A Personal Voyage: Journeys in Space and Time, by Carl Sagan

A Personal Voyage: Journeys in Space and Time, by Carl Sagan

Postby admin » Thu Mar 17, 2016 9:15 am

by Carl Sagan
[Part of the "Cosmos" series, transcribed by Tara Carreon]



We are drifting in a great ocean of space and time. In that ocean, the events that shape the future are working themselves out. Each creature in every world, to the remotest star, owes their existence to the great, coursing, implacable forces of nature, but also to minor happenstance. We are carried with our planet around the sun. The earth has made more than 4 billion circuits of our star since its origin. The sun itself travels about the core of the Milky Way galaxy. Our galaxy is moving among the other galaxies. We have always been space travelers.

These fine sand grains are all more or less uniform in size. They have been produced from bigger rocks through ages of jostling and rubbing, abrasion and erosion, driven in part by the distant moon and sun. So the roots of the present lie buried in the past. We are also travelers in time.

But trapped on earth, we've had little to say about where we're going in time and space, or how fast. But now we're thinking about true journeys in time, and real voyages to the distant stars.

A handful of sand contains about 10,000 grains, more than the total 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 with a few more distant, bright stars thrown in for good measure. Meanwhile, the cosmos is rich beyond measure. The total number of stars in the universe is larger than all the grains of sand on all the beaches of the planet earth.

Long ago, before we had figured out that the stars are distant suns, they seemed to us to make pictures in the sky. Just follow the dots. The constellation called the Big Dipper today in North America has had many other incarnations. Every culture, ancient and modern, has placed its totems and concerns among the stars, from a Chinese bureaucrat to a German wagon. But very ancient cultures would have seen different constellations because the stars move with respect to one another. We can give a computer the present three-dimensional positions and motions of nearby stars and then run the patterns back into time.

Every constellation is a single frame in a cosmic movie, but because our lives are so short, because the star patterns change so slowly, we tend not to notice it's a movie. A million years ago there was no big dipper. Our ancestors, looking up and wondering about the stars, saw some other pattern in the northern skies.

We can also run a constellation, Leo the Lion say, forward in time, and see what the patterns in the stars will be in the future. A million years from now, Leo might be renamed the constellation of the radio telescope, although I suspect that radio telescopes then will be as obsolete as stone spears are now. Or here's the constellation of Cetus the whale. A million years ago it may have been called something else, perhaps the spear.

Now let's run fast forward through a billion nights. Millions of years from now, some other, very different image will be featured in this cosmic movie. In Orion the Hunter, things are changing not only because the stars are moving, but also because the stars are evolving. Many of Orion's stars are hot, young and short-lived. They are born, live and die within a span of only a few million years. If we run Orion forward in time, we see the births and explosive deaths of dozens of stars flashing on and winking off like fireflies in the night. If we wait long enough, we see the constellations change. But if we go far enough, we also see the star patterns alter. The two-dimensional constellations are only the appearance of stars strewn through three dimensions. Some are dim and near; others are bright but farther away.

Could a space traveler actually see the patterns of the constellations change? For that, you must travel roughly as far as the constellation is from us. Here, we're traveling hundreds of light years, circling all around the stars of the Big Dipper. The inhabitants of planets around other stars will see very different constellations than we do, because their vantage points are different.

Here we are in the constellation Andromeda -- or at least a model of it -- next to the constellation Perseus. Andromeda, in the Greek myth, was the maiden who was saved by Perseus from a sea monster. This star just above me is Beta Andromedae, the second brightest star in the constellation, 75 light years from earth. The light by which we see this star has spent 75 years traversing interstellar space on its journey to the earth. In the unlikely event that Beta Andromedae blew itself up a week ago Tuesday, we will not know of it for another 75 years as this interesting information traveling at the speed of light travels the enormous interstellar distances. When the light we see from this star set out on its long interstellar voyage, the young Albert Einstein, working as a Swiss patent clerk, had just published his epochal Special Theory of Relativity here on earth.

We see that space and time are intertwined. We cannot look out into space without looking back into time. The speed of light is very fast, but space is very empty, and the stars are very far apart. In fact, the distances that we've been talking about up to now are very small by the usual astronomical standards. In fact, the distance from the earth to the center of the Milky Way galaxy is 30,000 light years. From our galaxy to the nearest spiral galaxy like our own, called M-31, and which is also within, that means behind, the constellation Andromeda, is 2 million light years. When the light we see today from M-31 left on its journey for earth, there were no human beings on the earth, although our ancestors were nicely evolving and very rapidly to our present form.

There are much greater distances in astronomy. The distance from the earth to the most distant quasar is 8 or 10 billion light years. We see them as they were before the earth itself accumulated, before the Milky Way galaxy was formed. The fastest space vehicles ever launched by the human species are the Voyager spacecraft. They are traveling so fast that it is only 10,000 times slower than the speed of light. The Voyager spacecraft will take 40,000 years to go the distance to the nearest stars, and it is not even headed toward the nearest stars. But is there a method by which we could travel in a conveniently short time to the stars? Can we travel close to the speed of light. And what's magic about the speed of light? Can't we travel faster than that? It turns out that there is something very strange about the speed of light, something that provides a key to our understanding of time and space. The story of its discovery takes us to Tuscany in Northern Italy.

There's something almost timeless about this place. A century ago it probably looked very much the same. If you had traveled these roads in the summer of 1895, you might have come upon a 16-yr. old German high school drop out. His teacher had told him that he would never amount to anything, that his attitude destroyed classroom discipline, that he'd be better off out of school. So he left and came here, where he enjoyed wandering these roads and giving his mind free reign to explore.

One day he began to think about light, about how fast it travels. In our everyday life, we always measure the speed of a moving object relative to something else. I'm moving at about 10 km/hr. relative to the ground. But the ground isn't at rest. The earth is turning at more than 1600 km/hr. The earth itself is in orbit around the sun. The sun is moving among the drifting stars, and so on. It was hard for the young man to imagine some absolute standard to measure all these relative motions against.

He knew that sound waves are a vibration of the air, and their speed is measured relative to the air itself. But sunlight travels across the vacuum of empty space. Do light waves move relative to something else? And if so, he wondered, relative to what?

That teenage dropout's name was Albert Einstein, and his ruminations changed the world. He had been fascinated by Bernstein's 1869 "People's Book of Natural Science." Here, on its very first page, it describes the astonishing speed of electricity through wires, and light through space. Einstein wondered, perhaps for the first time here in Northern Italy, what the world would look like if you could travel on a wave of light, to travel at the speed of light. What an engaging and magical thought for a teenage boy on the road where the countryside is dappled and rippling in sunlight.

You couldn't tell you were on a light wave if you were traveling 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 funny happens at the speed of light.

The more Einstein thought about such questions, the more troubling they became. Paradoxes seemed to pop up all over if you could travel at the speed of light. Certain ideas had been accepted as true without sufficiently careful thought. One of these ideas had to do with the light from a moving object.

The images by which we see the world are made of light and are carried at the speed of light: 300,000 km/sec. You might think that the image of me should be moving out ahead of me at the speed of light plus the speed of the bicycle. If I'm moving towards you faster than a horse and cart, then my image should be approaching you exactly that much faster. My image ought to arrive earlier. But in reality, you don't see any time delay. In a near-collision, for example, you always see everything happen at once: horse, cart, swerve, bicycle, all simultaneous.

But how would it look if it were proper to add the velocities? Since I'm heading towards you, you would add my speed to the speed of light. So my image ought to arrive before the image of the horse and cart. I'd be cycling toward you quite normally. To me, the collision would suddenly seem imminent. But you would see me swerve for no apparent reason and have a collision with nothing.

Now the horse and cart aren't headed toward you. Their image would arrive only at the speed of light. Could it seem to me that I just missed colliding, while to you it wasn't even close? In precise laboratory experiments scientists have never observed any such thing. If the world is to be understood, if we are to avoid logical paradoxes when traveling at high speeds, then there are certain rules which must be obeyed. Einstein called these rules the special theory of relativity. Light from a moving object travels at the same speed no matter whether the object is at rest or in motion. Thou Shalt Not Add My Speed To The Speed of Light. Also, no material object can travel at or beyond the speed of light. There is nothing in physics that prevents you from traveling as close to the speed of light as you like, 99.9% the speed of light is just fine. But no matter how hard you try, you can never gain that last decimal point. For the world to be logically consistent, there must be a cosmic speed limit.

The crack of a whip is due to its tip moving faster than the speed of sound. It makes a shock wave, a small sonic boom in the Italian countryside. A thunder clap has a similar origin. So does the sound of a supersonic airplane. So why is the speed of light a barrier, any more than the speed of sound? The answer is not just that light travels about a million times faster than sound. It's not merely an engineering problem, like the supersonic airplane. Instead, the light barrier is a fundamental law of nature, as basic as gravity. Einstein found his absolute framework for the world, this sturdy pillar among all the relative motions of the cosmos, light travels just as fast no matter how it's source is moving. The speed of light is constant relative to everything else. Nothing can ever catch up to light.

Einstein's prohibition against traveling faster than light seems to clash with our common sense notions. But why should we expect our common sense notions to have any reliability in a matter of this sort? Why should our experience at 10 km/hr. constrain the laws of nature at 300,000 km/sec? Relativity sets limits on what humans ultimately can do. The universe is not required to be in perfect harmony with human ambition.

Imagine a place where the speed of light is not its true value of 300,000 km/sec, but something a lot less. Let's say like 40 km/hr., and strictly enforced.

Just as in the real world we can never reach the speed of light, the commandment here is still Thou Shalt Not Travel Faster Than Light. But we can do thought experiments on what happens near the speed of light, here, 40 km/hr., the speed of a motor-scooter. You can't break the laws of nature. There are no penalties for doing so. The real world, and this one, are merely so arranged that transgressions can't happen. The job of physics is to find out what those laws are.

Before Einstein, physicists thought there were privileged frames of reference, some special places and times against which everything else had to be measured. Einstein encountered a similar notion in human affairs, the idea that the customs of a particular nation, his native Germany or Italy or anywhere, are the standard against which all other societies must be measured. But Einstein rejected the strident nationalism of his time. He believed every culture had its own validity, and also in physics he understood that there are no privileged frames of reference. Every observer, in any place, time or motion must deduce the same laws of nature. A speed is simply how much space you cover in a given time. As any kid on a motor-scooter knows.

Since near the velocity of light we cannot simply add speeds, the familiar notions of absolute space and absolute time independent of your relative motion must give way. That's why, as Einstein showed, funny things have to happen close to the speed of light. There are conventional perspectives of space and time that strangely change.

