There are many popular myths concerning black holes, many of them perpetuated by Hollywood. Television and movies have portrayed them as time-traveling tunnels to another dimension, cosmic vacuum cleaners sucking up everything in sight, and so on. It can be said that black holes are really just the evolutionary end point of massive stars. But somehow, this simple explanation makes them no easier to understand or less mysterious.
Black Holes: What Are They?
Black holes are the evolutionary ends of stars at least 10 to 15 times as massive as our Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned out stellar remnant.
With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a "singularity ".
As the density increases, the path of light rays emitted from the star are bent and eventually wrapped irrevocably around the star, as shown in the header image. Any emitted photons are trapped into an orbit by the intense gravitational field; they will never leave it. Because no light escapes after the star reaches this infinite density, it is called a black hole.
But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, the earth's orbit around the Sun would be unchanged. (Of course the Earth's temperature would change, and there would be no solar wind or solar magnetic storms affecting us.) To be "sucked" into a black hole, one has to cross inside the Schwarzschild radius. At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape.
The idea of an object with gravity strong enough to prevent light from escaping was proposed in 1783 by John Michell, an amateur British astronomer.
In the year that Galileo died -- 1642, Sir Isaac Newton was born on Christmas Day. While not directly speculating on the existence of Black Holes, his work in Mathematics established the basis for which 18th, 19th and early 20th century astronomers would base their work. Can you imagine a work that stood for almost 300 years before being modified (by Einstein)? Newton struggled mightily to reconcile his development of higher mathematics -- he is credited with inventing Calculus, with his sense of Theology. In 1684, three members of the Royal Society, Sir Christopher Wren, Robert Hooke and Edmond Halley, argued as to whether the elliptical orbits of the planets could result from a gravitational force towards the sun proportional to the inverse square of the distance. Halley writes: "Mr. Hook said he had had it, but that he would conceal it for some time so that others, triing and failing might know how to value it, when he should make it publick".
Halley went up to Cambridge, and put the problem to Newton, who said he had solved it four years earlier, but couldn’t find the proof among his papers. Three months later, he sent an improved version of the proof to Halley, and devoted himself full time to developing these ideas, culminating in the publication of the Principia in 1686. This was the book that really did change man’s view of the universe.
In 1795, Pierre-Simon Laplace, a French physicist independently came to the same conclusion.
However, such "Newtonian black holes" are very different from black holes in general relativity.
They prevent only light from escaping (not, for example, a rocket ship) and only in certain Newtonian models of light (such as an emission theory).
Black holes, as currently understood, are described by the general theory of relativity. This theory predicts that when a large enough amount of mass is present in a sufficiently small region of space, all paths through space are warped inwards towards the center of the volume, preventing all matter and radiation within it from escaping.
So, black holes are really still just about proven, but by their nature, (being completely devoid of light), they cannot be directly imaged by our telescopes.