by Carl Sagan
A helpful way to understand black holes is to think about the curvature of space. Consider a flat, flexible, lined two-dimensional surface, like a piece of graph paper made of rubber. If we drop a small mass, the surface is deformed or puckered. A marble rolls around the pucker in a orbit like that of a planet around the Sun. In this interpretation, which we owe to Einstein, gravity is a distortion in the fabric of space. In our example, we see two-dimensional space warped by mass into a third physical dimension. Imagine we live in a three-dimensional universe, locally distorted by matter into a fourth physical dimension that we cannot perceive directly. The greater the local mass, the more intense the local gravity, and the more severe the pucker, distortion or warp of space. In this analogy, a black hole is a kind of bottomless pit. What happens if you fall in? As seen from the outside, you would take an infinite amount of time to fall in, because all your clocks—mechanical and biological—would be perceived as having stopped. But from your point of view, all your clocks would be ticking away normally. If you could somehow survive the gravitational tides and radiation flux, and (a likely assumption) if the black hole were rotating, it is just possible that you might emerge in another part of space-time—somewhere else in space, some when else in time. Such worm holes in space, a little like those in an apple, have been seriously suggested, although they have by no means been proved to exist. Might gravity tunnels provide a kind of interstellar or intergalactic subway, permitting us to travel to inaccessible places much more rapidly than we could in the ordinary way? Can black holes serve as time machines, carrying us to the remote past or the distant future? The fact that such ideas are being discussed even semi-seriously shows how surreal the universe may be.
We are, in the most profound sense, children of the Cosmos. Think of the Sun’s heat on your upturned face on a cloudless summer’s day; think how dangerous it is to gaze at the Sun directly. From 150 million kilometers away, we recognize its power. What would we feel on its seething self-luminous surface, or immersed in its heart of nuclear fire? The Sun warms us and feeds us and permits us to see. It fecundated the Earth. It is powerful beyond human experience. Birds greet the sunrise with an audible ecstasy. Even some one-celled organisms know to swim to the light. Our ancestors worshiped the Sun,* and they were far from foolish. And yet the Sun is an ordinary, even a mediocre star. If we must worship a power greater than ourselves, does it not make sense to revere the Sun and stars? Hidden within every astronomical investigation, sometimes so deeply buried that the researcher himself is unaware of its presence, lies a kernel of awe.
The Galaxy is an unexplored continent filled with exotic beings of stellar dimensions. We have made a preliminary reconnaissance and have encountered some of the inhabitants. A few of them resemble beings we know. Others are bizarre beyond our most unconstrained fantasies. But we are at the very beginning of our exploration. Past voyages of discovery suggest that many of the most interesting inhabitants of the galactic continent remain as yet unknown and unanticipated. Not far outside the Galaxy there are almost certainly planets, orbiting stars in the Magellanic Clouds and in the globular clusters that surround the Milky Way. Such worlds would offer a breathtaking view of the Galaxy rising—an enormous spiral form comprising 400 billion stellar inhabitants, with collapsing gas clouds, condensing planetary systems, luminous supergiants, stable middle-aged stars, red giants, white dwarfs, planetary nebulae, novae, supernovae, neutron stars and black holes. It would be clear from such a world, as it is beginning to be clear from ours, how our matter, our form and much of our character is determined by the deep connection between life and the Cosmos.
*It had previously been thought that the protons were uniformly distributed throughout the electron cloud, rather than being concentrated in a nucleus of positive charge at the center. The nucleus was discovered by Ernest Rutherford at Cambridge when some of the bombarding particles were bounced back in the direction from which they had come. Rutherford commented: “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch [cannon] shell at a piece of tissue paper and it came back and hit you.”
