BLACK HOLES ONE: A Brief History

A black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. Any object close enough will eventually be pulled into the black hole, never to escape – the gravity is so strong that not even light can escape its pull.

The idea of a body so massive that even light could not escape dates back to 1784. Geologist John Mitchell in a paper published by the Royal Society, computed that a body with 500 times the radius of the Sun and of the same density would have an escape velocity at its surface that would exceed the speed of light, and would be invisible. Quote:

“If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae (inertial mass), with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.”

In 1915, Albert Einstein developed the theory of gravity called general relativity, having shown earlier that gravity does influence light. Even though his theory predicts the existence of such objects, Einstein didn’t like the singularity predicted at the center of a black hole. In 1935 he and Rosen found another solution to the equations for black holes. The solution comes in the form of what is called an Einstein-Rosen bridge, or wormhole that connects two different regions of space-time. Most scientists today believe its highly unlikely such wormholes exist, and if they did their life-spans would be extremely short due to their highly unstable nature – an object travelling through it would destabilize and collapse the wormhole.

In general relativity, gravity is a manifestation of the curvature of space-time. Massive objects distort the space and time (space-time) around them so the usual rules of geometry don’t apply anymore. Because of the mass, the distortion of space around a black hole is extremely severe, as a result black holes have some very strange properties. One of those properties is something called an event horizon. This spherical surface marks the boundary of a black hole. The fundamental feature of the event horizon is that it allows an object to pass in through it, but the object cannot ever escape or get back out. Even light cannot escape from the inside of the event horizon.