“The first range of hills that encircles the scanty vale of human life is the horizon for the majority of its inhabitants. On its ridges the common sun is born and departs. From them the stars rise, and touching them they vanish.
—Samuel Taylor Coleridge
Human perceptions of the universe are warped by the fact that no humans have ever ventured very far from earth. To the collective mind of mankind, the earth is “here” and the universe “out there.”
Yet the earth is not just a platform from which to observe the cosmos. It is part of the cosmos. We live in the universe. The earth and all the things upon it are particles of the universe, as are the stars and galaxies.
One way to think about this is to leave the earth behind, go elsewhere and take a look around.
- Neighborhood Space Triton, a Moon of Neptune
The thin crescent of Neptune dominates the sky, ten times the size of the moon as seen from earth. Every six days, as Triton completes its orbit, this crescent swells to a nearly full globe and then diminishes again to a sickle. A thick soup of cold gas covers Neptune in broad bands of color. Whirlpools and knots of gas the size of continents form and dissipate among the bands as days go by. The planet glows with a chilly green light.
Elsewhere in the sky one star shines much brighter than all the others, a pinpoint of light so brilliant it casts shadows. This is the sun, three billion miles away.
Except for the remote Pluto, about which little is known, the small planets of the Solar System all lie relatively close to the sun. Sunlight passes the orbits of Mercury and Venus and reaches the earth within three minutes, Mars in five minutes.
Further out is the realm of the giants. Jupiter is 15 light-minutes from the sun, Saturn 30 light-minutes, Uranus three times that. By the time sunlight reaches Neptune, the outermost of the giants, it has been traveling (at 186,000 miles per second) for four hours.
From that distance it would almost seem that we had left the Solar System behind. There is little sign of the other planets. The two closest, Uranus and Pluto, are too dim to be seen without a telescope. Jupiter and Saturn, the largest planets, can be made out with the unaided eye, but their orbits are so much closer to the sun that they never stray far from its glare. With a single outstretched hand we can at once block out the sun and both the visible planets; this one hand, in effect, removes the Solar System from the universe.
As for the earth, with a powerful telescope on Triton it could sometimes be seen, but even a telescope equal to the largest humans have built would show no sign of life.
- Interstellar Space: An Imagined Planet of Sirius B
Two suns hang in the sky. The dim parent star, Sirius B, sheds a sort of vivid moonlight over the land. More distant but much brighter, Sirius A appears as a tiny, bright white disk, flooding the sky and blotting out the stars. As seasons pass the two suns draw apart in the sky until for a time there is no real night—Sirius A illuminates the days and Sirius B the duskier but still luminous nights. Then they approach each other again, casting double shadows, and at night the stars come out.
The seasons are complicated by a 50-year cycle during which Sirius A first grows brighter and hotter, bringing cyclones and blistering summers, then shrinks away.
A light year is a considerable distance—5.8 million million miles—and Sirius is 8.7 light years from the earth. So upon arriving there it should not be startling to find the sky somewhat changed from that of earth.
Some of the stars have moved, now that we are in the Sirian System. Procyon, a bright star about three light years from Sirius, has shifted well away from the constellation of Canis Minor to now reside instead in Cancer. Alpha Centauri, nearest star to the sun, has crawled a substantial distance across the sky.
In Hercules, not far from the much brighter red Altair and the brilliant blue-white Vega, we can find the sun, a medium-bright star.
Otherwise the constellations look much as they do from earth. Most of the familiar stars are hundreds or thousands of light years away, so even a voyage to Sirius—a trip which would take existing spaceships hundreds of thousands of years—does little to change our perspective on them.
But local conditions in an alien star system can be quite strange. The situation described above—double suns oscillating in the sky, a 50-year cycle of the seasons, and so forth—might be found on an earth-like planet in the Sirian System. Whether such a planet actually exists is unknown because men do not yet have telescopes large enough to detect anything so small as a planet at such a distance. Rhythmic variations in the motion through space of a half dozen nearby stars indicates that they may have planets, but how many planets there are or what they are like we do not know.