Your nose is just a little closer to me than your ears. Light reflected off your nose reaches me just an instant in time before your ears. But suppose I had a magic camera so that I could see your nose and your ears at precisely the same instant. With such a camera you could take some pretty interesting pictures.

Paolo says goodbye to his little brother Vincenzo and rides off. He's now going more than half the speed of light. He's almost catching up with his own light rays. This compresses the light waves in front of him and his image becomes blue. The shorter wave length is what makes blue light waves blue. Also, Paolo becomes skinny in the direction of motion. This isn't just some optical illusion. It really happens when you travel near the speed of light. As he roars away, he leaves his own light waves stretched out behind him. Long light rays are red. We say his receding image is red-shifted.

Now Paolo leaves for a short tour of the countryside. He experiences something even stranger. Everything he can see is squeezed into a moving window just ahead of him. Blue shifted at the center, red shifted at the edges. To a passerby, Paolo appears blue shifted when approaching and red shifted when receding, but to him the entire world is both coming and going at nearly the speed of light. Roadside houses and trees that he has already gone past still appear to him at the edge of his forward field of view, but distorted and red shifted. When he slows down, everything again looks normal. Only very close to the speed of light does the visible world get squeezed into a kind of tunnel. You would really see these distortions if you could travel near the speed of light. Someday perhaps interstellar navigators will take their bearings on stars behind them whose images have all crowded together on the forward view screen.

The most bizarre aspect of traveling near the speed of light is that time slows down. All clocks, mechanical and biological, tick more slowly near the speed of light, but stationary clocks tick at their usual rate. If we travel close to light speed, we age more slowly than those we left behind. Paolo's watch, and his internal sense of time, show that he's been gone from his friends for only a few minutes, but from their point of view he's been away for many decades. His friends have grown up, moved on and died. And his younger brother has been patiently waiting for him all this time. The two brothers experience the paradox of time dilation. They have encountered Einstein's Special Relativity.

This was just a thought experiment, but atomic particles traveling near the speed of light do decay more slowly than stationary particles. As strange and counter-intuitive as it seems, time dilation is a law of nature.

Traveling close to the speed of light is a kind of elixir of life. Because time slows down close to the speed of light, Special Relativity provides us with a means of going to the stars.

This region of northern Italy is not only the cauldron of some of the thinking of the young Albert Einstein, it is also the home of another great genius who lived 400 years earlier: Leonardo de Vinci.

Leonardo delighted in climbing these hills and viewing the ground from a great height as if he were soaring like a bird. He drew the first aerial views of landscapes, villages, fortifications. I've been talking about Einstein in and around this town of Vinci in which Leonardo grew up. Einstein greatly respected Leonardo, and their spirits, in some sense, inhabit this countryside still.

Among Leonardo's many accomplishments in painting, sculpture, architecture, natural history, anatomy, geology, civil and military engineering, he had a great passion: he wished to construct a machine which would fly. He made sketches of such machines, built miniature models, constructed great, full-scale prototypes, and not a one of them ever worked. Mainly because there were no machines of adequate capacity available in his time. The technology was just not ready. The designs, however, were brilliant. For example, this bird-like machine here in the Leonardo museum in the town of Vinci. Leonardo's great designs encouraged engineers in later epochs, although Leonardo himself was very depressed at these failures. But it's not his fault. He was trapped in the 15th century.

A somewhat similar case occurred in 1939 when a group of engineers calling themselves the British Interplanetary Society decided to design a ship that would carry people to the moon. Now it was by no means the same design as the Apollo ship which actually took people to the moon some years later, but that design suggested that a mission to the moon might one day be a practical engineering possibility. Today, we have preliminary designs of ships which will take people to the stars. They are constructed in earth orbit and from there they venture on their great interstellar journeys. One of them is called Project Orion. It utilizes nuclear weapons, hydrogen bombs against an inertial plate, each explosion providing a kind of put-put, a fast nuclear motorboat in space. Orion seems entirely practical, and was under serious development in the U.S. until the signing of the international treaty forbidding nuclear weapons explosions in space. Personally, the Orion starship is the best use of nuclear weapons I can think of, provided the ships don't depart from very near the earth. Project Daedalus is a recent design of the British Interplanetary Society. It assumes the existence of a nuclear fusion reactor, something much safer, as well as more efficient, than the existing nuclear fission power plants. We do not yet have fusion reactors. One day quite soon we may.

Orion and Daedalus might go 10% the speed of light. So a trip to Alpha Centauri -- 4-1/2 light years away -- would take 45 years -- less than a human lifetime. Such ships could not travel close enough to the speed of light for the time slowing effects of special relativity to become important. It does not seem likely that such ships would be built before the middle of the 21st century, although we could build an Orion starship now. For voyages beyond the nearest stars, something must be added. Perhaps they could be used as multi-generation ships, so those arriving would be the remote descendants of those who had originally set out centuries before. Or perhaps some safe means of human hibernation might be found so that the space travelers might be frozen and then thawed out when they arrive at their destination centuries later. But fast interstellar space flight, approaching the speed of light, is much more difficult. That's an objective not for a hundred years, but for a thousand, or ten thousand. But it also is possible. A kind of interstellar ramjet has been proposed which scoops up the hydrogen atoms that float between the stars, accelerates them into an engine, and spits them out the back. But in deep space, there is only one atom for every 10 cubic centimeters of space. For the ramjet to work, it has to have a frontal scoop hundreds of kilometers across. When the ship reaches relativistic velocities, the hydrogen atoms will be moving with respect to the interstellar spaceship at close to the speed of light. If precautions aren't taken, the passengers will be fried by these induced cosmic rays. There's a proposed solution: a laser is used to strip electrons off of the atoms and make them electrically charged while they are still some distance away. And an extremely strong magnetic field is used to deflect the charged atoms into the scoop and away from the rest of the spacecraft. This is engineering on a scale so far unprecedented on the earth. We are talking of engines the size of small worlds.

Suppose that the spacecraft is designed to accelerate at 1 G, so we'd be comfortable aboard it. We'd be going closer and closer to the speed of light until the midpoint of the journey. Then, the spacecraft is turned around and we decelerate at 1 G to the destination. For most of the trip, the velocity would be very close to the speed of light, and time would slow down enormously. By how much? Barnard Star could be reached by such a ship in eight years, ship-time. The center of the Milky Way galaxy, in 21 years. The Andromeda galaxy in 28 years. Of course, the people left behind on the earth would see things somewhat differently. Instead of 21 years to the center of the galaxy, they would measure it as 30,000 years. When we got back, very few of our friends would be around to greet us.

In principle, such a journey -- mounting the decimal points closer and closer to the speed of light -- would even permit us to circumnavigate the known universe in 56 years ship-time. We would return tens of billions of years in the far future, with the earth a charred cinder and the sun dead. Relativistic space flight makes the universe accessible to advanced civilizations, but only to those who go on the journey, not to those who stay home.

These designs are probably further from the actual interstellar spacecraft of the future than Leonardo's models are from the supersonic transports of the present. But if we do not destroy ourselves, I believe that we will one day venture to the stars. When our solar system is all exploded, the planets of other stars will beckon.

Space travel and time travel are connected. To travel fast into space is to travel fast into the future. We travel into the future, although slowly, all the time. But what about the past? Could we journey into yesterday? Many physicists think that this is fundamentally impossible, that there is no way we could build a device which would carry us backwards into time. Some say that even if we were to build such a device, it wouldn't do us much good, that we couldn't significantly affect the past.

For example, suppose you traveled into the past and somehow or other prevented your own parents from meeting. Why then you would probably never have been born, which is something of a contradiction, isn't it, since you're clearly there? Other people think that the two alternative histories have equal validity, that they are parallel threads, skeins of time, that they could exist side by side: the history in which you were never born and the history that you know all about. Perhaps time itself has many potential dimensions, despite the fact that we are condemned to experience only one of those dimensions.

Now suppose you could go back into the past and really change it by, oh, let's say, something like persuading Queen Isabella not to bankroll Christopher Columbus. Then you would have set into motion a different sequence of historical events, which those people you left behind you in our time would never get to know about. If that kind of time travel were possible, then every imaginable sequence of alternative history might in some sense really exist.

Would it be possible for a time traveler to change the course of history in a major way? Well, let's think of that. History consists, for the most part, of a complex multitude of deeply interwoven threads: biological, economic and social forces that are not so easily unraveled. The ancient Greeks imagined the course of human events to be a kind of tapestry, created by three goddesses: the fates. Random minor events generally have no long-range consequences but some, which occur at critical junctures, may alter the weave of history. There may be cases where profound changes can be made by relatively trivial adjustments. The further in the past such an event is, the more powerful its influence.

What if your time traveler had persuaded Queen Isabella that Columbus's geography was wrong? Almost certainly some other European would have sailed west to the new world soon after. There were many inducements: the lure of the spice trade, improvements in navigation, competition among rival European powers. The discovery of America around 1500 was inevitable. Of course, then there wouldn't be any postage stamps showing Columbus, and the Republic of Columbia would have some other name. But the big picture would have turned out more or less the same. In order to affect the future profoundly, a time traveler would have to pick and choose. He would probably have to intervene in a number of events which are very carefully selected so that he could change the weave of history.

It's a lovely fantasy to explore those other worlds that never were. If you had H.G. Wells' time machine, maybe you could understand how history really works if an apparently pivotal person had never lived: Paul the Apostle, or Peter the Great, or Pythagoras. How different would the world really be? What if the scientific tradition of the ancient Ionian Greeks had prospered and flourished? It would have required many social factors of the time to have been different, including the common feeling that slavery was right and natural. But what if that light that had dawned on the eastern Mediterranean some 2,500 years ago had not flickered out? What if the scientific method and experiment had been vigorously pursued 2,000 years before the industrial revolution, our industrial revolution? What if the power of this new mode of thought, the scientific method, had been generally appreciated? I think we might have saved ten or twenty centuries. Perhaps the contribution that Leonardo made would have been made a thousand years earlier and the contribution of Einstein 500 years ago. Not that it would have been those people who would have made those contributions, because they live only in our timeline.

If the Ionians had won, we might be now, I think, be going to the stars. We might at this moment have the first survey ships returning with astonishing results from Alpha Centauri and Barnard Star, Sirius and (Ta Siti?) There would now be great fleets of interstellar transports being constructed in earth orbit, small unmanned survey ships, liners for immigrants, perhaps, great trading ships to ply the spaces between the stars. On all of these ships there would be symbols and inscriptions on the side. The inscriptions, if we look closely, would be written in Greek. The symbol, perhaps, would be the dodecahedron, and the inscription on the sides to the ships to the stars something like Starship Theodorus of the planet earth.