*The spirit of this calculation is very old. The opening sentences of Archimedes’ The Sand Reckoner are: “There are some, King Gelon, who think that the number of the sand is infinite in multitude: and I mean by the sand not only that which exists about Syracuse and the rest of Sicily, but also that which is found in every region, whether inhabited or uninhabited. And again, there are some who, without regarding it as infinite, yet think that no number has been named which is great enough to exceed its multitude.” Archimedes then went on not only to name the number but to calculate it. Later he asked how many grains of sand would fit, side by side, into the universe that he knew. His estimate: 1063, which corresponds, by a curious coincidence, to 1083 or so atoms.
*Silicon is an atom. Silicone is a molecule, one of billions of different varieties containing silicon. Silicon and silicone have different properties and applications.
*The Earth is an exception, because our primordial hydrogen, only weakly bound by our planet’s comparatively feeble gravitational attraction, has by now largely escaped to space. Jupiter, with its more massive gravity, has retained at least much of its original complement of the lightest element.
*Stars more massive than the Sun achieve higher central temperatures and pressures in their late evolutionary stages. They are able to rise more than once from their ashes, using carbon and oxygen as fuel for synthesizing still heavier elements.
†The Aztecs foretold a time “when the Earth has become tired …, when the seed of Earth has ended.” On that day, they believed, the Sun will fall from the sky and the stars will be shaken from the heavens.
*Moslem observers noted it as well. But there is not a word about it in all the chronicles of Europe.
†Kepler published in 1606 a book called De Stella Nova, “On the New Star,” in which he wonders if a supernova is the result of some random concatenation of atoms in the heavens. He presents what he says is “… not my own opinion, but my wife’s: Yesterday, when weary with writing, I was called to supper, and a salad I had asked for was set before me. ‘It seems then,’ I said, ‘if pewter dishes, leaves of lettuce, grains of salt, drops of water, vinegar, oil and slices of eggs had been flying about in the air for all eternity, it might at last happen by chance that there would come a salad.’ ‘Yes,’ responded my lovely, ‘but not so nice as this one of mine.’ ”
*1 g is the acceleration experienced by falling objects on the Earth, almost 10 meters per second every second. A falling rock will reach a speed of 10 meters per second after one second of fall, 20 meters per second after two seconds, and so on until it strikes the ground or is slowed by friction with the air. On a world where the gravitational acceleration was much greater, falling bodies would increase their speed by correspondingly greater amounts. On a world with 10 g acceleration, a rock would travel 10 × 10 m/sec or almost 100 m/sec after the first second, 200 m/sec after the next second, and so on. A slight stumble could be fatal. The acceleration due to gravity should always be written with a lowercase g, to distinguish it from the Newtonian gravitational constant, G, which is a measure of the strength of gravity everywhere in the universe, not merely on whatever world or sun we are discussing. (The Newtonian relationship of the two quantities is F = mg = GMm/r2; g = GM/r2, where F is the gravitational force, M is the mass of the planet or star, m is the mass of the falling object, and r is the distance from the falling object to the center of the planet or star.)
*The early Sumerian pictograph for god was an asterisk, the symbol of the stars. The Aztec word for god was Teotl, and its glyph was a representation of the Sun. The heavens were called the Teoatl, the godsea, the cosmic ocean.
CHAPTER X
THE EDGE OF FOREVER
There is a way on high, conspicuous in the clear heavens, called the Milky Way, brilliant with its own brightness. By it the gods go to the d
welling of the great Thunderer and his royal abode … Here the famous and mighty inhabitants of heaven have their homes. This is the region which I might make bold to call the Palatine [Way] of the Great Sky.
—Ovid, Metamorphoses (Rome, first century)
Some foolish men declare that a Creator made the world. The doctrine that the world was created is ill-advised, and should be rejected.
If God created the world, where was He before creation?… How could God have made the world without any raw material? If you say He made this first, and then the world, you are faced with an endless regression … Know that the world is uncreated, as time itself is, without beginning and end.
