. Since no light can escape we say the region is black hence the name black hole. Near singularities the known laws of physics break down. For this reason considerable time and effort is being spent studying this strange phenomena.

In 1916, German astrophysicist Karl Schwarzschild derived an equation for the radius of a black hole. This Schwarzschild radius, also called the event horizon, is proportional to the mass of the black hole, M, and may be written as Rs = (1.48 x 10-27) times M. This radius is the point or distance from the singularity at which light can still escape from the region. At a distance less than the event horizon radius, everything disappears. Although the matter at the center is really a singularity, we say it is a black hole the size of the event horizon distance.

When matter falls into or comes closer than the event horizon of a black hole, it becomes isolated from the rest of space-time. It can never leave that region. For all practical purposes the matter has disappeared from the universe. Once inside the black hole’s event horizon, matter will be torn apart into its smallest subatomic components and eventually be squeezed into the singularity. As the singularity accumulates more and more matter, the size of the black hole’s event horizon increases proportionally.

While everyone thought matter in a black hole was gone forever, Stephen Hawking in 1974 predicted that black holes could radiate energy away (Hawking radiation). Thus a black hole may give up some of what it had absorbed, but it does so very, very slowly. Theoretically then we say a black hole may evaporate over time by the emission of energy, but the time it would take for even a small black hole to evaporate is beyond our comprehension, on the order of a one with 67 zeros of years. Considerable work is needed to understand these strange phenomena, but for now that is where our understanding of physics has taken us.