by Carl Sagan
Since Aristarchus, every step in our quest has moved us farther from center stage in the cosmic drama. There has not been much time to assimilate these new findings. The discoveries of Shapley and Hubble were made within the lifetimes of many people still alive today. There are those who secretly deplore these great discoveries, who consider every step a demotion, who in their heart of hearts still pine for a universe whose center, focus and fulcrum is the Earth. But if we are to deal with the Cosmos we must first understand it, even if our hopes for some unearned preferential status are, in the process, contravened. Understanding where we live is an essential precondition for improving the neighborhood. Knowing what other neighborhoods are like also helps. If we long for our planet to be important, there is something we can do about it. We make our world significant by the courage of our questions and by the depth of our answers.
We embarked on our cosmic voyage with a question first framed in the childhood of our species and in each generation asked anew with undiminished wonder: What are the stars? Exploration is in our nature. We began as wanderers, and we are wanderers still. We have lingered long enough on the shores of the cosmic ocean. We are ready at last to set sail for the stars.
*This sense of fire as a living thing, to be protected and cared for, should not be dismissed as a “primitive” notion. It is to be found near the root of many modern civilizations. Every home in ancient Greece and Rome and among the Brahmans of ancient India had a hearth and a set of prescribed rules for caring for the flame. At night the coals were covered with ashes for insulation; in the morning twigs were added to revive the flame. The death of the flame in the hearth was considered synonymous with the death of the family. In all three cultures, the hearth ritual was connected with the worship of ancestors. This is the origin of the eternal flame, a symbol still widely employed in religious, memorial, political and athletic ceremonials throughout the world.
*The exclamation point is a click, made by touching the tongue against the inside of the incisors, and simultaneously pronouncing the K.
*As an aid to confusion, Ionia is not in the Ionian Sea; it was named by colonists from the coast of the Ionian Sea.
*There is some evidence that the antecedent, early Sumerian creation myths were largely naturalistic explanations, later codified around 1000 B.C. in the Enuma elish (“When on high,” the first words of the poem); but by then the gods had replaced Nature, and the myths offers a theogony, not a cosmogony. The Enuma elish is reminiscent of the Japanese and Ainu myths in which an originally muddy cosmos is beaten by the wings of a bird, separating the land from the water. A Fijian creation myth says: “Rokomautu created the land. He scooped it up out of the bottom of the ocean in great handfuls and accumulated it in piles here and there. These are the Fiji Islands.” The distillation of land from water is a natural enough idea for island and seafaring peoples.
*And astrology, which was then widely regarded as a science. In a typical passage, Hippocrates writes: “One must also guard against the risings of the stars, especially of the Dog Star [Sirius], then of Arcturus, and also of the setting of the Pleiades.”
†The experiment was performed in support of a totally erroneous theory of the circulation of the blood, but the idea of performing any experiment to probe Nature is the important innovation.
*The frontiers of the calculus were also later breached by Eudoxus and Archimedes.
*The sixth century B.C. was a time of remarkable intellectual and spiritual ferment across the planet. Not only was it the time of Thales, Anaximander, Pythagoras and others in Ionia, but also the time of the Egyptian Pharaoh Necho who caused Africa to be circumnavigated, of Zoroaster in Persia, Confucius and Lao-tse in China, the Jewish prophets in Israel, Egypt and Babylon, and Gautama Buddha in India. It is hard to think these activities altogether unrelated.
*Although there were a few welcome exceptions. The Pythagorean fascination with whole-number ratios in musical harmonies seems clearly to be based on observation, or even experiment on the sounds issued from plucked strings. Empedocles was, at least in part, a Pythagorean. One of Pythagoras’ students, Alcmaeon, is the first person known to have dissected a human body; he distinguished between arteries and veins, was the first to discover the optic nerve and the eustachian tubes, and identified the brain as the seat of the intellect (a contention later denied by Aristotle, who placed intelligence in the heart, and then revived by Herophilus of Chalcedon). He also founded the science of embryology. But Alcmaeon’s zest for the impure was not shared by most of his Pythagorean colleagues in later times.
*A Pythagorean named Hippasus published the secret of the “sphere with twelve pentagons,” the dodecahedron. When he later died in a shipwreck, we are told, his fellow Pythagoreans remarked on the justice of the punishment. His book has not survived.
*Copernicus may have gotten the idea from reading about Aristarchus. Recently discovered classical texts were a source of great excitement in Italian universities when Copernicus went to medical school there. In the manuscript of his book, Copernicus mentioned Aristarchus’ priority, but he omitted the citation before the book saw print. Copernicus wrote in a letter to Pope Paul III: “According to Cicero, Nicetas had thought the Earth was moved … According to Plutarch [who discusses Aristarchus] … certain others had held the same opinion. When from this, therefore, I had conceived its possibility, I myself also began to meditate upon the mobility of the Earth.”
*Huygens actually used a glass bead to reduce the amount of light passed by the hole.
*This supposed privileged position of the Earth, at the center of what was then considered the known universe, led A. R. Wallace to the anti-Aristarchian position, in his book Man’s Place in the Universe (1903), that ours may be the only inhabited planet.
