by Carl Sagan
More and more elaborate reproductive systems arose. Those systems that copied better produced more copies. Those that copied poorly produced fewer copies. Soon most of the molecules were organized into molecular collectives, into self-replicating systems. It was not that any molecules had the glimmering of an idea or the ghostly passage of a need or want or aspiration; merely, those molecules that copied did, and soon the face of the planet became transformed by the copying process. In time, the seas became full of these molecular collectives, forming, metabolizing, replicating … forming, metabolizing, replicating … forming, metabolizing, mutating, replicating… Elaborate systems arose, molecular collectives exhibiting behavior, moving to where the replication building blocks were more abundant, avoiding molecular collectives that incorporated their neighbors. Natural selection became a molecular sieve, selecting out those combinations of molecules best suited by chance to further replication.
All the while the building blocks, the foodstuffs, the parts for later copies, were being produced, mainly by sunlight and lightning and thunder – all driven by the nearby star. The nuclear processes in the insides of the stars drove the planetary processes, which led to and sustained life.
As the supply of foodstuffs gradually was exhausted, a new kind of molecular collective arose, one able to produce molecular building blocks internally out of air and water and sunlight. The first animals were joined by the first plants. The animals became parasites upon the plants, as they had been earlier on the stellar manna falling from the skies. The plants slowly changed the composition of the atmosphere; hydrogen was lost to space, ammonia transformed to nitrogen, methane to carbon dioxide. For the first time, oxygen was produced in significant quantities in the atmosphere – oxygen, a deadly poisonous gas able to convert all the self-replicating organic molecules back into simple gases like carbon dioxide and water.
But life met this supreme challenge: In some cases by burrowing into environments where oxygen was absent, but – in the most successful variants – by evolving not only to survive the oxygen but to use it in the more efficient metabolism of foodstuffs.
Sex and death evolved – processes that vastly increased the rate of natural selection. Some organisms evolved hard parts, climbed onto, and survived on the land. The pace of production of more complex forms accelerated. Flight evolved. Enormous four-legged beasts thundered across the steaming jungles. Small beasts emerged, born live, instead of in hard-shelled containers filled with replicas of the early oceans. They survived through swiftness and cunning – and increasingly long periods in which their knowledge was not so much preprogrammed in self-replicating molecules as learned from parents and experiences.
All the while, the climate was variable. Slight variations in the output of sunlight, the orbital motion of the planet, clouds, oceans, and polar icecaps produced climatic changes – wiping out whole groups of organisms and causing the exuberant proliferation of other, once insignificant, groups.
And then … the Earth grew somewhat cold. The forests retreated. Small arboreal animals climbed down from the trees to seek a livelihood on the savannas. They became upright and tool-using. They communicated by producing compressional waves in the air with their eating and breathing organs. They discovered that organic material would, at a high enough temperature, combine with atmospheric oxygen to produce the stable hot plasma called fire. Postpartum learning was greatly accelerated by social interaction. Communal hunting developed, writing was invented, political structures evolved, superstition and science, religion and technology.
And then one day there came to be a creature whose genetic material was in no major way different from the self-replicating molecular collectives of any of the other organisms on his planet, which he called Earth. But he was able to ponder the mystery of his origins, the strange and tortuous path by which he had emerged from star-stuff. He was the matter of the cosmos, contemplating itself. He considered the problematical and enigmatic question of his future. He called himself Man. He was one of the starfolk. And he longed to return to the stars.
38. Starfolk
II. A Future
The story of the preceding chapter is a kind of scientific fable. It is more or less what many modern scientists believe on the basis of available evidence. It is the outline of the emergence of man, a process wending through billions of years of time and driven by gravitation and nuclear physics, by organic chemistry and natural selection. It tells how the matter of which we are made was generated in another place and another time, in the insides of a dying star five billion or more years ago.
There are three aspects of the fable that I find particularly interesting. First, that the universe is put together in such a way as to permit, if not guarantee, the origin of life and the development of complex creatures. It is easy to imagine laws of physics that would not permit appropriate nuclear reactions, or laws of chemistry that would not permit appropriate configurations of molecules to be assembled. But we do not live in such universes. We live in a universe remarkably hospitable to life.
Second, there is in the fable no step unique to our Solar System or to our planet. There are 250 billion suns in our Milky Way Galaxy, and billions of other galaxies in the heavens. Perhaps half of these stars have planets at biologically appropriate distances from the local sun. The initial chemical constituents for the origin of life are the most abundant molecules in the universe. Something like the processes that on Earth led to man must have happened billions of other times in the history of our Galaxy. There must be other starfolk.
The evolutionary details would not be the same, of course. Even if the Earth were started over again and only random forces again operated, nothing like a human being would be produced – because human beings are the end product of an exquisitely complicated evolutionary pathway full of false starts and dead ends and statistical accidents. But we might well expect, if not human beings, organisms functionally not very different from ourselves. Since there are second- and third-generation stars much older than our Sun, there must be, I think, many places in the Galaxy where there are beings far more advanced than we in science and technology, in politics, ethics, poetry, and music.
