The Collapsing Universe: The Story of Black Holes
Space, Time, and Gravity: The Theory of the Big Bang and Black Holes
The Key to the Universe: A Report on the New Physics
Space and Time in the Modern Universe
Ten Faces of the Universe
The Iron Sun: Crossing the Universe Through Black Holes
White Holes: Cosmic Gushers in the Universe
Black holes are hot. Although this is literally true (according to the latest theories) of some black holes, I mean they are hot as a topic. The above books are only fragments of this year’s crop that deal entirely or in part with black holes. Why such obsessive interest in astronomical objects that may not even exist, and that in any case cannot be fully understood without knowing general relativity theory and quantum mechanics?
Let the first paragraph of Isaac Asimov’s book set the tone for what I believe is the answer.
Since 1960 the universe has taken on a wholly new face. It has become more exciting, more mysterious, more violent, and more extreme as our knowledge concerning it has suddenly expanded. And the most exciting, most mysterious, most violent, and most extreme phenomenon of all has the simplest, plainest, calmest, and mildest name—nothing more than a “black hole.”
Black. Black is beautiful, black is ominous, black is awesome, black is apocalyptic, black is blank. “A hole is nothing,” Asimov continues, “and if it is black, we can’t even see it. Ought we to get excited over an invisible nothing?”
Nothing. Why does anything exist? Why not just nothing? This is the super-ultimate metaphysical question. Obviously no one can answer it, yet there are times (for some people) when the question can overwhelm the soul with such power and anguish as to induce nausea. Indeed, that is what Sartre’s great novel, Nausea, is all about.
Suddenly we are being told that if a star is sufficiently massive it eventually will undergo a runaway collapse that ends with the star’s matter crushed completely out of existence. Not only that, but our entire universe may slowly stop expanding, go into a contracting phase, and finally disappear into a black hole, like an acrobatic elephant jumping into its anus. There is speculation (not taken seriously by the experts) that every black hole is joined to a “white hole”—a hole that gushes energy instead of absorbing it. The two holes are supposedly connected by an “Einstein-Rosen bridge” or “wormhole.” When a huge sun collapses into a black hole, so goes the conjecture, a companion white hole instantly appears at some other spot in spacetime. This could explain the incredible outpouring of energy from the quasars, those mysterious objects, apparently far beyond our galaxy, that nobody yet understands. Was the big bang which created our universe the white hole that exploded into existence after a previous universe collapsed into its black hole?
It is easy to understand why the religiously inclined are excited by such wild, speculative cosmology. The heavens declare the glory of God and the firmament showeth his handiwork. Nor is it hard to understand why those who are into Eastern philosophy, pseudoeastern cults, parapsychology, and unorthodox science are also fascinated. If the universe can be that crazy, so goes the argument, then why be disturbed when the Maharishi announces, as he recently did, that transcendental meditation can enable one to levitate and become invisible? Black holes are the latest symbols of unfathomable mystery. Public interest in them is, I am persuaded, no indication of interest in science, but rather a peculiar byproduct of the specter of the supernatural that is now haunting North America.
For the reader with no understanding of relativity and quantum theory—that is, the average reader—Asimov’s book is the best of the lot. The old maestro writes with his unfailing clarity, humor, informality, and enthusiasm. Like all top science-fiction writers he knows exactly where to draw the line between serious science and fantasy. Periodically he reminds his readers that there is as yet no clear observational evidence that black holes exist, and that “almost anything some astronomers suggest about a black hole is denied by other astronomers.”
Cautiously, step by step, Asimov sketches the necessary background for understanding a black hole’s properties. He begins with gravity, that gentle, all-pervasive, poorly understood force that holds together the matter of galaxies, stars, and planets. At the centers of planet-size bodies the pressure of gravity is insufficient to overcome the opposing electromagnetic force that binds the molecules of the matter at the core, and the matter remains intact. However, if the body is large enough (about the size of Jupiter) the pressure of gravity becomes so strong it triggers a hydrogen fusion reaction. The body becomes a sun.
There are three ways a sun can die. If a star is close to the size of our sun it will exhaust its hydrogen fuel, expand to a red giant, then slowly contract to a white dwarf. Eventually it will cool to a black dwarf, a permanently embalmed corpse that never changes unless it happens to be eaten by a black hole.
If a star is moderately greater in mass than our sun, its fate is more interesting. It is likely to explode into a supernova; then part of its mass instantly shrinks to a size smaller than the earth. So great is the density of this body that its gravitational force overcomes the opposing electromagnetic force and the structure of the star’s matter disintegrates. It becomes a fast-spinning neutron star.
Most astronomers are convinced that pulsars are neutron stars. These are small stellar objects inside our galaxy that send out absolutely regular beeps of radio waves, sometimes beeps of visible light. There are probably millions of them in the Milky Way that are within the range of today’s radio telescopes.