If you were a really ambitious time traveler, you might not dally with human history, or even pause to examine the evolution of life on earth. Instead, you would journey back to witness the origin of our solar system from the gas and dust between the stars. Five billion years ago, an interstellar cloud was collapsing to form our solar system. Most clumps of matter gravitated towards the center and were destined to form the sun. Smaller, peripheral clumps would become the planets. Long ago, there was a kind of natural selection among the worlds. Those with highly elliptical orbits tended to collide and be destroyed, but planets in circular orbits tended to survive. But if events had been only a little different, the earth would never have formed and some other planet at some other distance from the sun would be around. We owe the existence of our world to random collisions in a long vanished cloud. Soon, the central mass became very hot. Thermonuclear reactions were initiated and the sun turned on, flooding the solar system with light. But the growing smaller lumps would never achieve such high temperatures, and would never generate thermonuclear reactions. They would become the earth and the other planets heated, not from within, but mainly by the distant sun. The accretion continued until almost all the gas and dust and small worldlets were swept up by the surviving planets. Our time traveler would witness the collision that made the worlds. Except for the comets and asteroids, the chaos of the early solar system was reduced to a remarkable simplicity: nine or so principle planets in almost circular orbits and a few dozen moons.

Now let's take a different look. If we view the solar system edge-on, and move the sun off-screen to the left, we see that the small terrestrial planets, the ones about as massive as the earth, tend to be close to the sun. The big Jupiter-like planets tend to be much further from the sun. But is that the way it has to be? Computer studies suggest that there may be many similar systems about other stars, with the terrestrials in close and the jovian planets farther away. But some systems might have jovians and terrestrials mixed together. There may be great worlds like Jupiter looming in other skies. Rarely, the jovian planets may form close to the star, the terrestrials trailing away towards interstellar space. Our familiar arrangement of planets is only one, perhaps typical, case in the vast expanse of systems. Often, one fledgling planet accumulates so much gas and dust that thermonuclear reactions do occur. It becomes a second sun. A binary star system has formed. For most of these worlds, the vistas will be dazzling. Not a one of them will be hospitable. Many will appear hostile. Where there are two suns in the sky, every object will cast two shadows.

What wonders are waiting for us on the planets of the nearby stars? Are there radically different kinds of worlds? Unimaginably exotic forms of life? Perhaps in another century or two, when our solar system is all explored, we will also have put our own planet in order. Then we will set sail for the stars and the beckoning worlds around them. On that day, our machines and our descendants, approaching the speed of light, will skim the light years, leaping ahead through time, seeking new worlds. Einstein has shown us that it's possible. We will journey simultaneously to distant planets and to the far future. Some worlds, like this one, will look out onto a vast gaseous nebula, the remains of a star that once was and is no longer. In all those skies, rich and distant and exotic constellations, there may be a faint yellow star, perhaps barely visible to the naked eye, perhaps seen only through the telescope, the home star of a fleet of interstellar transports exploring this tiny region of the great Milky Way galaxy.

The themes of space and time are intertwined. Worlds and stars, like people, are born, live and die. The lifetime of a human being is measured in decades, but the lifetime of a sun is a hundred million times longer. Matter is much older than life. Billions of years before the sun and earth even formed, atoms were being synthesized in the insides of hot stars, and then returned to space when the stars blew themselves up. Newly formed planets were made of this stellar debris. The earth and every living thing are made of star stuff.

But how slowly in our human perspective life evolved, from the molecules of the early oceans, to the first bacteria. The reason evolution is not immediately obvious to everybody is because it moves so slowly and takes so long. How can creatures who live for only 70 years detect events that take 70 million years to unfold, or 4 billion? By the time one-celled animals had evolved, the history of life on earth was half over. Not very far along to us, you might think, but by now almost all of the basic chemistry of life had been established. Forget our human time perspective. From the point of view of a star, evolution was weaving intricate new patterns from the star stuff on the planet earth, and very rapidly. Most evolutionary lines became extinct. Many lines became stagnant. If things had gone a little differently, a small change of climate, say, or a new mutation, or the accidental death of a different humble organism, the entire future history of life might have been very different. Perhaps the line to an intelligent, technological species would have passed through worms. Perhaps the present masters of the planet would have had ancestors who were tunicate eggs. We might not have evolved. Someone else, someone very different, would be here now in our stead, maybe pondering their origins.

But that's not what happened. There was a particular sequence of environmental accidents and random mutations in the hereditary material. One particular timeline for life on earth in this universe. As a result, the dominant organisms on the planet today come from fish. Along the way, many more species became extinct than now exist. If history had a slightly different weave, some of those extinct organisms might have survived and prospered. But occasionally, a creature thought to have become extinct hundreds of millions of years ago turns out to be alive and well. The (selokin?), for example. For 3-1/2 billion years, life had lived exclusively in the water. But now, in a great breathtaking adventure, it took to the land. But if things had gone a little differently, the dominant species might still be in the ocean, or they might have developed spaceships to carry them off the planet altogether.

From our ancestors the reptiles, there developed many successful lines, including the dinosaurs. Some were fast, dexterous and intelligent. A visitor from another world or time might have thought them the wave of the future, but after nearly 200 million years they were suddenly all wiped out. Perhaps it was a great meteorite colliding with the earth, spewing debris into the air, blotting out the sun and killing the plants that the dinosaurs ate. I wonder when they first sensed that something was wrong.

The successors to the dinosaurs came from the same reptilian stock, but they were able to survive the catastrophe that destroyed their cousins. Again, there were many branches which became extinct and again, had events been only a little different, those branches might have led to the dominant form today. For 40 million years, a visitor wouldn't have been much impressed by these timid little creatures, but they led to all the familiar mammals of today. And that includes the primates. About 20 million years ago, a space-time traveler might have recognized these guys as promising, bright, quick, agile, sociable, curious. Their ancestors were once atoms made in stars, then simple molecules, single cells, polyps stuck to the ocean floor, fish, amphibians, reptiles, shrews. But then they came down from the trees and stood upright. They grew an enormous brain. They developed culture, invented tools, domesticated fire. They discovered language and writing. They developed agriculture. They built cities and forged metal, and ultimately, they set out for the stars from which they had come 5 billion years earlier. We are star stuff which has taken its destiny into its own hands. The loom of time and space works the most astonishing transformations of matter. Our own planet is only a tiny part of the vast cosmic tapestry, a starry fabric of worlds yet untold.

Those worlds in space are as countless as all the grains of sand on all the beaches of the earth. Each of those worlds is as real as ours. In every one of them there is a succession of incidents, events, occurrences which influence its future. Countless worlds, numberless moments, an immensity of space and time. And our small planet, at this moment here, we face a critical branch-point in history. What we do with our world right now will propagate down through the centuries and powerfully affect the destiny of our descendants. It is well within our power to destroy our civilization, and perhaps our species as well. If we capitulate to superstition, or greed, or stupidity, we can plunge our world into a darkness deeper than the time between the collapse of classical civilization and the Italian Renaissance. But we are also capable of using our compassion and our intelligence, our technology and our wealth, to make an abundant and meaningful life for every inhabitant of this planet, to enhance enormously our understanding of the universe and to carry us to the stars.

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Re: A Personal Voyage: Journeys in Space and Time, by Carl S

Postby admin » Thu Mar 17, 2016 9:17 am

by Carl Sagan
[Part of the "Cosmos" series, transcribed by Tara Carreon]




Ma Kepler [Katherine] was released, after fourteen months of imprisonment. She could not return to Leonberg, though, because the populace threatened to lynch her. Six months later she died.

It was against this background that Kepler wrote the Harmony of the World.

-- ARTHUR KOESTLER, The Watershed

She was carried out of her house in an oak linen chest, and taken to the prison in Leonberg. She was then seventy-three years of age. (He writes that God himself "was too kind to remain idle and began to play the game of signatures, signing his likeness into the world.") There were forty-nine accusations against her, and numerous supplementary charges. She was said to have failed to weep when the Holy Scriptures were read to her. (He resolves the harmonies into regular polygons.) Katherine Kepler replied that she had shed so many tears in her life, she had none left. (The irregular polygons, and all figures that cannot be constructed by compass and ruler, he says, are unclean because they defy the intellect. Inscibilis. Ineffabilis. Non-entia. Unspeakable. Nonexistences. And this is the reason, he writes, "why God did not employ the septagon and the other figures of this species to embellish the world.") Her son Johannes Kepler answered the Act of Accusation by an Act of Contestation, which was refuted by an Act of Acceptation to which was submitted an Act of Exception and Defense which was answered by an Act of Deduction and Confutation. Finally, in her defense her son submitted an Act of Conclusion, one hundred and twenty-eight pages long. (He then discovers regular polygons inscribed in the movements of heavenly bodies. And of these he writes: ''The heavenly motions are nothing but a continuous song for several voices ... a music which ... sets landmarks in the immeasurable flow of time.")

After that, her case, by order of the duke, was sent to her son's university, where the faculty found that Katherine should be questioned under torture, but suggested that proceedings stop at the territio -- questioning under threat of torture. (As part of the harmony of the world, Kepler reveals that the ratio which exists between the periodic times of any two planets is precisely one and one half of the power of their mean distances.) She was led to the place of torture; the executioner was presented to her; all his instruments shown her and their effect on the body described. Great pain and dolor awaited her if she did not confess, she was told. The terror of the place had wrought confessions from many before her, but she said that even if they tore her veins from her body one by one, she had nothing to confess. (On discovering that the heavenly bodies move in ellipses rather than perfect circles, Kepler apologizes for having to bring a small cartload of dung into the universe in order to rid it of a far vaster quantity of dung.) She fell on her knees then and asked God to give a sign if she was a witch or a monster, and then said she was willing to die, that God would reveal the truth after her death. (He writes: "Yes, I give myself up to holy ravings. I mockingly defy all mortals with this open confession; I have robbed the golden vessel of the Egyptians to make out of them a tabernacle for my God....") In this way, and due to the efforts of her son, and the respect he commanded in the world, Katherine Kepler was released.

-- Woman and Nature: The Roaring Inside Her, by Susan Griffin

There are two ways to view the stars: as they really are and as we might wish them to be. These are the Pleides, a group of young stars astronomers recognize as leaving their stellar nurseries of gas and dust, and this is the Crab Nebula, a stellar graveyard where gas and dust are being dispersed back into the interstellar medium. Inside it is a dying pulsar. Both the Pleides and the Crab Nebula are in a constellation that astrologers long ago named Taurus the Bull. They imagined it to influence our daily lives.