CHAPTER VIII
TRAVELS IN SPACE AND TIME
We have loved the stars too fondly to be fearful of the night.
—Tombstone epitaph of two amateur astronomers
The rising and falling of the surf is produced in part by tides. The Moon and the Sun are far away. But their gravitational influence is very real and noticeable back here on Earth. The beach reminds us of space. Fine sand grains, all more or less uniform in size, have been produced from larger rocks through ages of jostling and rubbing, abrasion and erosion, again driven through waves and weather by the distant Moon and Sun. The beach also reminds us of time. The world is much older than the human species.
A handful of sand contains about 10,000 grains, more than the number of stars we can see with the naked eye on a clear night. But the number of stars we can see is only the tiniest fraction of the number of stars that are. What we see at night is the merest smattering of the nearest stars. Meanwhile the Cosmos is rich beyond measure: the total number of stars in the universe is greater than all the grains of sand on all the beaches of the planet Earth.
Despite the efforts of ancient astronomers and astrologers to put pictures in the skies, a constellation is nothing more than an arbitrary grouping of stars, composed of intrinsically dim stars that seem to us bright because they are nearby, and intrinsically brighter stars that are somewhat more distant. All places on Earth are, to high precision, the same distance from any star. This is why the star patterns in a given constellation do not change as we go from, say, Soviet Central Asia to the American Midwest. Astronomically, the U.S.S.R. and the United States are the same place. The stars in any constellation are all so far away that we cannot recognize them as a three-dimensional configuration as long as we are tied to Earth. The average distance between the stars is a few light-years, a light-year being, we remember, about ten trillion kilometers. For the patterns of the constellations to change, we must travel over distances comparable to those that separate the stars; we must venture across the light-years. Then some nearby stars will seem to move out of the constellation, others will enter it, and its configuration will alter dramatically.
Our technology is, so far, utterly incapable of such grand interstel
lar voyages, at least in reasonable transit times. But our computers can be taught the three-dimensional positions of all the nearby stars, and we can ask to be taken on a little trip—a circumnavigation of the collection of bright stars that constitute the Big Dipper, say—and watch the constellations change. We connect the stars in typical constellations, in the usual celestial follow-the-dots drawings. As we change our perspective, we see their apparent shapes distort severely. The inhabitants of the planets of distant stars witness quite different constellations in their night skies than we do in ours—other Rorschach tests for other minds. Perhaps sometime in the next few centuries a spaceship from Earth will actually travel such distances at some remarkable speed and see new constellations that no human has ever viewed before—except with such a computer.
The Big Dipper, as seen from the Earth (top left), from the back (top right) and from the side (right). The last two views would be seen if we were able to travel to the proper vantage points, about 150 light-years away.
The appearance of the constellations changes not only in space but also in time; not only if we alter our position but also if we merely wait sufficiently long. Sometimes stars move together in a group or cluster; other times a single star may move very rapidly with respect to its fellows. Eventually such stars leave an old constellation and enter a new one. Occasionally, one member of a double-star system explodes, breaking the gravitational shackles that bound its companion, which then leaps into space at its former orbital velocity, a slingshot in the sky. In addition, stars are born, stars evolve, and stars die. If we wait long enough, new stars appear and old stars vanish. The patterns in the sky slowly melt and alter.
Even over the lifetime of the human species—a few million years—constellations have been changing. Consider the present configuration of the Big Dipper, or Great Bear. Our computer can carry us in time as well as in space. As we run the Big Dipper backwards into the past, allowing for the motion of its stars, we find quite a different appearance a million years ago. The Big Dipper then looked quite a bit like a spear. If a time machine dropped you precipitously in some unknown age in the distant past, you could in principle determine the epoch by the configuration of the stars: If the Big Dipper is a spear, this must be the Middle Pleistocene.
Computer-generated images of the Big Dipper as it would have been seen on Earth one million years ago and half a million years ago. Its present appearance is shown at bottom.
We can also ask the computer to run a constellation forward into time. Consider Leo the Lion. The zodiac is a band of twelve constellations seemingly wrapped around the sky in the apparent annual path of the Sun through the heavens. The root of the word is that for zoo, because the zodiacal constellations, like Leo, are mainly fancied to be animals. A million years from now, Leo will look still less like a lion than it does today. Perhaps our remote descendants will call it the constellation of the radio telescope—although I suspect a million years from now the radio telescope will have become more obsolete than the stone spear is now.
The (nonzodiacal) constellation of Orion, the hunter, is outlined by four bright stars and bisected by a diagonal line of three stars, which represent the belt of the hunter. Three dimmer stars hanging from the belt are, according to the conventional astronomical projective test, Orion’s sword. The middle star in the sword is not actually a star but a great cloud of gas called the Orion Nebula, in which stars are being born. Many of the stars in Orion are hot and young, evolving rapidly and ending their lives in colossal cosmic explosions called supernovae. They are born and die in periods of tens of millions of years. If, on our computer, we were to run Orion rapidly into the far future, we would see a startling effect, the births and spectacular deaths of many of its stars, flashing on and winking off like fireflies in the night.