And it is based on the principles …
—The Mahapurana (The Great Legend),
Jinasena (India, ninth century)
Ten or twenty billion years ago, something happened—the Big Bang, the event that began our universe. Why it happened is the greatest mystery we know. That it happened is reasonably clear. All the matter and energy now in the universe was concentrated at extremely high density—a kind of cosmic egg, reminiscent of the creation myths of many cultures—perhaps into a mathematical point with no dimensions at all. It was not that all the matter and energy were squeezed into a minor corner of the present universe; rather, the entire universe, matter and energy and the space they fill, occupied a very small volume. There was not much room for events to happen in.
In that titanic cosmic explosion, the universe began an expansion which has never ceased. It is misleading to describe the expansion of the universe as a sort of distending bubble viewed from the outside. By definition, nothing we can ever know about was outside. It is better to think of it from the inside, perhaps with grid lines—imagined to adhere to the moving fabric of space—expanding uniformly in all directions. As space stretched, the matter and energy in the universe expanded with it and rapidly cooled. The radiation of the cosmic fireball, which, then as now, filled the universe, moved through the spectrum—from gamma rays to X-rays to ultraviolet light; through the rainbow colors of the visible spectrum; into the infrared and radio regions. The remnants of that fireball, the cosmic background radiation, emanating from all parts of the sky can be detected by radio telescopes today. In the early universe, space was brilliantly illuminated. As time passed, the fabric of space continued to expand, the radiation cooled and, in ordinary visible light, for the first time space became dark, as it is today.
The early universe was filled with radiation and a plenum of matter, originally hydrogen and helium, formed from elementary particles in the dense primeval fireball. There was very little to see, if there had been anyone around to do the seeing. Then little pockets of gas, small nonuniformities, began to grow. Tendrils of vast gossamer gas clouds formed, colonies of great lumbering, slowly spinning things, steadily brightening, each a kind of beast eventually to contain a hundred billion shining points. The largest recognizable structures in the universe had formed. We see them today. We ourselves inhabit some lost corner of one. We call them galaxies.
About a billion years after the Big Bang, the distribution of matter in the universe had become a little lumpy, perhaps because the Big Bang itself had not been perfectly uniform. Matter was more densely compacted in these lumps than elsewhere. Their gravity drew to them substantial quantities of nearby gas, growing clouds of hydrogen and helium that were destined to become clusters of galaxies. A very small initial nonuniformity suffices to produce substantial condensations of matter later on.
As the gravitational collapse continued, the primordial galaxies spun increasingly faster, because of the conservation of angular momentum. Some flattened, squashing themselves along the axis of rotation where gravity is not balanced by centrifugal force. These became the first spiral galaxies, great rotating pinwheels of matter in open space. Other protogalaxies with weaker gravity or less initial rotation flattened very little and became the first elliptical galaxies. There are similar galaxies, as if stamped from the same mold, all over the Cosmos because these simple laws of nature—gravity and the conservation of angular momentum—are the same all over the universe. The physics that works for falling bodies and pirouetting ice skaters down here in the microcosm of the Earth makes galaxies up there in the macrocosm of the universe.
Within the nascent galaxies, much smaller clouds were also experiencing gravitational collapse; interior temperatures became very high, thermonuclear reactions were initiated, and the first stars turned on. The hot, massive young stars evolved rapidly, profligates carelessly spending their capital of hydrogen fuel, soon ending their lives in brilliant supernova explosions, returning thermonuclear ash—helium, carbon, oxygen and heavier elements—to the interstellar gas for subsequent generations of star formation. Supernova explosions of massive early stars produced successive overlapping shock waves in the adjacent gas, compressing the intergalactic medium and accelerating the generation of clusters of galaxies. Gravity is opportunistic, amplifying even small condensations of matter. Supernova shock waves may have contributed to accretions of matter at every scale. The epic of cosmic evolution had begun, a hierarchy in the condensation of matter from the gas of the Big Bang—clusters of galaxies, galaxies, stars, planets, and, eventually, life and an intelligence able to understand a little of the elegant process responsible for its origin.