Astronomers say that the planet Saturn is an immense globe of hydrogen and helium encircled by a ring of snowballs 50,000 km. wide, and that Jupiter's great red spot is a giant storm raging for perhaps a million years. But the astrologers see the planets as affecting human character and fate. Jupiter represents a regal bearing and a gentle disposition, and Saturn, the gravedigger, fosters, they say, mistrust, suspicion and evil. To astronomers Mars is a place as real as the Earth, a world awaiting exploration. But the astrologers see Mars as a warrior, the instigator of quarrels, violence and destruction. Astronomers and astrologers were not always so distinct. For most of human history the one encompassed the other. But there came a time when astronomy escaped from the confines of astrology. The two traditions began to diverge in the life and mind of Johannes Kepler. It was he who demystified the heavens by discovering that a physical force lay behind the motions of the planets. He was the first astrophysicist and the last scientific astrologer.

The intellectual foundations of astrology were swept away 300 years ago, and yet astrology is still taken seriously by a great many people. Have you ever noticed how easy it is to find a magazine on astrology? Virtually every newspaper in America has a daily column on astrology. Almost none of them have even a weekly column on astronomy. People wear astrological pendants, check their horoscopes before leaving the house, even our language preserves an astrological consciousness. For example, take the word "disaster." It comes from the Greek for "bad star." The Italians once believed that disease was caused by the influence of the stars. It's the origin of our word "influenza." The zodiacal signs used by the astrologers even ornament this statue of Prometheus in New York City. Prometheus who stole fire from the gods.

What is all this astrology business? Fundamentally, it's the contention that which constellations the planets are in at the moment of your birth profoundly influence your future. A few thousand years ago the idea developed that the motions of the planets determine the fates of kings, dynasties, empires. Astrologers studied the motion of the planets and asked themselves what had happened last time, let's say, that Venus was rising in the constellation of the goat? Maybe something similar would happen this time as well. It was a subtle and risky business. Astrologers became employed only by the state. In many countries, it became a capital offense for anyone but the official astrologer to read the portents in the skies. Why? Because a good way to overthrow a regime was to predict its downfall. Chinese court astrologers who made inaccurate predictions were executed. Others simply doctored the record so that afterwards they were in perfect conformity with events. Astrology developed into a strange discipline, a mixture of careful observations, mathematics and record keeping with fuzzy thinking and pious fraud.

Nevertheless, astrology survived and flourished. Why? Because it seems to lend a cosmic significance to the routine of our daily lives. It pretends to satisfy our longing to feel personally connected to the universe. Astrology suggests a dangerous fatalism. If our lives are controlled by a set of traffic signals in the sky, why try to change anything?

Look at this. Here's two different newspapers published in the same city on the same day. Let's see what they do about astrology. Suppose you were a Libra, that is born between September 23 and October 22. According to the astrologer for the New York Post, compromise will help ease tension. Well, maybe, it's sort of vague. According to the New York Daily News' astrologer, demand more of yourself. Well, also vague, but also pretty different. It's interesting that these predictions are not predictions. They tell you what to do, they don't say what's going to happen. They are consciously designed to be so vague that they could apply to anybody, and they disagree with each other.

Astrology can be tested by the lives of twins. There are many real cases like this. One twin is killed in childhood, in say a riding accident, or is struck by lightning, but the other lives to a prosperous old age. Suppose that happened to me. My twin and I would be born precisely in the same place and within minutes of each other. Exactly the same planets would be rising at our births. If astrology were valid, how could we have such profoundly different fates? It turns out that astrologers can't even agree among themselves what a given horoscope means. In careful tests, they are unable to predict the character and future of people they know nothing else about except the time and place of birth. Also, how could it possibly work? How could the rising of Mars at the moment of my birth affect me, then or now? I was born in a closed room. Light from Mars couldn't get in. The only influence of Mars which could affect me is its gravity, but the gravitational influence of the obstetrician was much larger than the gravitational influence of Mars. Mars is a lot more massive but the obstetrician was a lot closer.

The desire to be connected with the cosmos reflects a profound reality. We are connected. Not in the trivial ways that pseudo-science of astrology promises, but in the deepest ways. Our little planet is under the influence of a star. The sun warms us, it drives the weather, it sustains all living things. Four billion years ago it brought forth life on Earth. But our sun is only one of a billion trillion stars within the observable universe. And those countless suns all obey natural laws, some of which are already known to us.

How did we discover that there are such laws? If we lived on a planet where nothing ever changed, there wouldn't be much to do. There would be nothing to figure out. There would be no impetus for science. And if we lived in an unpredictable world where things changed in random or very complex ways, we wouldn't be able to figure things out, and again there would be no such thing as science. But we live in an inbetween universe where things change alright, but according to patterns, rules, or as we call them, laws of nature. If I throw a stick up in the air, it always falls down. If the sun sets in the west, it always rises again the next morning in the east. And so it's possible to figure things out. We can do science and with it we can improve our lives.

Human beings are good at understanding the world. We always have been. We were able to hunt game or build fires only because we had figured something out. There once was a time before television, before motion pictures, before radio, before books. The greatest part of human existence was spent in such a time. And then, over the dying embers of the campfire, on a moonless night, we watched the stars.

The night sky is interesting. There are patterns there. If you look closely, you can see pictures. One of the easiest constellations to recognize lies in the northern skies. In North America it's called "The Big Dipper." The French have a similar idea. They call it "La Casserole." The Casserole. In Medieval England, this same pattern of stars reminded people of a simple wooden plow. The ancient Chinese had a more sophisticated notion. To them, these stars carried the celestial bureaucrat on his rounds about the pole of the sky, seated on the clouds and accompanied by his eternally hopeful petitioners. The people of northern Europe imagined yet another pattern. To them it was Charles' wane, or wagon, a medieval cart. But other cultures saw these seven stars as part of a larger picture. It was the tail of a great bear which the ancient Greeks and Native Americans saw instead of the handle of a dipper. But surely the most imaginative interpretation of this larger group of stars was that of the ancient Egyptians. They made out a curious procession of a bull and a reclining man followed by a strolling hippopotamus with a crocodile on its back. What a marvelous diversity in the images various cultures saw in this particular constellation. But the same is true for all the other constellations.

Some people think these things are really in the night sky. But we put these pictures there ourselves. We were hunter folk so we put hunters and dogs, lions and young women up in the skies, all manner of things of interest to us. When 17th Century European sailors first saw the southern skies they put all sorts of things of 17th Century interest up there: microscopes and telescopes, compasses, and the sterns of ships. If the constellations had been named in the 20th Century, I suppose we'd put there refrigerators and bicycles, rock stars, maybe even mushroom clouds. A new set of human hopes and fears placed among the stars.

But there is more to the stars than just pictures. For example, stars always rise in the east and always set in the west, taking the whole night to cross the sky if they pass overhead. There are different constellations in different seasons. The same constellations always rise at say at the beginning of autumn. It never happens that a new constellation suddenly appears out of the east, one that you never saw before. There's a regularity, a permanence, a predictability about the stars. In a way, they are almost comforting.

The return of the sun after a total eclipse. Its rising in the morning after its troublesome absence at night and the reappearance of the crescent moon after the new moon, all spoke to our ancestors of the possibility of surviving death. Up there in the skies was a metaphor of immortality.

Almost a thousand years ago in the American southwest, the Anasazi people built a stone temple, an astronomical observatory to mark the longest day of the year. Dawn on that day must have been a joyous occasion, a celebration of the generosity of the sun. They built this ceremonial calendar so that the sun's rays would penetrate a window and enter a particular niche on this day alone. That kind of precision is a triumph of human intelligence. It outlives its creators. Today, this is a lonely place. The Anasazi people are no more. They had learned to predict the changing of the seasons; they could not predict the changing of the climate and the failure of the rains. But their temple continues to catch the sun's first rays on the Summer Solstice.

I imagine the Anasazi people gathered in these pews every June 21st, dressed with feathers and turquoise to celebrate the power of the sun. These upper niches -- there are 28 of them -- may represent the number of days for the moon to reappear in the same constellation. These people paid a lot of attention to the sun and the moon and the stars. And other devices based on somewhat similar designs can be found in Angkor Wat in Cambodia, Stonehenge in England, Abu Simbel in Egypt, Chichenetza in Mexico, and in the great plains of North America.

Now why did people all over the world go to such great trouble to teach themselves astronomy? It was literally a matter of life and death to be able to predict the seasons. We hunted antelope or buffalo whose migrations ebbed and flowed with the seasons. Fruits and nuts were ready to be picked at some times and not at others. When we invented agriculture, we had to take care and sow our seeds and harvest our crops at just the right season. Annual meetings of far flung nomadic peoples were set for prescribed times.

Now some alleged calendrical devices might be due to chance; for example, the accidental alignment of a window and a niche. But there are other devices that are wonderfully different. Today, only the dry ruins of the great Anasazi cities have survived the ravages of time. Not far from these ancient cities, in an almost inaccessible location, there is another solstice marker, this one of singular and unmistakable purpose. The deliberate arrangement of three great stone slabs allows a sliver of sunlight to pierce the heart of a carved spiral, only at noon on the longest day of the year.

The wind whips through the canyons here in the American southwest and there's no one to hear it but us, a reminder of the 40,000 generations of thinking men and women who preceded us, about whom we know next to nothing, upon whom our society is based.

When our prehistoric ancestors studied the sky after sunset, they observed that some of the stars were not fixed with respect to the constant pattern of the constellations. Instead, five of them moved, slowly forward across the sky, then backward for a few months, then forward again, as if they couldn't quite make up their minds. We call them planets, the Greek word for "wanderers." These planets presented a profound mystery. The earliest explanation was that they were living beings. How else to explain their strange looping behavior. Later they were thought to be gods, and then disembodied astrological influences. But the real solution to this mystery is that the planets are worlds, that the Earth is one of them, and that they all go around the sun according to precise mathematical laws. This discovery has led directly to our modern global civilization. The merging of imagination with observation produced an exact description of the solar system. Only then could you answer the fundamental question that is at the root of modern science, "What makes it all go?" Two thousand years ago, no such question would even have been asked. The prevailing view had then been formulated by Claudius Ptolemy, an Alexandrian astronomer and also the preeminent astrologer of his time.