Sirius B, a white dwarf star of great density only about twice the size of earth, orbits the much larger and brighter Sirius A in an ellipse that is the cause of the 50-year climate cycle conjectured above. Other star systems consist of three or even four stars, and life on one of their planets could be almost infinitely complicated. For one thing, stars in a single system are often of different composition and color—red, blue, yellow, green—and this could be expected to influence not only conditions on their planets, but the perceptions of the planets’ inhabitants.
Humans see the way they do because of the sun. The spectrum of light visible to people centers at about the middle of the spectrum of solar light that reaches the surface of the earth. A hotter star like Sirius appears to us blue-white; a dimmer star like Antares looks red.
But to inhabitants of those star systems things would look different. (We are considering now only mammal-like creatures on earth-like planets; the possibilities are of course far greater if we start to imagine, say, intelligent jellyfish swimming in a methane sea.) To a native of one of Sirius’ planets whose eyes evolved in its light, our sun might appear as yellow as a kerosene lamp and Antares nearly invisible. To someone from Antares the sun could be cold and harsh and Sirius almost unbearable.
This represents only a modest alternative perspective. Stars radiate in radio wavelengths as naturally as they do in light; clouds of gas in space and whole galaxies pour out radio waves as well. It is not impossible to imagine creatures who “see” in radio; their perceptions would be coarse and unfocused by the standards of light, because radio wavelengths are much longer, but unlike humans they would be able to see vast distances in space through clouds of dust and gas that block out light. Individual stars are not prominent in radio wavelengths, but great arcs and fields of energy are; the universe is festooned with these fields.
We know this because of the development of radio “telescopes.” These are actually highly sensitive, high-resolution radio receivers tuned to frequencies in which the cosmos broadcasts (for example the 21-centimeter wavelength, where are found the primary radiations of hydrogen gas).
Other instruments have made it possible to map the skies in other wavelengths—in infrared and ultraviolet light, in gamma rays and X-rays. Intelligent creatures living in conditions far different from our own may have devised means to see all of these, as well as what we call “visible” light and perhaps other unknown radiations.
None of these perceptions has any special claim to represent reality. We do not “see” reality. We see only one of countless facets of reality, and the universe itself is at least the sum of all these potential and realized perspectives.
3. Deep Space the Edge of the Galaxy
Half the sky is filled with the light of the Milky Way. We are viewing it edgewise, flattened like a sunfish whose markings are spiral arms full of stars, coiling out from the center. A few scattered stars, the outposts of the galaxy, appear in the foreground; the rest blend into the soft glow of the spiral, the combined light of a thousand million stars.
Any tiny region within this spiral, an area perhaps the size of an outstretched fingernail, represents the whole cosmos visible to the naked eye of a planet like earth. The stars of our skies and the worlds we would visit in centuries of space travel all lie within one such tiny neighborhood.
The other half of the sky is void. There are no stars, since we are perched on the perimeter of our galaxy. The nearly absolute darkness is broken at intervals by dull smudges of light, which are other galaxies. Some, large and fairly bright, lie within a few million light years and belong to a cluster of galaxies which incorporates our own. Other clusters lie hundreds of millions of light years farther out, and these themselves may be grouped into metagalaxies, in which galaxies number as stars.
That humans even know they live in a galaxy is astonishing. It is as though a society of plankton confined to an inlet in the Philippines had managed to draw a rough but accurate chart of the entire Pacific Ocean.
As charted by people, the Milky Way galaxy is a flat pinwheel, 100,000 light years in diameter and 15,000 thick. It is composed of stars, dust and gas, but mostly space: Two galaxies could pass through one another without a collision between stars. The system is rotating; the sun, moving 140 miles per second, is carried through one complete revolution every 200 million years. So 25 of these galactic years have passed since the sun was born, 15 since life appeared on earth, but only 1/25th since the genesis of man.
The sun lies in the suburbs of the Milky Way, 30,000 light years out from the crowded stars of the center and 20,000 light years in from the region where the galaxy trails off into intergalactic space.
