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By — McGraw-Hill Professional
Updated on Sep 17, 2011

Introduction to Black Holes

Some neutron stars, as they collapse under their own weight, apparently do not stop even when all the space has been removed from between the subatomic particles. If the mass of such a star is great enough, calculations show that gravitation will become so powerful that no other force in the Cosmos can overcome it, not even the forces within neutrons and other particles.

Basic Anatomy

When a neutron star gets going on the runaway frenzy of gravitational collapse that nothing can stop, the object will, in theory, continue to shrink until it becomes a geometric point that contains all the mass of the neutron star from which it formed but zero volume. There is an overwhelming gravitational field at its “surface” and a slowing down of time (because of relativistic effects, to be dealt with in a later chapter) to a complete stop relative to the outside Universe. This object is called a space-time singularity .

Surrounding the singularity is a spherical zone within which nothing can escape, not even visible light or other EM radiation. It is a “zone of no return” because the escape velocity is greater than the speed of light in free space. The outer boundary of this zone is called the event horizon (Fig. 14-6). To an outside observer, the object would appear as a black sphere having the radius of the event horizon. The edge of the sphere would glow faintly because of starlight that has been almost, but not quite, captured and pulled in. The background of stars near such a black hole would appear distorted because of space warping caused by gravitation.

Extreme Objects in Our Galaxy Matter And
Antimatter Basic Anatomy

Figure 14-6. Simplified drawing of a black hole. This illustration does not take relativistic spatial distortion into account.

Gravity’s Ultimate Victory

From a simplistic standpoint, gravitation can become so powerful that it will not let anything escape, not even the energy packets called photons that represent all forms of EM radiation. As the gravitational field at the surface of a collapsing neutron star becomes increasingly powerful, EM rays are bent downward significantly as they leave the surface. At a certain point, the rays leaving in an almost horizontal direction fall back. The star continues to collapse, and the gravitational field becomes more powerful still; rays fall back at ever-increasing angles. When the radius of the object gets so small that the gravitational field at the surface reaches critical intensity, only those photons traveling straight up from the surface manage to get away. However, things don’t stop there. The collapsar keeps shrinking within the event horizon; then all photons are trapped. No known form of energy can propagate faster than photons, and so the event horizon represents a one-way membrane : Things can get in, but nothing can get out.

This idea is not new. As long as there has been a particle theory of light, imaginative scientists have theorized that black holes can exist. When Albert Einstein revolutionized physics with his theory of relativity in the early 1900s, new evidence arose for the existence of black holes. However, nobody had ever seen an object in space that fit the description. The nature of black holes, assuming that they exist, is such that they are invisible at all wavelengths. The fantastic nature of black holes, along with the apparent fact that they can never be observed directly, originally caused some scientists to scoff. How could we say, for example, that angels were not dancing on the surfaces of neutron stars that had collapsed to within their event horizons? After all, no one could take a look and disprove such an idea! Recently, however, most astronomers have come to believe that black holes are not only plausible but real. They’re out there.

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