The third point is the most arresting. It is the intimate connection between stars and life. Our planet was formed from the dregs of star-stuff. The atoms necessary for the origin of life were cooked in the interiors of red giant stars. These atoms were forced together, to form complex organic molecules, by ultraviolet light and thunder and lightning, all produced by the radiation of our neighboring Sun. When the food supply ran short, green-plant photosynthesis developed, driven again by sunlight, the sunlight off which almost all the organisms on Earth, and certainly everyone we know, live out their days.
But this cannot be the end of the fable. Our Sun is only approaching vigorous middle age. It has perhaps another five billion or ten billion years of stellar life ahead of it.
And what of life on Earth and man? They, too, for all we know, may have a future. And if not, there are billions of other stars and probably billions of other inhabited planets in our Galaxy. What is the interaction between stars and life later on?
The death of stars is taking astronomers into unexpected and almost surreal celestial landscapes. One of these is the supernova explosion, the death throes of a star slightly more massive than our Sun. In a brief period of a few weeks to a few months, such an exploding star may become brighter than the rest of the galaxy in which it resides. In super-novae, elements like gold and uranium are generated from iron. The supernovae are the long-sought Philosopher's Stone, converting base metals into precious metals.
Having blown away most of its star-stuff – destined, some of it, to go into later generations of formation of stars and planets and life – the star settles down to a quiet old age, its fires spent, as a white dwarf. A white dwarf is constituted of matter in a state that physicists, with no moral imputation intended, call degenerate. Electrons are stripped off the nuclei of atoms. The protecti
ve shields of negative electricity are removed. The nuclei can move much closer together, and a state of extraordinary density results. Typical degenerate matter weighs about a ton per thimbleful. Some white dwarfs, properly considered, are vast stellar crystals able to hold up the weight of the overlying layers of the star. Some white dwarfs are largely carbon. We may speak of a star made of diamond.
But for more massive stars, the white dwarfs – their embers slowly fading, decaying into black dwarfs – are not the end state. Degenerate matter cannot hold up the weight of a more massive star, and another cycle of stellar contraction thus ensues – the matter being crushed together to more and more incredible densities, until some new regime of physics is entered, until some new force surfaces to stop the stellar collapse. There is only one further such force known: It is the nuclear force that holds the nucleus of the atom together. This nuclear force is responsible for the stability of atoms and, therefore, for all of chemistry and biology. It is also responsible for the thermonuclear reactions in the insides of stars that make stars shine and, thereby, in a quite different way, drives planetary biology.
Imagine a star more or less like the Sun, but a little more massive, near the end of its days of converting simple nuclei into more complex nuclei. It produces the last series of complex nuclear reactions it is able to – and then collapses. As its size decreases, it spins more and more rapidly, like a pirouetting ice skater bringing in her arms. Only when the density in its interior becomes comparable to the density of matter inside the atomic nucleus does the collapse stop. It is a simple matter in elementary physics to calculate at what stage the collapse will end. It ends when the star is about a mile across and rotating about ten times a second.
Such an object is a rapidly rotating neutron star. It is, in truth, a giant atomic nucleus a mile across. Neutron star matter is so dense that a speck of it – just barely visible – would weigh a million tons. The Earth would not be able to support it. A piece of neutron star matter, if it could be transported to the Earth without falling apart, would sink effortlessly through the crust, mantle, and core of our planet like a razor blade through warm butter.
Such neutron stars were theoretical constructs, the imaginings of speculative physicists – until the pulsars were discovered. Pulsars are sources of radio emission. Some of them are associated with old supernova explosions. They blink at us as if the beam of some cosmic lighthouse swept by us ten times a second. The details of the emission from pulsars are best understood if they are the fabled neutron stars. Because of the loss of energy to space that we observe, the rotation rate of an isolated neutron star must slowly decline, even though it is a stellar timekeeper of extraordinary accuracy. The observed decay of pulsar periods is just about what is expected from neutron star physics.
The first pulsar to be detected was called, by its discoverers, only half impishly, LGM-1. The LGM stood for "little green men." Was it, they wondered, a beacon from an advanced extraterrestrial civilization? My own view, when I first heard about pulsars, was that they were perfect interstellar navigation beacons, the sorts of markers that an interstellar spacefaring society would want to place throughout the Galaxy for time- and space-fixes for their voyages. There is now little doubt that pulsars are neutron stars. But I would not exclude the possibility that if there are interstellar space-faring societies, the naturally formed pulsars are used as navigation beacons and for communications purposes.
The state of matter in the inside of such a neutron star is still not understood. We do not know if a surface crust comprising a neutron crystal lattice overlies a core of liquid neutrons. Should the core be solid, starquakes are expected – a shifting of the matter under enormous stress in the interior of the star. Such starquakes should produce a discontinuous change in the period of rotation of the neutron star. Such changes, called "glitches," are observed.
Some were disappointed to learn that pulsars were neutron stars and not interstellar radio communication channels. But pulsars are hardly uninteresting. Indeed, a star more massive than the Sun that fits into a sphere a mile across and rotates ten times every second is, in a certain sense, much more exotic than a civilization slightly more advanced than we, on the planet of another star.
