Theory on Black Holes

In 1973, Hawking left Gonville and Caius College and joined the department of applied mathematics and theoretical physics, known as DAMPT, at Cambridge University. In this same year, Hawking published The Large Scale Structure of Spacetime, which was a complex collaboration between him and associate George Ellis dealing with classical cosmology. By now, Hawking relied upon a wheelchair, though this did not hinder his career. His mind was the most important tool he needed. For everything else there was help, be it in the form of a wheelchair or, as the future would hold, a computerized speech device.

Another major contribution made by Hawking was his work on black holes. (In 1969, American theoretical physicist John Wheeler had first coined the term black hole.) The knowledge of the existence

BLACK HOLE CURVATURE

BLACK HOLE CURVATURE

Hawking theorized that in an expanding universe, black holes could occur in different sizes. The drawing on the left is of a small black hole with a sudden curvature. The drawing on the right is of a large black hole with a gradual curvature.

of a "dark star"—a region of space thought to be so grossly warped by gravitational forces that nothing could escape from it, not even light—had been around for about two centuries. In 1796, the French scientist Pierre-Simon Laplace (1749-1827), working from Newton's theory of gravitation, theorized that since light consisted of particles it must therefore be affected by gravity. Laplace put forward that some stars may be too massive to radiate and that the largest bodies in the universe may be invisible.

The boundary of a black hole from which nothing can escape is known as the event horizon. For years, the conjecture has been that all light and matter that falls past the event horizon and into the black hole is lost forever. During the early 1970s, in a wave of revolutionary research, Hawking showed that that was not the case and that black holes in fact emit radiation. He also theorized that since they were emitting particles, they were also losing mass and would therefore eventually cease to exist. He came to this theory by combining three very different areas of physics: that of general relativity—which describes gravity and the very large—that of quan-

In 1978, the first object seen by High Energy Astronomy Observatory (HEAO)-2, an X-ray space telescope also known as the Einstein Observatory, was of a possible black hole in the constellation Cygnus. Named Cygnus X-1, this is the first black hole ever discovered. The color in this image is reversed for clarity. (Photo courtesy of NASA-MSFC)

In 1978, the first object seen by High Energy Astronomy Observatory (HEAO)-2, an X-ray space telescope also known as the Einstein Observatory, was of a possible black hole in the constellation Cygnus. Named Cygnus X-1, this is the first black hole ever discovered. The color in this image is reversed for clarity. (Photo courtesy of NASA-MSFC)

tum mechanics—which describes particles and the very small—and that of thermodynamics—heat theory—to form a single theory. Hawking's findings provided the first positive steps toward suggesting a unity in the separate branches of physics, something scientists had been trying to do for the past 50 years. Hawking also put forth that because of the big bang theory, black holes could have an alternative beginning (other than stellar collapse) and that they could be of varying sizes. Before Hawking, the common thinking was that the creation of a huge black hole occurred when a dead star too heavy to be a neutron star collapsed under the force of its own gravity. Hawking suggested that the big bang expansion of the universe could squeeze pockets of matter into "primordial black holes" that could have a mass as little as roughly 10-5 gram.

At first, as with many scientific breakthroughs, Hawking's discovery concerning the properties of black holes was not broadly accepted, and it caused widespread controversy throughout the scientific community. Eventually, the controversy grew into acceptance. The radiation emitted by black holes is now known as Hawking radiation. Originally identified in 1965 as an X-ray source, Cygnus X-1 was identified in 1978 by the Einstein Observatory as the first possible candidate for being a black hole.

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