By an enormous extension of technology, starships may one day be built which can navigate the 20,000 light years from the earth to the outskirts of the galaxy, but the two-way trip would consume 40,000 to 50,000 years of earth time. This is a serious business considering that civilization on earth is only about 10,000 years old.
Fortunately for deep-space astronauts, the elapsed time on board the starship would be far shorter. This is because of the effects of relativity. At velocities approaching the speed of light, the relative passage of time on a spaceship slows down. The faster the speed, the slower the passage of time (though the clock never stops altogether). A starship could therefore travel thousands of light years, while thousands of years passed on earth, in an on-board time of only a few years. The reasons for this cannot be explained briefly, but the important thing to note is that it is not an illusion. Time-dilation is a real phenomenon, verified in a variety of experiments since predicted by Einstein.
Great advantage of this relativity effect could be taken by powerful space vehicles. Imagine a starship which could maintain an acceleration equal to 1G, the force of gravity on earth. Such a ship would accelerate constantly at 1G for half its journey, then turn end-for-end and decelerate at the same rate until it reached its destination, maintaining a comfortable artificial gravity throughout the trip.
This 1G starship could travel the 20,000 light years to the edge of the galaxy in only 20 years, on-board time. It could cross the entire galaxy in about 25 years, and even strike out and reach a neighboring galaxy in less than 30 years.
To build and operate such a ship would require advances in technology far beyond anything even remotely possible in the near future, but there are no known theoretical barriers to it. With the addition of suspended animation techniques, these starships could make voyages virtually across the universe.
Of course, if these vehicles can be built, someone may have already built them. Starships may be crossing through space by the millions and they may have been doing so for millions of years in the past. We have made almost no attempt to look for other civilizations, and our own is so young that we can hardly expect it to have come to the attention of another. (The most conspicuous evidence of life on earth—radio, TV and radar broadcasts—have by now reached no more than about 40 light years into space, a distance encompassing a few hundred of the galaxy’s 100 billion stars.) Conceivably the galaxy is united in a single civilization, harbors millions of friendly or hostile societies, or is linked telepathically into one great mind. We just do not know.
4. Claustrophobic Space: The Center of a Globular Star Cluster
The sky is filled with bright stars. Some shine even in the daytime. At night they cloud the sky, thousands of them as brilliant as the brightest few we see from earth. The whole universe seems to be jammed with stars, with us at its center.
If there is a library of the galaxy somewhere containing the most hapless early cosmologies, some of them are likely to have come from the inhabitants of the centers of globular clusters. Others could have come from people on planets like Venus, where the opaque atmosphere might be expected to relegate all early cosmology to the realm of the imagination.
Globulars are aggregates of stars apparently formed from knots of gas left behind when the galaxy was born. A big globular may contain 100,000 stars, loosely associated at the edges but closely packed—by the standards of space, meaning an average separation of a couple of light years—in the center.
A people which evolved on a planet near the center of such a cluster would be presented with the strange and beautiful spectacle of thousands of bright stars in all directions. With such a perspective, they might forgivably think themselves to be in the center of the universe.
As a compensation, they would have extraordinary opportunities for interstellar travel. An earth spaceship with a range of ten light years could reach only one of seven stars, but in the close precincts of a globular cluster, the same ship could choose from hundreds. Creatures in the center of a globular might develop like earth’s Polynesians, who acquired great navigational skills by exploring their neighboring islands.
5. Extragalactic Space: The Edge of a Distant Globular Cluster
Here the whole galaxy is spread flat before us; it fills the sky. Stars of endless color and variety, clouds of luminous gas, dark rifts of dust are set out like smears on a microscope slide. There can be no better place from which to begin the ceaseless study of what a galaxy is.