But there is another and much more profound way in which neutron stars and supernova explosions are connected with life. In a supernova explosion, as we have already mentioned, vast quantities of atoms from the surface of the star are ejected at very high velocities into interstellar space. In the case of the neutron star, there is, because of its rapid rotation, a zone, not far from its surface, which is rotating at almost the speed of light. Particles are ejected from that zone at velocities so great that the theory of relativity must be taken into account to describe them. Both supernova explosions and the high-velocity zone surrounding neutron stars must produce cosmic rays – the very fast charged particles (mostly protons, but containing all the other elements as well) that pervade the space between the stars.
Cosmic rays fall on the Earth's atmosphere. The less energetic particles are absorbed by the atmosphere or deflected by Earth's magnetic field. But the more energetic particles, the ones produced by supernovae or neutron stars, penetrate to the surface of the Earth. And here they collide with life. Some cosmic rays penetrate through the genetic material of life forms on the surface of our planet. These random, unpredictable cosmic rays produce changes, mutations, in the hereditary material. Mutations are variations in the blueprints, the hereditary instructions, contained in our self-replicating molecules. Like a fine watch repeatedly hit with a hammer, the functioning of life is unlikely to improve under such random pummelings. But as sometimes happens with watches or bulky television sets, a random pummeling does occasionally improve the functioning. The vast bulk of mutations are harmful, but the small fraction of mutations that are an improvement provide the raw material for evolutionary advance. Life would be at a dead end without mutation. Thus, in yet another way, life on Earth is intimately bound to stellar events. Human beings are here because of the paroxysms in dying stars thousands of light-years away.
The births of stars generate the planetary nurseries of life. The lives of stars provide the energy upon which life depends. The deaths of stars produce the implements for the continued development of life in other parts of the Galaxy. If there are on the planets of dying stars intelligent beings unable to escape their fate, they may at least derive some comfort from the thought that the death of their star, the event that will cause their own extinction, will, nevertheless, provide the means for continued biological advance of the starfolk on a million other worlds.
39. Starfolk
III. The Cosmic Cheshire Cats
The neutron star is not the most exotic inhabitant of the stellar bestiary. A star larger than three times the mass of our Sun is too big even for nuclear forces to stop its collapse. Once the collapse begins, there is nothing to stop it. The star contracts to one mile across and continues to shrink. The density passes that of the nucleus of an atom, and matter is further crushed together. The gravitational field in the vicinity of such a massive dying star continues to increase. Eventually, the gravity is so strong that not only is matter unable to leave the star, but light is also trapped. A photon traveling at the speed of light away from the star is constrained to follow a curved path and fall back upon the star, just as the Earth's gravity is too strong for any of us to throw a rock to escape velocity. Such stars are too massive for even photons to escape. Consequently, they are dark. They cannot be seen directly, because no light emanates from them. They are present gravitationally, but not optically. They are called "black holes." They are beasts akin to the smile on the Cheshire cat. They are enormous stars that have winked out but are still there.
Black holes are basically theoretical concepts. They may sound no more likely than, and not nearly as charming as, the elves of Middle Earth. But they are probably there. In fact, much of the mass of the Galaxy may reside not in stars we can see, nor in th
e gas and dust between the stars, but in black holes peppering the Galaxy like the holes in an Emmenthaler cheese.
The first black hole may have been found. Cygnus X-1 is a rapidly varying source of X-rays, visible light, and radio waves. Its X-ray emission was monitored from NASA's UHURU satellite, launched from an Italian launch complex off the coast of Kenya. All the clues point to Cyg X-1's being a binary star, two stars revolving around each other in a regular and intricate waltz. From the motion of the one star we see, we can deduce the mass of the star we cannot. It turns out to be a massive star, perhaps ten times the mass of our Sun. Such a massive star should ordinarily be very bright. Yet there is no optical hint of its presence. The bright star in Cyg X-1 is revolving about a massive object that is present gravitationally but not optically. It is very likely a black hole a few thousand light-years from Earth.
Black holes may have their uses. What we know about them till now is entirely theoretical, not tested against the skeptical standards of observation. There are some strange possibilities that have been suggested for black holes. Since there is no way to get out of a black hole, it is, in a sense, a separate universe.
In fact, our own universe is very likely itself a vast black hole. We have no knowledge of what lies outside our universe. This is true by definition, but also because of the properties of black holes. Objects that reside in them cannot ordinarily leave them. In a strange sense, our universe may be filled with objects that are not here. They are not separate universes. They do not have the mass of our universe. But in their separateness and their isolation they are autonomous universes.
There is an even more bizarre prospect. In one speculative view (Chapter 36), an object that plunges down a rotating black hole may re-emerge elsewhere and elsewhen – in another place and another time. Black holes may be apertures to distant galaxies and to remote epochs. They may be shortcuts through space and time. If such holes in the fabric of the space-time continuum exist, it is by no means certain that it would ever be possible for an extended object like a spacecraft to use a black hole for travel through space or time. The most serious obstacle would be the tidal force exerted by the black hole during approach – a force that would tend to pull any extended matter to pieces. And yet it seems to me that a very advanced civilization might cope with the tidal stresses of a black hole.