Ptolemy believed that the Earth was at the center of the universe, that the sun and the moon and the planets, like Mars, went around the Earth. It's the most natural idea in the world. The Earth seems steady, solid, immobile, while we can see the heavenly bodies rising and setting every day. But then, how to explain the loop de loop motion of the planets in the sky? Mars, for example? This little machine shows Ptolemy's model. The planets were imagined to go around the Earth attached to perfect crystal spheres, but not attached directly to the spheres, but indirectly through a kind of off-center wheel. The sphere turns and the little wheel rotates and as seen from the Earth, Mars does its loop de loop. This model permitted reasonably accurate predictions of planetary motion, where a planet would be on a given day. Certainly good enough predictions for the precision of measurement in Ptolemy's time and much later.

Supported by the church through the Dark Ages, Ptolemy's model effectively prevented the advance of astronomy for 1,500 years. Finally, in 1543, a quite different explanation of the apparent motion of the planets was published by a Polish cleric named Nicholas Copernicus. Its most daring feature was the proposition that the sun, not the Earth, was at the center of the universe. The Earth was demoted to just one of the planets. The retrograde, or loop de loop motion happens as the Earth overtakes Mars in its orbit, and see that from the standpoint of the Earth, Mars is now going slightly backwards, and now it is going in its original direction. This Copernican model worked at least as well as Ptolemy's crystal spheres, but it annoyed an awful lot of people. The Catholic Church later put Copernicus' work on its list of forbidden books, and Martin Luther described Copernicus in these words. He said, "People give ear to an upstart astrologer. This fool wishes to reverse the entire science of astronomy." The confrontation between the two views of the cosmos, earth-centered and sun-centered, reached its climax with a man who like Ptolemy was both an astronomer and an astrologer.

He lived in a time when the human spirit was fettered and the mind chained, when angels and demons and crystal spheres were imagined up there in the skies. Science still lacked the slightest notion of physical laws underlying nature. But the brave and lonely struggle of this man was to provide the spark that ignited the modern scientific revolution. Johannes Kepler was born in Germany in 1571. He was sent to the Protestant Seminary School in the provincial town of Maulbronn to be educated for the clergy. It was a strict, disciplined life, up before dawn to begin a long day of prayer and study. This was the age of the Reformation. Maulbronn was a kind of education and ideological bootcamp, training young Protestants in the use of theological weaponry against the fortress of Roman Catholicism.

There was little reassurance or comfort here for a sensitive boy like Kepler. He was intelligent and he knew it. That, together with his stubbornness and his fierce independence, served to isolate him from the other boys. Kepler made few friends in his two years at Maulbronn, so he kept to himself, withdrawn into the world of his own thoughts which were often concerned with his imagined unworthiness in the eyes of God. He despaired of ever attaining salvation. But God to him was more than punishment. God was also the creative power of the universe, and the young Kepler's curiosity about God was even greater than his fear. He wanted to know God's plan for the world. He wanted to read the mind of God.

This was his obsession. It was to inspire all of his great achievements. It was to take him and Europe out of the cloister of medieval thought. In places like Maulbronn, the faint echoes of the genius of antiquity still reverberated. Here, in addition to theology, Kepler was exposed to Greek and Latin and music and mathematics. And it was in geometry that he thought he glimpsed the image of perfection. He was later to write, "Geometry existed before the creation. It is co-eternal with the mind of God. Geometry provided God with a model for the creation. Geometry is God himself."

But the real world of Kepler's time was far from perfect. It was haunted by fear, pestilence, famine and war. Superstition's a natural refuge for people who are powerless. Only one thing seemed certain: the stars themselves. It was remembered that in ancient times the astrologer Ptolemy and the sage Pythagoras had taught that the heavens were harmonious and changeless. And Ptolemy had said that the motions of the planets through the stars of the zodiac were portents of events here below. Was it the influence of Mars and Venus that made his father a brutal man, a mercenary who had abandoned him? Did an unfortunate conjunction of planets in an adverse sign make his mother a mischievous and quarrelsome woman? If such things were fated by the stars, then perhaps there were hidden patterns underlying the unpredictable chaos of daily life. Patterns as constant as the stars.

But how could you discover them? Where would you begin? If the world and everything in it was created by God, then shouldn't you begin with a careful study of physical reality? Was not all of creation an expression of the harmonies in the mind of God? The book of nature had waited 1,500 years for a reader.

In 1589, Kepler left Maulbronn to continue his studies at the great university in Tubingen. It was a liberation to find himself amidst the most vital intellectual currents of the time. One of his teachers revealed to him the revolutionary ideas of Copernicus. Kepler relished this urbane, scholarly community. Here, his genius was recognized at last.

Kepler was not to be ordained after Tubingen. Instead, to his great surprise, he found himself summoned to Graz in Austria to become a teacher of high school mathematics. Kepler was not a very good teacher. The first year in Graz's mathematic's class had only a handful of students. The second year, none. He mumbled. He digressed. He was at times utterly incomprehensible. He was distracted by an incessant clamor of speculations and associations that ran through his head. And one pleasant summer afternoon, with his students longing for the end of the lecture, he was visited by a revelation that was to alter radically the future course of astronomy and the world.

There were only six planets known in his time: Mercury, Venus, Earth, Mars, Jupiter and Saturn. For some time, Kepler had been wondering why only six planets? Why not 20 planets or 100? And why this particular spacing between their orbits? No one had ever asked such questions before. In the course of a lecture on astrology, Kepler inscribed within the circle of the zodiac a triangle with three equal sides. He then noticed quite by accident that a smaller circle inscribed within the triangle bore the same relationship to the outer circle as did the orbit of Jupiter to the orbit of Saturn. Could a similar geometry relate the orbits of the other planets? Now Kepler remembered the perfect solids of Pythagoras. Of all the possible three-dimensional shapes, there were five and only five whose sides were regular polygons. He believed that the two numbers were connected, that the reason there were only six planets was that there were only five regular solids, and these perfect solids nested one within the other. He believed he had discovered the invisible supports for the spheres of the six planets. And this connection between geometry and astronomy could, he thought, admit only one explanation: The Hand of God Mathemetician.

"The intense pleasure I received from this discovery can never be told in words," he said. "Now I no longer became weary at work. Days and nights I passed in mathematical labors until I could see whether my hypothesis would agree with the orbits of Copernicus or if my joy should vanish into thin air." But no matter how hard he tried, the perfect solids and the planetary orbits did not agree with each other very well. Why didn't it work? Because unfortunately, it was wrong. The true orbital size of the planets, we now know, have absolutely nothing to do with the five perfect solids, as the later discovery of Uranus, Neptune and Pluto shows. But Kepler spent the rest of his life pursuing this geometrical phantasm. He couldn't abandon it, and he couldn't make it work. His frustration must have been enormous. Finally, he decided it must be the long accepted planetary observations that were inaccurate, and not his model of the nested solids. There was only one man in the world who had access to more precise observations. That man was Tycho Brahe who, as chance would have it, had recently written Kepler to come and join him. Kepler was reluctant at first, but he had no choice.

In 1598, a wave of oppression enveloped Graz. It was spearheaded by the local archduke who vowed to restore the Catholic faith to the province, and in his own words, "would rather make a desert of the country than rule over heretics." Kepler's school was closed, people were forbidden to worship or to sing hymns, or to own books of a heretical nature. Those who refused to embrace Catholicism were fined 10% of their assets and exiled from the country on pain of death. Kepler chose exile.

"Hypocrisy I have never learned. I am in earnest about faith. I do not play with it!" For Kepler, it was only the first in a series of religious exiles forced upon him by religious fanatics.

Now he decided to accept Tycho Brahe's open invitation. Brahi, a wealthy Danish nobleman, lived in great splendor, and had recently been appointed Imperial Mathemetician at Prague. Kepler left Graz with his wife and step-daughter and set out on the difficult journey. Kepler's wife was not a happy woman. She was chronically ill and had recently lost two young children. The marriage itself was no comfort. She had no understanding of her husband's work, and regarded his profession with contempt.

Kepler was married to his work, and every tedious mile was bringing him closer to the great Tycho Brahe, whose observations he devoutly hoped would confirm his theory. Kepler envisioned Tycho's domain as a sanctuary from the evils of the time. He aspired to be a worthy colleague to the illustrious Tycho who, for 35 years, had been immersed in exact measurements of a clockwork universe, ordered and precise.

But Tycho's court was not at all what Kepler had expected. Tycho himself was a flamboyant figure, adorned with a gold nose. The original had been lost in a student duel fought over who was the superior mathematician, and he maintained a circus-like entourage of assistants, distant relatives and assorted hangers-on.

Kepler had no use for the endless revelry. He was impatient to see Tycho's data, but Tycho would give him only a few scraps at a time. "Tycho," he said, "gave me no opportunity to share his studies. He would only, in the course of a meal and in-between other matters, mention, as if in passing, today the figure of the apogee of one planet, tomorrow the nodes of another." Kepler was ill-suited for such games, and the general climate of intrigue offended his sense of propriety. Their cruel mockery of the pious and scholarly Kepler depressed and saddened him. "My opinion of Tycho is this: He's superlatively rich, but knows not how to make proper use of it. Tycho possesses the best observations. He also has collaborators. He lacks only the architect who would put all of this to use."

Tycho was unable to turn his observations into a coherent theory of the solar system. He knew he needed the brilliant Kepler's help. But simply to hand over his life's work to a potential rival? That was unthinkable. Tycho was the greatest observational genius of the age, and Kepler the greatest theoretician. Either man alone could not achieve the synthesis which both felt was now possible. The birth of modern science, which is the fusion of observation and theory, teetered on the precipice of their mutual distrust.

The two repeatedly quarreled and were reconciled, until a few months later, Tycho died of his habitual overindulgence in food and wine. Kepler wrote to a friend on the last night of Tycho's gentle delirium: "He repeated over and over again these words, like someone composing a poem: 'Let me not seem to have lived in vain; let me not seem to have lived in vain.'" And he did not. Eventually, after Tycho's death, Kepler contrived to extract the observations from Tycho's reluctant family, observations of the apparent motion of Mars through the constellations obtained over a period of many years. The data from the last few decades before the invention of the telescope were by far the most precise ever obtained up to that time. Kepler worked with a kind of passionate intensity to understand Tycho's observations. What real motions of the Earth and Mars about the sun could explain to the precision of measurement the apparent motion as seen from the Earth of Mars in the sky? And why Mars? Because Tycho had told Kepler that the apparent motion of Mars was the most difficult to reconcile with a circular orbit. After years of calculation, he believed that he had found the correct values for a Martian circular orbit which matched ten of Tycho Brahe's observations within two minutes of arc.