If a star is much more massive than our sun, it is expected to expire in a manner so bizarre that its fate is still shrouded in mystery. After it completes its catastrophic implosion, not even neutrons can withstand the enormous gravitational compression. All particles are totally destroyed, and the laws of physics no longer have meaning. The star has entered a black hole.
Black holes were crudely anticipated in 1798 by the French mathematician Pierre Simon de Laplace. His predecessor Isaac Newton believed that light consists of particles that are affected by gravity. If a star is large enough, Laplace pointed out, its gravitational force will prevent all light from escaping from it. This is not strictly true. In Newtonian physics the speed of light approaching a star, no matter how massive, would be so greatly accelerated that it could bounce off a reflecting surface and escape.
In relativity theory, light also consists of particles (photons) that are influenced by gravity, but their speed is a constant that cannot be exceeded. A few months after Einstein published his general relativity theory a German astronomer, Karl Schwarzschild, made exact calculations of what is now called the “Schwarzschild radius.” This is the radius of a body, given its mass, below which gravity is strong enough to prohibit light, matter, or any kind of signal from escaping. It is the critical radius below which matter becomes an invisible black hole. For a mass equal to our sun’s, the radius is a few kilometers. For a mass equal to the earth’s, it is the radius of a large pea.
In 1939 J. Robert Oppenheimer and his student Hartland Snyder made some surprising calculations. Assuming the truth of relativity, there are no laws to prevent the gravitational collapse of a sufficiently massive sun from compressing the sun’s matter within the Schwarzschild radius and forming a black hole. Moreover, the calculations lead to something even more mind boggling. At the core of every black hole there has to be a spacetime “singularity”—a term mathematicians use for a point at which something catastrophic happens to the solution of an equation. In this case, calculations show that spacetime curvature becomes infinite, which is to say that it becomes a single point. At that point, gravitational force and density (mass per unit volume) also become infinite.
If a spacetime singularity actually occurs and can be observed, it is called a “naked singularity.” So far, no one has seen a naked singularity. Perhaps its equations tell only part of the story and there are forces not yet understood which prevent singularities from existing. Roger Penrose, a brilliant theoretical physicist at Oxford University and a chief architect of black holes, believes that spacetime singularities can occur, but a “cosmic censor” prevents them from becoming naked. It conceals them, so to speak, inside an “event horizon” that prevents them from interacting in any way with the universe.
For twenty years the calculations of Oppenheimer and Snyder were considered no more than eccentric exercises for graduate students. Then in 1962 the quasars were discovered, and five years later the pulsars. Suddenly astrophysicists realized that maybe they were seeing objects in the final stages of just the sort of catastrophic collapse that had been worked out on paper. At first it was hoped that if the collapsing mass of a big star were a bit lopsided, the singularity could be avoided. But Penrose proved otherwise. The singularity is unavoidable. Regardless of the size, shape, or chemical constitution of a sun, if it is massive enough to collapse into a black hole, it will have that awful singularity at its center. As for the hole itself, all the structural peculiarities of the sun that formed it will be obliterated. “Black holes have no hair,” so goes a theorem. It means that all black holes, aside from mass, spin, and electric charge, are identical.
It is possible, as Philip Morrison and other cosmologists have emphasized, that laws not yet known may prevent the formation of black holes. True, there are some spots in the sky where astronomers think they see something going on that can be explained only by a black hole—notably the strong X-ray radiation coming from the vicinity of a giant star in the constellation of Cygnus (the Swan)—but what they see may have conventional explanations. There is no hard evidence, though the prevailing opinion is that black holes do exist. Some astronomers suspect that a giant black hole squats at the center of every galaxy, spinning silently while it slowly gobbles up nearby suns. As of today, however, black holes are theoretical constructions supported mainly by the fact that relativity theory requires them, by the rule that anything not excluded by theory probably exists, and by observed phenomena in the heavens that cannot be explained in any better way. Either black holes are real, it is said, or there are holes in relativity.
There is, of course, nothing wrong in building theoretical models for structures before they are observed. Sir Arthur Stanley Eddington once remarked, only half in jest, “You cannot believe in astronomical observations before they are confirmed by theory.” Eddington, by the way, a few years before Oppenheimer’s calculations, came remarkably close to constructing a model of a black hole.
“The star,” Eddington wrote, “apparently has to go on radiating and radiating and contracting and contracting until, I suppose, it gets down to a few kilometers radius, when gravity becomes strong enough to hold the radiation, and the star can at last find peace.” So far, so prophetic! But Eddington went on: “I felt driven to the conclusion that this was almost a reductio ad absurdum of the relativistic degeneracy formula. Various accidents may intervene to save the star, but I want more protection than that. I think that there should be a law of nature to prevent the star from behaving in this absurd way.”