Clusters of galaxies fill the universe today. Some are insignificant, paltry collections of a few dozen galaxies. The affectionately titled “Local Group” contains only two large galaxies of any size, both spirals: the Milky Way and M31. Other clusters run to immense hordes of thousands of galaxies in mutual gravitational embrace. There is some hint that the Virgo cluster contains tens of thousands of galaxies.
On the largest scale, we inhabit a universe of galaxies, perhaps a hundred billion exquisite examples of cosmic architecture and decay, with order and disorder equally evident: normal spirals, turned at various angles to our earthly line of sight (face-on we see the spiral arms, edge-on, the central lanes of gas and dust in which the arms are formed); barred spirals with a river of gas and dust and stars running through the center, connecting the spiral arms on opposite sides; stately giant elliptical galaxies containing more than a trillion stars which have grown so large because they have swallowed and merged with other galaxies; a plethora of dwarf ellipticals, the galactic midges, each containing some paltry millions of suns; an immense variety of mysterious irregulars, indications that in the world of galaxies there are places where something has gone ominously wrong; and galaxies orbiting each other so closely that their edges are bent by the gravity of their companions and in some cases streamers of gas and stars are drawn out gravitationally, a bridge between the galaxies.
Some clusters have their galaxies arranged in an unambiguously spherical geometry; they are composed chiefly of ellipticals, often dominated by one giant elliptical, the presumptive galactic cannibal. Other clusters with a far more disordered geometry have, comparatively, many more spirals and irregulars. Galactic collisions distort the shape of an originally spherical cluster and may also contribute to the genesis of spirals and irregulars from ellipticals. The form and abundance of the galaxies have a story to tell us of ancient events on the largest possible scale, a story we are just beginning to read.
The development of high-speed computers makes possible numerical experiments on the collective motion of thousands or tens of thousands of points, each representing a star, each under the gravitational influence of all the other points. In some cases, spiral arms form all by themselves in a galaxy that has already flattened to a disk. Occasionally a spiral arm may be produced by the close gravitational encounter of two galaxies, each of course composed of billions of stars. The gas and dust diffusely spread through such galaxies will collide and become warmed. But when two galaxies collide, the stars pass effortlessly by one another, like bullets through a swarm of bees, because a galaxy is made mostly of nothing and the spaces between the stars are vast. Nevertheless, the configuration o
f the galaxies can be distorted severely. A direct impact on one galaxy by another can send the constituent stars pouring and careening through intergalactic space, a galaxy wasted. When a small galaxy runs into a larger one face-on it can produce one of the loveliest of the rare irregulars, a ring galaxy thousands of light-years across, set against the velvet of intergalactic space. It is a splash in the galactic pond, a temporary configuration of disrupted stars, a galaxy with a central piece torn out.
The unstructured blobs of irregular galaxies, the arms of spiral galaxies and the torus of ring galaxies exist for only a few frames in the cosmic motion picture, then dissipate, often to be reformed again. Our sense of galaxies as ponderous rigid bodies is mistaken. They are fluid structures with 100 billion stellar components. Just as a human being, a collection of 100 trillion cells, is typically in a steady state between synthesis and decay and is more than the sum of its parts, so also is a galaxy.
The suicide rate among galaxies is high. Some nearby examples, tens or hundreds of millions of light-years away, are powerful sources of X-rays, infrared radiation and radio waves, have extremely luminous cores and fluctuate in brightness on time scales of weeks. Some display jets of radiation, thousand-light-year-long plumes, and disks of dust in substantial disarray. These galaxies are blowing themselves up. Black holes ranging from millions to billions of times more massive than the Sun are suspected in the cores of giant elliptical galaxies such as NGC 6251 and M87. There is something very massive, very dense, and very small ticking and purring inside M87—from a region smaller than the solar system. A black hole is implicated. Billions of light-years away are still more tumultuous objects, the quasars, which may be the colossal explosions of young galaxies, the mightiest events in the history of the universe since the Big Bang itself.