Now there are 60 minutes of arc in an angular degree, and of course 90 degrees from horizon to zenith. So a few minutes of arc is a very small quantity to measure, especially without a telescope. But Kepler's ecstasy of discovery soon crumbled into gloom because two further observations of Tycho were inconsistent with his orbit by as much as eight minutes of arc. Kepler wrote, "If I had believed that we could ignore these eight minutes, I would have patched up my hypothesis accordingly. But since it was not permissible to ignore them, those eight minutes pointed the road to a complete reformation of astronomy.

The difference between a circular orbit and the true orbit of Mars could be distinguished only by precise measurement and by a courageous acceptance of the facts. Kepler was profoundly annoyed at having to abandon a circular orbit. It shook his faith in God as the maker of a perfect celestial geometry. "Having cleaned the stable of astronomy of circles and spirals," he said, 'I was left with only a single cartful of dung." He tried various oval-like curves, calculated away, made some arithmetical mistakes which caused him, in fact, to reject the correct answer, and months later, in some desperation, tried the formula for the first time for an ellipse. The ellipse matched the observations of Tycho beautifully. In such an orbit, the sun is not at the center, but is offset. It's at one focus of the ellipse. When a given planet is at the far point in its orbit from the sun, it goes more slowly. As it approaches the near point, it speeds up. Such motion is why we describe the planets as forever falling toward the sun, but never reaching it. Kepler's First Law of Planetary Motion is simply this: A planet moves in an ellipse, with the sun at one focus.

As the planet moves along its orbit, it sweeps out, in a given period of time, an imaginary wedge-shaped area. When the planet is far from the sun, the area is long and thin. When the planet is far from the sun, the area is short and squat. Although the shapes of these wedges are different, Kepler found that their areas are exactly the same. This provided a precise mathematical description of how a planet changes its speed in relation to its distance from the sun. Now, for the first time, astronomers could predict exactly where a planet would be in accordance with a simple and invariable law. Kepler's Second Law is this: A planet sweeps out equal areas in equal times.

Kepler's first two laws of planetary motion may seem a little remote and abstract. Alright, planets move in ellipses and they sweep out equal areas in equal times. So what? It's not as easy to grasp as circular motion. We might have a tendency to dismiss it, to say it's a mere mathematical tinkering, something removed from everyday life. But these are the laws our planet itself obeys as we, glued by gravity to the surface of the Earth, hurtle through space. We move in accordance with laws of nature which Kepler first discovered. When we send spacecraft to the planets, when we observe double stars, when we examine the motion of distant galaxies, we find that all over the universe, Kepler's laws are obeyed.

Many years later Kepler came upon his third, and last law of planetary motion, a law which relates the motion of the various planets with each other, which lays out correctly the clockwork of the solar system. He discovered a simple mathematical relationship between the size of a planet's orbit and the average speed with which it travels around the sun. This confirmed his long-held belief that there must be a force in the sun that drives the planets, a force stronger for the inner, fast-moving planets, and weaker for the outer, slow-moving planets. Isaac Newton later identified that force as gravity, answering at last the fundamental question, "What makes the planets go?"

Kepler's Third, or Harmonic Law, states that the squares of the periods of the planets, the time for them to make one orbit, are proportional to the cubes, to the third power, of their average distances from the sun. So the further away a planet is from the sun, the slower it moves, but according to a precise mathematical law. Kepler was the first person in the history of the human species to understand correctly and quantitatively how the planets move, how the solar system works.

The man who sought harmony in the cosmos was fated to live at a time of exceptional discord on Earth. Exactly eight days after Kepler's discovery of his Third Law, there occurred in Prague an incident that unleashed the devastating Thirty Years War. The war's convulsions shattered the lives of millions of people. Kepler lost his wife and young son to an epidemic spread by the soldiery. His royal patron was deposed and he was excommunicated from the Lutheran Church for his uncompromising independence on questions of belief. He was a refugee once again.

The conflicts, portrayed on both sides as a holy war, was more an exploitation of religious bigotry by those hungry for land and power. This war introduced organized pillage to keep armies in the field. The brutalized population of Europe stood by helpless as their ploughshares and pruning hooks were literally beaten into swords and spears. Rumor and paranoia swept through the countryside enveloping especially the powerless. Among the many scapegoats chosen were elderly women living alone who were charged with witchcraft. Kepler's mother was taken away in the middle of the night in a laundry chest. It took Kepler six years of unremitting effort to save her life.

In Kepler's little home town, about three women were arrested, tortured and killed as witches every year between 1615 and 1629, and Katarina Kepler was a cantankerous old woman. She engaged in disputes which annoyed the local nobility, and she sold drugs. Poor Kepler thought that he himself had contributed inadvertently to his mother's arrest. It came about because he had written one of the first works of science fiction. It was intended to explain and popularize science, and was called "The Somnium." The Dream. He imagined a journey to the moon, with the space travelers standing on the lunar surface looking up to see, rotating slowly above them, the lovely planet Earth.

Part of the basis for the charge of witchcraft was that in his Dream, Kepler used his mother's spells to leave the Earth. But he really believed that one day human beings would launch celestial ships with sails adapted to the breezes of heaven, filled with explorers who, he said, would not fear the vastness of space. He speculated on the mountains, valleys, craters, climate, and possible inhabitants of the moon. Before Kepler, astronomy had little connection with physical reality, but with Kepler came the idea that a physical force moves the planets in their orbits. He was the first to combine a bold imagination with precise measurements, to step out into the cosmos. It changed everything.

This fusion of facts with dreams opened the way to the stars. As a boy Kepler had been captured by a vision of cosmic splendor, a harmony of the worlds, which he sought so tirelessly all his life. Harmony in this world eluded him. His Three Laws of Planetary Motion represent, we now know, is a real harmony of the worlds, but to Kepler, they were only incidental to his quest for a cosmic system based on the perfect solids, a system which, it turns out, existed only in his mind. Yet, from his work we have found that scientific laws pervade all of nature, that the same rules apply on Earth as in the skies, that we can find a resonance, a harmony between the way we think and the way the world works.

When he found that his long cherished beliefs did not agree with the most precise observations, he accepted the uncomfortable facts. He preferred the hard truth to his dearest illusions. That is the heart of science.

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Re: A Personal Voyage: Journeys in Space and Time, by Carl S

Postby admin » Thu Mar 17, 2016 9:17 am

A Personal Voyage: The Backbone of Night
by Carl Sagan
[Part of the "Cosmos" series, transcribed by Tara Carreon]



The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars.

There was a time when the stars seemed an impenetrable mystery. Today, we have begun to understand them. In our personal lives also, we journey from ignorance to knowledge. Our individual growth reflects the advancement of the species. The exploration of the cosmos is a voyage of self discovery.

When I was a child, I lived here, in the Bensonhurst section of Brooklyn in the city of New York. I knew my immediate neighborhood intimately: every candy store, front stoop, backyard, empty lot, and wall for playing Chinese handball. It was my whole world. But more than a few blocks away, north of the raucous traffic and the elevated railway on 86th Street, was an unknown territory, off-limits to my wanderings. It could have been Mars for all I knew.

Even with an early bedtime, in the winter you could occasionally see the stars. I would look up at them and wonder what they were, and ask other kids and adults, and they would answer: "They are lights in the sky, kid." Well, I could tell they were lights in the sky, but what were they? There had to be some deeper answer.

I remember I was issued my first library card -- I think it was some library over there on 85th Street -- anyway, it was in alien territory, and I asked the librarian for a book on stars. She gave me a funny kind of picture book with portraits of men and women with names like Veronica Lake and Alan Ladd. I explained that wasn't what I wanted at all, and for some reason then obscure to me, she smiled and got me another book, the right kind of book. I was so excited to know the answer that I opened the book breathlessly right there in the library, and the book said something astonishing, a very big thought: Stars, it said, were suns, but very far away. The sun was a star but close up.

How, I wondered, could anybody know such things for sure? How did they figure it out? Where did they even begin? I was ignorant of the idea of angular size. I didn't know a thing about the inverse square law of the propagation of light. I didn't have the ghost of a chance of calculating the distance to the stars. But I could tell that if the stars were suns, they had to be awfully far away, further away than 86th Street, further away than Manhattan, further away probably than New Jersey. The universe had become much grander than I had ever guessed.

And then I read another astonishing fact: the earth -- which includes Brooklyn -- was a planet that went around the sun, that there were other planets -- they also went around the sun -- some closer to the sun, some further from the sun. But planets didn't shine by their own light the way the sun does. No. The planets merely reflected the little bit of light that shines on them from the sun back to us. If you were a great distance away from the sun you wouldn't be able to see the earth or the other planets at all. Well then, it stood to reason I thought, that those other stars ought to have their own planets, and some of those planets ought to have life. Why not? And that life ought to be pretty different from life as we know it, life here in Brooklyn.

As a child, it was my immense good fortune to have parents and a few teachers who encouraged my curiosity. This was my 6th grade classroom. I came back here one afternoon to remember what it was like. I brought some of the breathtaking pictures of other worlds that had been radioed back by the Voyager Spacecraft in their encounters with Jupiter and its moons. This is Calisto. What is Calisto? It is the outermost big moon of Jupiter. Who is this guy? Europa. Another Europa. A black and white picture of a ring of Jupiter.

Everyone of us begins life with an open mind, a driving curiosity, a sense of wonder.

I thought it might be fun if we now had some questions.

Q. Why is the earth round? Why isn't it square or any other shape?

A. That's a good question. I like that question. That's a question I've asked myself, and the answer has to do with gravity. The earth has a strong gravity. If you were to make a mountain very high, higher than Everest -- you know it's the biggest mountain on the earth -- it would be crushed by its own weight. You see, gravity pulls everything toward the center. So any really big bump on the earth is crushed. But if you had a small object, a tiny world, the gravity is very low and then it can be very different from a sphere. Here, look at this one. See, it's lumpy. It's a lumpy world. It looks like a potato, right? There's a large potato orbiting the planet of Mars. This is one of the moons of Mars. And that's a perfect example. You can have big departures from a sphere if your gravity is low. Now, a question from the front.

Q. Is the sun considered part of the Milky Way galaxy?

A. Sure. You're considered part of the Milky Way galaxy. Everything except other galaxies is part of the Milky Way galaxy. The sun is one star. There are a few hundred billion stars in the Milky Way, and around each star maybe there's a whole bunch of planets, and on one of those planets is life, and one of the life forms on that planet is you. So you're a part of the Milky Way galaxy.

Sometimes I think how lucky we are to live in this time, the first moment in human history when we are in fact visiting other worlds and engaging in a deep reconnaissance of the cosmos. But if we had been born in a much earlier age, no matter how great our dedication, we could not have understood what the planets and stars are. We would not have known that there were other suns and other worlds. This is one of the great secrets, wrested from nature through a million years of patient observation and courageous thinking.