Globular clusters occupy a sphere of space concentric with the center of the galaxy. If we visualize the galaxy as a plate, the relationship is something like that of a melon sliced in two and pressed back together with the plate in the middle. Within this volume of space are hundreds of globulars, some falling inside the plane of the galaxy, others well out in space, held by the galaxy’s gravity but clear of the spiral itself. It is on the edge of one of these deep-space globular clusters, 10,000 light years from the galaxy, that we now place ourselves.
If life in the center of a deep-space globular encourages cosmic parochialism, life at its edge could almost limitlessly expand the mind. Imagine a journey outward from the center: As light years are crossed, stars are encountered less and less frequently, and the glowing backdrop of the Milky Way claims the eye. Finally we emerge from the cluster altogether, to confront a startling, face-on view of a galaxy we might previously not have guessed existed. This is a perspective radically different from that of those who live within the galaxy; they are like fish in the sea, while we are like people standing with our feet in the water, looking down.
The revelation of their relationship to the galaxy could challenge star travelers who had gained their skills navigating among the close worlds of their own cluster. Having learned to negotiate formidable distances of hundreds of light years, they would be offered the chance to explore the riches of the galaxy, waiting across 10,000 light years of empty space.
This combination of close neighbor stars with deep-space perspective is found only on the edges of globular clusters which lie clear of the galaxy. If there is a galactic civilization, its pioneer astronomers and astronauts may have come from these regions.
As mentioned earlier, a 1G starship could negotiate 10,000 light years in about 20 years of on-board time. Creatures accustomed to higher gravities might build 2G or 3G ships and comfortably make the trip in only five or ten on-board years. Of course, alien perceptions of time might assay this period to be fleeting moments, or eons; the proportion of time between starship and home is nonetheless the same. Every deep-space astronaut must live with the fact that time at home will pass much more rapidly than it will for him. The credo of all star travelers, whatever their physical constitution, is permanent farewell.
6. Terminal Space: A Black Hole
We are traveling swiftly through interstellar space when we realize something has gone wrong. The navigator reports unaccountable shifts in the apparent positions of his reference stars. Radio communication from the home planet shifts lower in frequency than the amount we calculate for our speed. The stars begin to turn red.
Unable to alter our course at such high velocity, we see that we are doomed. Radio voices from home slow down like sounds from a broken phonograph and finally stop. We tune to beacons from other worlds; they do the same. The stars, now cherry red, inch toward one another; ultimately they merge into a ball and vanish. It would seem that we are witnessing the death of the universe, but instead it is our own. We have fallen into a black hole. With our ship we will shortly be stripped to subatomic particles and will vanish from the universe.
A healthy star like the sun shines steadily because it maintains a balance of forces: Gravity pulls in and radiation (light, heat and so forth) pushes out.
But when radiation falters and no longer balances the force of gravity, a star may collapse. If it stops at one stage it becomes a white dwarf—small, massive, still radiating light. Many of these dwarf stars are known to exist; Sirius B is one.
If the collapse goes on the star may become so dense that its concentrated gravitational force exceeds even the force that binds atoms together. For years such neutron stars were postulated but none was found in the sky. Now it appears almost certain that pulsars, first observed in 1967, are in fact the predicted neutron stars.
The extreme case of gravitational collapse is the black hole. Here the star becomes so dense, its gravity so intense, that even its own light cannot leave it. In other words, the star’s escape velocity becomes greater than the speed of light, so nothing can escape.
Invisible, a black hole will suck in anything that passes too close, whether it is light, gas, dust or starships. A spaceman caught in the grip of a black hole can relinquish hope, whether he has 20 arms, telepathic consciousness, or bones of solid gold.
What happens in the vicinity of a black hole is nothing less than the collapse of space and time. In the theory of relativity, the stars are seen to reside not in the three-dimensional space of Newton but in the four-dimensional space-time continuum of Einstein. Stars (and all other matter) create what might be called eddies in the continuum; these eddies are equivalent to what we call—in conventional, Newtonian terms—gravitational fields.
