Three Hundred Years of Gravitation, Black Holes and Time Warps: Einstein's Outrageous Legacy
A black hole in space has a gravitational force so strong that nothing can escape from it, not even light. Hence no one can see a black hole, but astronomers are convinced they exist. An early intellectual precursor to the concept of black holes may be found in John Michell's dark stars. Michell noted in 1783 that if bodies with densities not less than the Sun's, and hundreds of times greater in diameter, really existed, gravity would prevent their light particles from reaching us. Their existence might be inferred from the motions of luminous bodies around them. Michell's idea was ignored even before wave theory, in which gravity does not act on light, overthrew the particle theory from which he reasoned.
Collapsed stars make another intellectual precursor to black holes. After 1915 and Einstein's relativity theory, gravity again could act on light. In the 1930s, Subrahmanyan Chandrasekhar, recently arrived from India to study in England, modeled stellar structures. He found that stars of less than 1.4 solar mass shrink until they become white dwarfs, but more massive stars continue contracting. The British astrophysicist Arthur Eddington noted that at high compression, gravity would be so great that radiation could not escape, a situation he regarded as absurd. Others accepted Chandrasekhar's mathematics but believed that continuous or catastrophic mass ejection would act as a universal regulating mechanism to bring stars below the critical mass. Also, massive stars might evolve into stars composed of neutrons. In 1939, however, the American physicist J. Robert Oppenheimer established a mass limit for neutron stars (see Kurchatov, Igor Vasilievich, and J. Robert Oppenheimer). When it has exhausted thermonuclear sources of energy, a sufficiently heavy star will collapse indefinitely, unless it can reduce its mass.
World War II and then the hydrogen bomb project diverted research away from stellar structure. Nuclear weapons programs, though, did develop a deeper understanding of physics and more powerful computational techniques, and in the 1960s, computer programs that simulated bomb explosions were modified to simulate implosions of stars. A renewed theoretical assault followed on “black holes,” as they were named by the American nuclear physicist John Wheeler in 1967. In contrast to collapsed or frozen stars, black holes are now known to be dynamic, evolving, energy-storing, and energy-releasing objects.
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Because no light escapes from black holes, detection of them requires observing manifestations of their gravitational attraction. From a companion star, a black hole captures and heats gas to millions of degrees, hot enough to emit X rays. Because the Earth's atmosphere absorbs X rays, devices to detect them must be lofted on rockets or satellites. A few black holes probably have been found, although other explanations of the observational data are possible.
In another predicted manifestation, two black holes spiral together, gyrate wildly while coalescing, and then become steady. Outward ripples of curvature of spacetime, also called gravitational waves, would carry an unequivocal black-hole signature. Gravitational waves should propagate through matter, diminishing in intensity with distance. On Earth, they should create tides the size of an atom's nucleus, in contrast to lunar tides of about a meter. Gravitational-wave detectors may be operational early in the twenty-first century.
Meanwhile, without benefit of prediction and intent, we may already have observed manifestations of black holes. Extraordinarily strong radio emissions from both the centers of galaxies and from quasars (compact, highly luminous objects) may be powered by the rotational energy of gigantic black holes, either coalesced from many stars or from the implosion of a single supermassive rotating star a hundred million times heavier than our sun. Other possible explanations for radio galaxies and quasars, however, do not require black holes.