Human beings have always asked questions about the stars. It's as natural as breathing. But imagine a time before science had found out the answers. Imagine what it was like, say, hundreds of thousands of years ago soon after the discovery of fire. We were just as smart and just as curious then as we are now. Sometimes it seems to me that there were people then who thought like this: "We are wandering hunter folk. Fire keeps us warm. Its light makes holes in the darkness. It keeps hungry animals away. In the darkness we can see each other and talk. We take care of the flame. The flame takes care of us. The stars are not near to us. When we climb a hill or a tree, they are no closer. They flicker with a strange, cold, white, far-away light -- many of them, all over the sky, but only at night. I wonder what they are."

One night I thought: "The stars are flames. They give a little light at night as fire does. Maybe the stars are campfires which other wanderers light at night. The stars give a much smaller light than campfires, so they must be very far away. I wonder if our campfires look like stars to the people in the sky. But why don't those campfires and the wanderers who made them fall down at our feet? Why don't strange tribes drop from the sky? Those beings in the sky must have great powers."

I don't suppose that every hunter-gatherer had such thoughts about the stars, but we know from contemporary hunter-gatherer communities that very imaginative ideas arise. The Kung Bushmen of the Kalahari desert in the Republic of Botswana have an explanation of the Milky Way. At their latitude it's often overhead. They call it the Backbone of Night. They believe it holds the sky up. They believe that if not for the Milky Way, pieces of sky would come crashing down at our feet. So the Milky Way, in their view, has some practical value. The Backbone of Night.

Later on, metaphors about campfires or backbones or holes through which a flame could be seen, were replaced in most human communities by another idea. The powerful beings in the sky were promoted to Gods. They were given names and relatives and special responsibilities for the cosmic services they were expected to perform. There was a god for every human concern. God ran nature. Nothing happened without the direct intervention of some god. If the gods were happy, there was plenty of food and humans were happy. But if something displeased the gods, and it didn't take much, the consequences were awesome: droughts, floods, storms, war, earthquakes, volcanic eruptions, epidemics. The gods had to be propitiated, and a vast industry of priests arose to make the gods less angry. But because the gods were capricious you couldn't be sure what they'd do. Nature was a mystery. It was hard to understand the world.

Our ancestors groped in darkness to make sense of their surroundings. Powerless before nature, they invented rituals and myths, some desperate and cruel, others imaginative and benign. The ancient Greeks explained the brightness in the night sky as the milk of the goddess Hera squirted from her breast across the heavens. We still call it the Milky Way.

In gratitude for the many gifts of the gods, our ancestors created works of surpassing beauty. This is all that remains of the ancient temple of Hera, Queen of Heaven. A single marble column standing in a vast field of ruins on the Greek island of Samos. It was one of the wonders of the world built by people with an extraordinary eye for clarity and symmetry. But those who thronged to that temple were also the architects of a bridge from their world to ours. We were moving once again in our voyage of self discovery on our journey to the stars.

Here, 25 centuries ago, on the island of Samos, and in the other Greek colonies that had grown up in the busy Aegean sea, there was a glorious awakening. Suddenly, there were people who believed everything was made of atoms, that human beings and other animals had evolved from simpler forms, that diseases were not caused by demons or the gods, that the earth was only a planet going around a sun which was very far away.

This revolution made Cosmos out of Chaos. Here, in the 6th Century B.C., a new idea developed, one of the great ideas of the human species. It was argued that the universe was knowable. Why? Because it was ordered, because there are regularities in nature which permitted secrets to be uncovered. Nature was not entirely unpredictable. There were rules that even she had to obey. This ordered and admirable character of the universe was called Cosmos, and it was set in stark contradiction to the idea of Chaos. This was the first conflict of which we know between science and mysticism, between nature and the gods.

By why here? Why in these remote islands and inlets of the Eastern Mediterranean? Why not in the great cities of India or Egypt, Babylon, China, Mesoamerica? Because they were all at the center of old empires. They were set in their ways. Hostile to new ideas. But here in Ionia were a multitude of newly colonized islands and city states. Isolation, even if incomplete, promotes diversity. No single concentration of power could enforce conformity. Free inquiry became possible. They were beyond the frontiers of the empires. The merchants and tourists and sailors of Africa, Asia, and Europe met in the harbors of Ionia to exchange goods and stories and ideas. It was a vigorous and heady interaction of many traditions, prejudices, languages and gods.

These people were ready to experiment. Once you are open to questioning rituals and time-honored practices, you find that one question leads to another. What do you do when you're faced with several different gods, each claiming the same territory? The Babylonian Marduk and the Greek Zeus were each considered King of the Gods, Master of the Sky. You might decide that since they otherwise had rather different attributes, that one of them was merely invented by the priests. But if one, why not both?

So here it was that the great idea arose, the realization that there might be a way to know the world without the god hypothesis, that there might be principles, forces, laws of nature through which the world might be understood without attributing the fall of every sparrow to the direct intervention of Zeus. This is the place where science was born. That's why we're here.

This great revolution happened between 600 and 400 B.C. It was accomplished by the same practical and productive people who made the society function. Political power was in the hands of the merchants who promoted the technology on which their prosperity depended. The earliest pioneers of science were merchants and artisans and their children.

The first Ionian scientist was named Thales. He was born over there, in the city of Miletus, across this narrow strait. He had traveled in Egypt and was conversant with the knowledge of Babylon. Like the Babylonians, he believed that the world had once all been water. To explain the dry land, the Babylonians added that their god Marduk had placed a mat on the face of the waters and had piled dirt up on top of it. Thales had a similar view but he left Marduk out. Yes, the world had once been mostly water, but it was a natural process which explained the dry land. Thales thought it was similar to the silting up that he had observed at the delta of the River Nile.

Whether Thales' conclusions were right or wrong is not nearly as important as his approach. The world was not made by the gods, but instead was the result of material forces interacting in nature. Thales brought back from Babylon and Egypt the seeds of new sciences: astronomy and geometry, sciences which would sprout and grow in the fertile soil of Ionia.

Anaxamander of Miletus was a friend and colleague of Thales, one of the first people that we know of to have actually done an experiment. By examining the moving shadow cast by a vertical stick, he determined accurately the length of the year and the length of the seasons. For age, men had used sticks to club and spear each other. Anaxamander used a stick to measure time.

In 540 B.C. or thereabouts, on the island of Samos, there came to power a tyrant named Polycrates. He seems to have started out as a caterer and then went on to international piracy. His loot was unloaded on this very breakwater. But he suppressed his own people, he made war on his neighbors, he quite rightly feared invasion. So Polycrates surrounded his capital city with an impressive wall whose remains stand to this day.

To carry water from a distant spring through the fortifications, he ordered this great tunnel built. A kilometer long, it pierces a mountain. Two cuttings were dug from either side which met almost perfectly in the middle. The project took some 15 years to complete. It is a token of the civil engineering of its day, and an indication of the extraordinary practical capability of the Ionians.

The enduring legacy of the Ionians is the tools and techniques they developed which remain the basis of modern technology. This was the time of Theodorus, the master engineer of the age, a man who is credited with the invention of the key, the ruler, the carpenter's square, the level, the lathe, bronze casting. Why are there no monuments to this man? Those who dreamt and speculated and deduced about the laws of nature talked to the engineers and the technologists. They were often the same people. The practical and the theoretical were one.

This new hybrid of abstract thought and everyday experience blossomed into science. When these practical men turned their attention to the natural world, they began to uncover hidden wonders and breathtaking possibilities. Anaxamander studied the profusion of things, and saw their interrelationships, and he concluded that they originated in water and mud and then colonized the dry land. Human beings, he said, must have evolved from simpler forms. This insight had to wait 24 centuries until its truth was demonstrated by Charles Darwin.

Nothing was excluded from the investigations of these first scientists. Even the air became the subject of close examination by a Greek from Sicily named Empedocles. He made an astonishing discovery with a household implement that people had used for centuries. This is the so-called "Water Thief." It's a brazen sphere with a neck and a hole at the top and a set of little holes at the bottom. It was used as a kitchen ladle. You fill it by immersing it in water. If, when it's been in there a little bit, you pull it out with the neck uncovered, then the water trickles out of the little holes making a small shower. Instead, if you pull it out with the neck covered, the water is retained. Now try to fill it with the neck covered with my thumb. Nothing happens. Why not? There is something in the way, some material is blocking the access of the water into the sphere. I can't see any such material. What could it be? Empedocles identified it as air. What else could it be? A thing you can't see can exert pressure, can frustrate my wish to fill this vessel with water if I were dumb enough to keep my thumb on the neck. Empedocles had discovered the invisible. Air, he thought, must be matter in a form so finely divided that it couldn't be seen.

This whiff, this hint of the existence of atoms, was carried much further by a contemporary named Democritus. Of all the ancient scientists, it is he who speaks most clearly to us across the centuries. The few surviving fragments of his scientific writings reveal a mind of the highest logical and intuitive powers. He believed that a large number of other worlds wander through space, that worlds are born and die, that some are rich in living creatures, and that others are dry and barren. He was the first to understand that the Milky Way is an aggregate of the light of innumerable faint stars, beyond campfires in the sky, beyond the milk of Hera, beyond the backbone of night. The mind of Democritus soared. He saw deep connections between the heavens and the earth. Man, he said, is a microcosm, a little cosmos.

Democritus came from the Ionian town of Abdera on the Northern Aegean shore. In those days, Abdera was the butt of jokes. If around the year 400 B.C., in an outdoor restaurant like this, you told a story about someone from Abdera, you were guaranteed a laugh. It was in a way the Brooklyn of its time. For Democritus, all of life was to be enjoyed and understood. In fact, for him understanding and enjoyment were pretty much the same thing. He said, "a life without festivity is a long road without an inn." Democritus may have come from Abdera, but he was no dummy.

Democritus understood that the complex forms, changes and motions of the material world all derived from the interaction of very simple moving parts. He called these parts atoms. All material objects are collections of atoms intricately assembled, even we. When I cut this apple, the knife must be passing through empty space between the atoms, Democritus argued. If there were no such empty spaces, no void, then the knife would encounter some impenetrable atom and the apple would not be cut. Let's compare the cross sections of the two pieces. Are the exposed areas exactly equal? No, said Democritus. The curvature of the apple forces this slice to be slightly shorter than the rest of the apple. If they were equally tall, then we'd have a cylinder and not an apple. No matter how sharp the knife, these two pieces have unequal cross sections. But why? Because on the scale of the very small, matter exhibits some irreducible roughness, and this fine scale of roughness, Democritus of Abdera identified with the world of atoms.