To visualize this four-dimensional effect is impossible, but we can create a three-dimensional model of it if we imagine each star sitting in the bottom of a whirlpool or depression. Planets orbit along the inside of the depression like ball bearings rolling around in a funnel. A beam of light which crosses part of the eddy departs on a slightly different path, like a billiard ball rolling across a hollow on a bad pool table. Each object follows its easiest course, or geodesic, across the contours of space-time.
On first encounter many people find the notion of a space-time continuum mysterious, because it cannot be visualized; or superfluous, because after all we can get along fine on earth, and even go to the moon, without worrying about it. But generally speaking, relativity is a tool for dealing with great volumes of space and extreme velocities. On earth its influence is subtle. In deep space it becomes essential. The most tradition-bound thinker, put at the controls of a starship thousands of light years from home, would have to quickly start thinking in terms of relativity or else reconcile himself to oblivion.
A black hole creates a very severe eddy. Falling into one, an astronaut would witness a profound warp in the space-time continuum, which is why it would look to him as if the whole universe were collapsing. In fact, the fabric through which he views the universe is what would be collapsing. Entering the fringes of this warp he could, with a powerful ship, still escape. After a certain point he could not. In all directions, the grid lines of space and time would converge, and he could no more evade destruction than he could stop the passage of time.
Some astronomers have been predicting the discovery of a black hole in space, and within the past few months one object has been observed which seems to be one. So the transition from theory to reality may be close at hand.
One troublesome thing about the black hole theory is that once something drops into one it disappears from the universe forever. Nothing short of the collapse of the entire universe—doomsday—can unlock the matter that has fallen into a black hole. This makes black holes unique in the known universe, and it makes physicists unhappy.
To deal with this quirk it has been suggested that matter which drops into black holes may reappear elsewhere in the universe through “white holes.” This might happen if the space-time warp of a black hole was so severe that it bent the continuum until two distant points in the universe touched like paper crumpled together. What vanished in one place might be spewed out at another, perhaps millions of light years away.
But even if this were true it would not make black holes less frightening to space voyagers. The forces involved in a black hole reduce everything to simple particles, and as particles everything would emerge. If black holes are a gate from one part of the universe to another, they are a gate through which no one can pass intact.
7. All Space: Cosmological Perspectives
“No brick and mortar, no steel girders are required to keep the universe in its place, because everything which is is in its place.”
The universe originated in a single explosion ten billion years ago. Condensed from the hurtling gas of this explosion, the galaxies today continue to rush away from one another. This expansion may go on indefinitely or it may eventually be overcome by the combined gravity of all the galaxies, which would then pull to a stop and ultimately gather back together into a single unity, again to explode, producing a new universe. We do not yet know whether this will happen, but if it does nothing will survive it; there could be no way of knowing how many times the universe has been born in the past or what new universes might be like in the future.
As the universe expands, space and time unfold with it. There are a finite number of galaxies in the universe, yet there is no boundary to it: Everyone everywhere sees galaxies in all directions, and anyone can travel endlessly in any direction without reaching an end to space, though eventually he will have visited every star. If the universe collapses in the future, space and time will collapse with it.
The ultimate question which can be asked about physical reality is the cosmological question: What is the universe like? The above model, rather widely if tentatively supported, represents a modern attempt to answer it.
In all likelihood this cosmology is wrong, though perhaps less wrong than others which have been put forward. All cosmologies are audacious; they are constructed out of the admittedly dwarfed, crippled pictures of the universe which even the most brilliant humans carry in their minds. The tortured effort by which men have built observation and thought to the relative sophistication at which these theories now stand is a credential which may be brought forth on the day humans encounter another race of beings.
The conviction remains that there is an ultimate reality to the universe and that it will be accessible to human thought. This reality may turn out to be alien to anything we have contemplated so far. We do not see the world as did the Egyptians, the Greeks or the Chinese, and it is unlikely that humans of the future will see it as we do.
People may learn to perceive within a framework unknown to us today. The entire human community may come to see and think as a single planetary brain, to which the mysteries of cosmology might become knowable. Or the answer may already be known somewhere in terms meaningful to us, by beings we need only contact to learn it. That would be another story.