The solar neighborhood, the immediate environs of the Sun in space, includes the nearest star system, Alpha Centauri. It is really a triple system, two stars revolving around each other, and a third, Proxima Centauri, orbiting the pair at a discreet distance. At some positions in its orbit, Proxima is the closest known star to the Sun—hence its name. Most stars in the sky are members of double or multiple star systems. Our solitary Sun is something of an anomaly.
The second brightest star in the constellation Andromeda, called Beta Andromedae, is seventy-five light-years away. The light by which we see it now has spent seventy-five years traversing the dark of interstellar space on its long journey to Earth. In the unlikely event that Beta Andromedae blew itself up last Tuesday, we would not know it for another seventy-five years, as this interesting information, traveling at the speed of light, would require seventy-five years to cross the enormous interstellar distances. When the light by which we now see this star set out on its long voyage, the young Albert Einstein, working as a Swiss patent clerk, had just published his epochal special theory of relativity here on Earth.
Space and time are interwoven. We cannot look out into space without looking back into time. Light travels very fast. But space is very empty, and the stars are far apart. Distances of seventy-five light-years or less are very small compared to other distances in astronomy. From the Sun to the center of the Milky Way Galaxy is 30,000 light-years. From our galaxy to the nearest spiral galaxy, M31, also in the constellation Andromeda, is 2,000,000 light-years. When the light we see today from M31 left for Earth, there were no humans on our planet, although our ancestors were evolving rapidly to our present form. The distance from the Earth to the most remote quasars is eight or ten billion light-years. We see them today as they were before the Earth accumulated, before the Milky Way was formed.
This is not a situation restricted to astronomical objects, but only astronomical objects are so far away that the finite speed of light becomes important. If you are looking at a friend three meters (ten feet) away, at the other end of the room, you are not seeing her as she is “now”; but rather as she “was” a hundred millionth of a second ago. [(3 m) / (3 × 108 m/sec) = 1/(108 / sec) = 10–8 sec, or a hundredth of a microsecond. In this calculation we have merely divided the distance by the speed to get the travel time.] But the difference between your friend “now” and now minus a hundred-millionth of a second is too small to notice. On the other hand, when we look at a quasar eight billion light-years away, the fact that we are seeing it as it was eight billion years ago may be very important. (For example, there are those who think that quasars are explosive events likely to happen only in the early history of galaxies. In that case, the more distant the galaxy, the earlier in its history we are observing it, and the more likely it is that we should see it as a quasar. Indeed, the number of quasars increases as we look to distances of more than about five billion light-years).
The two Voyager interstellar spacecraft, the fastest machines ever launched from Earth, are now traveling at one ten-thousandth the speed of light. They would need 40,000 years to go the distance to the nearest star. Do we have any hope of leaving Earth and traversing the immense distances even to Proxima Centauri in convenient periods of time? Can we do something to approach the speed of light? What is magic about the speed of light? Might we someday be able to go faster than that?
If you had walked through the pleasant Tuscan countryside in the 1890’s, you might have come upon a somewhat long-haired teenage high school dropout on the road to Pavia. His teachers in Germany had told him that he would never amount to anything, that his questions destroyed classroom discipline, that he would be better off out of school. So he left and wandered, delighting in the freedom of Northern Italy, where he could ruminate on matters remote from the subjects he had been force-fed in his highly disciplined Prussian schoolroom. His name was Albert Einstein, and his ruminations changed the world.
Einstein had been fascinated by Bernstein’s People’s Book of Natural Science, a popularization of science that described on its very first page the astonishing speed of electricity through wires and light through space. He wondered what the world would look like if you could t
ravel on a wave of light. To travel at the speed of light? What an engaging and magical thought for a boy on the road in a countryside dappled and rippling in sunlight. You could not tell you were on a light wave if you traveled with it. If you started on a wave crest, you would stay on the crest and lose all notion of it being a wave. Something strange happens at the speed of light. The more Einstein thought about such questions, the more troubling they became. Paradoxes seemed to emerge everywhere if you could travel at the speed of light. Certain ideas had been accepted as true without sufficiently careful thought. Einstein posed simple questions that could have been asked centuries earlier. For example, what do we mean when we say that two events are simultaneous?
Imagine that I am riding a bicycle toward you. As I approach an intersection I nearly collide, so it seems to me, with a horse-drawn cart. I swerve and barely avoid being run over. Now think of the event again, and imagine that the cart and the bicycle are both traveling close to the speed of light. If you are standing down the road, the cart is traveling at right angles to your line of sight. You see me, by reflected sunlight, traveling toward you. Would not my speed be added to the speed of light, so that my image would get to you considerably before the image of the cart? Should you not see me swerve before you see the cart arrive? Can the cart and I approach the intersection simultaneously from my point of view, but not from yours? Could I experience a near collision with the cart while you perhaps see me swerve around nothing and pedal cheerfully on toward the town of Vinci? These are curious and subtle questions. They challenge the obvious. There is a reason that no one thought of them before Einstein. From such elementary questions, Einstein produced a fundamental rethinking of the world, a revolution in physics.