His arguments are not those we use today, but they are elegant and subtle and derived from everyday experience, and his conclusions were fundamentally right. Democritus believed that nothing happens at random, that everything has a material cause. He said, "I would rather understand one cause than be King of Persia." He believed that poverty in a democracy was far better than wealth in a tyranny. He believed that the prevailing religions of his time were evil and that neither souls nor mortal gods existed. There is no evidence that Democritus was persecuted for his beliefs, but then again, he came from Abdera.

However, in his time, the brief tradition of tolerance for unconventional views was beginning to erode. For instance, the prevailing belief was that the moon and the sun were gods. Another contemporary of Democritus, named Anaxagoras, taught that the moon was a place made of ordinary matter, and that the sun was a red hot stone far away in the sky. For this, Anaxagoras was condemned, convicted, and imprisoned for impiety, a religious crime. People began to be persecuted for their ideas. A portrait of Democritus is now on the Greek 100 drachma note, but his ideas were suppressed and his influence on history made minor. The mystics were beginning to win.

You see, Ionia was also the home of another quite different intellectual tradition. Its founder was Pythagoras, who lived here on Samos in the 6th Century B.C. According to local legend, this cave was once his abode. Maybe that was once his living room. Many centuries later this small Greek Orthodox shrine was erected on his front porch. There's a continuity of tradition from Pythagoras to Christianity. Pythagoras seems to have been the first person in the history of the world to decide that the earth was a sphere. Perhaps he argued by analogy with the moon or the sun, or maybe he noticed the curved shadow of the earth on the moon during a lunar eclipse, or maybe he recognized that when ships leave Samos, their masts disappear last.

Pythagoras believed that a mathematical harmony underlies all of nature. The modern tradition of mathematical argument essential in all of science owes much to him. And the notion that the heavenly bodies move to a kind of music of the spheres was also derived from Pythagoras. It was he who first used the word Cosmos to mean a well ordered and harmonious universe, a world amenable to human understanding. For this great idea, we are indebted to Pythagoras, but there were deep ironies and contradictions in his thoughts. Many of the Ionians believed that the underlying harmony and unity of the universe was accessible through observation and experiment, the method that dominates science today. However, Pythagoras had a very different method. He believed that the laws of nature could be deduced by pure thought. He and his followers were not basically experimentalists, they were mathematicians, and they were thorough-going mystics.

They were fascinated by these five regular solids, bodies whose faces are all polygons, triangles, squares or pentagons. There can be an infinite number of polygons, but only five regular solids. Four of the solids were associated with earth, fire, air and water. The cube for example represented earth. These four elements, they thought, make up terrestrial matter. So the fifth solid they mystically associated with the Cosmos. Perhaps it was the substance of the heavens. This fifth solid was called the dodecahedron. Its faces are pentagons, twelve of them. Knowledge of the dodecahedron was considered too dangerous for the public. Ordinary people were to be kept ignorant of the dodecahedron. In love with whole numbers, the Pythagoreans believed that all things could be derived from them. Certainly all other numbers.

So a crisis in doctrine occurred when they discovered that the square root of two was irrational. That is, that the square root of two could not be represented as the ratio of two numbers, no matter how big they were. Irrational originally meant only that, that you can't express a number as a ratio. But for the Pythagoreans, it came to mean something else, something threatening, a hint that their world view might not make sense -- the other meaning of irrational. Instead of wanting everyone to share and know of their discoveries, the Pythagoreans suppressed the square root of two and the dodecahedron. The outside world was not to know.

The Pythagoreans had discovered in the mathematical underpinnings of nature one of the two most powerful scientific tools. The other, of course, is experiment. But instead of using their insight to advance the collective voyage of human discovery, they made of it little more than the hocus pocus of a mystery cult. Science and mathematics were to be removed from the hands of the merchants and the artisans. This tendency found its most effective advocate in a follower of Pythagoras named Plato. He preferred the perfection of these mathematical abstractions to the imperfections of everyday life. He believed that ideas were far more real than the natural world. He advised the astronomers not to waste their time observing the stars and planets. It was better, he believed, to just think about them. Plato expressed hostility to observation and experiment. He taught contempt for the real world and disdain for the practical application of scientific knowledge. Plato's followers succeeded in extinguishing the light of science and experiment that had been kindled by Democritus and the other Ionians. Plato's unease with the world as revealed by the senses was to dominate and stifle Western philosophy. Even as late as 1600, Johannes Kepler was still struggling to interpret the structure of the Cosmos in terms of Pythagorean solids and Platonic perfections.

Ironically, it was Kepler who helped reestablish the old Ionian method of testing ideas against observations. But why had science lost its way in the first place? What appeal could these teachings of Pythagoras and Plato have had for their contemporaries? They provided, I believe, an intellectually respectable justification for a corrupt social order.

The mercantile tradition which had led to Ionian science also led to a slave economy. You could get richer if you owned a lot of slaves. Athens in the time of Plato and Aristotle had a vast slave population. All of that brave Athenian talk about democracy applied only to a privileged few. Plato and Aristotle were comfortable in a slave society. They offered justifications for oppression. They served tyrants. They taught the alienation of the body from the mind, a natural enough idea, I suppose, in a slave society. They separated thought from matter. They divorced the earth from the heavens. Divisions which were to dominate Western thinking for more than 20 centuries. The Pythagoreans had won.

In the recognition by Pythagoras and Plato that the Cosmos is knowable, that there is a mathematical underpinning to nature, they greatly advanced the cause of science. BUT, in the suppression of disquieting facts, the sense that science should be kept for a small elite, the distaste for experiment, the embrace of mysticism, the easy acceptance of slave societies, their influence has significantly set back the human endeavor.

The books of the Ionian scientists are entirely lost. Their views were suppressed, ridiculed and forgotten by Platonists and by the Christians who adopted much of the philosophy of Plato. Finally, after a long mystical sleep in which the tools of scientific inquiry lay moldering, the Ionian approach was rediscovered. The Western world reawakened. Experiment and open inquiry slowly became respectable once again. Forgotten books and fragments were read once more. Leonardo and Copernicus and Columbus were inspired by the Ionian tradition.

The Pythagoreans and their successors held the peculiar notion that the earth was tainted, somehow nasty, while the heavens were pristine and divine. So the fundamental idea that the earth is a planet, that we are citizens of the universe, was rejected and forgotten. This idea was first argued by Aristarchus, born here on Samos three centuries after Pythagoras. He held that the earth moves around the sun. He correctly located our place in the solar system. For his trouble, he was accused of heresy. From the size of the earth's shadow on the moon during a lunar eclipse, he deduced that the sun had to be much much larger than the earth and also very far away. From this he may have argued that it was absurd for so large an object as the sun to be going around so small an object as the earth. So he put the sun, rather than the earth, at the center of the solar system, and he had the earth and the other planets going around the sun. He also had the earth rotating on its axis once a day. These are ideas that we ordinarily associate with the name Copernicus, but Copernicus seems to have gotten at least some hint of these ideas by reading about Aristarchus. In fact, in the manuscript of Copernicus' book, he refers to Aristarchus, but in the final version he suppressed the citation. Resistance to Aristarchus, a kind of geocentrism in everyday life, is still with us. We still talk about a sun rising and the sun setting.

It's 2200 years since Aristarchus, and the language still pretends that the earth does not turn, that the sun is not at the center of the solar system. Aristarchus understood the basic scheme of the solar system, but not its scale. He knew that the planets move in concentric orbits about the sun, and he probably knew their order out to Saturn, but he was much too modest in his estimates of how far apart the planets are. In order to calculate the true scale of the solar system, you need a telescope. It wasn't until the 17th Century that astronomers were able to get even a rough estimate of the distance to the sun. And once we knew the distance to the sun, what about the stars? How far away are they? There is a way to measure the distance to the stars, and the Ionians were fully capable of discovering it. Aristarchus had toyed with the daring idea that the stars were distant suns. That if a star were as near as the sun it would appear as big and as bright as the sun. Everyone knows that the farther away an object is, the smaller it seems. This inverse proportionality between apparent size and distance is the basis of perspective in art and photography.

So the further away we are from the sun, the smaller and dimmer it appears. How far from the sun would we have to be for it to appear as small and dim as a star? Or equivalently, how small a piece of sun would be as bright as a star? An experiment to answer this question was first performed in 17th Century Holland by Christiaan Huygens, and is very much in the Ionian tradition. Huygens drilled a number of holes in a brass plate and held the plate up to the sun, and he asked himself which hole seemed as bright as he remembered the bright star Sirius to have been the previous evening. Well, the hole that matched was apparently 1/28,000th the apparent size of the sun. So Sirius, he reasoned, must be 28,000 times further away than the sun, or about half a light year away. It's hard to remember just how bright a star is hours after you've looked at it, but Huygens remembered very well. In fact, if he had known that Sirius was intrinsically brighter than the sun, he would have got the answer exactly right. Sirius is 8.8 light years away from us.

Between Aristarchus and Huygens, people had answered that question which had so excited me as a young boy growing up in Brooklyn, the question, "What are the stars?" And the answer is that the stars are mighty suns light years away in the depths of interstellar space. And around those suns, are there other planets? And on those other worlds, are there beings who wonder as we do?

Here is a light bulb which is supposed to represent a nearby star. And next to it, and very hard to see because of the bright light, is a planet. Now we'll need a volunteer. Who would like to come up please? Ordinarily, you would have a hard time seeing the planet because it is so close that the star washes out the planet. But if we're able to put something in front of the star, to make an artificial eclipse, then we might be able to see the planet. So I'm going to stand over here. Imagine that I'm a telescope somewhere near the earth. And if Tab could slowly move the disk across, good, a little faster would be nice, good, now you are just beginning to cover over the star, I really can't see the planet at all, keep going, good. Now right there I can't see the star at all, and I see the planet lit by the light of the star. Now that is a method for looking for planets around nearby stars, and that method uses a spacecraft to hold the disk and scan the sky for another telescope to see if there are any planets. So Tab, you have successfully accomplished your mission to look for planets around other stars. Thank you for being our interplanetary spacecraft.

So this is one way, and there are spaceships that will be able to do this in the next ten years or so. And there is another way -- this has already been tried from the earth -- imagine that there's a nearby star that you can see, it's bright, and it has a dark companion, a planet, shining only by reflected light near it, so dim you can't see it. But imagine that this planet and its star are going around each other like that. You can see the star, you can't see the planet. So now I'm going to need two volunteers, because, just to save some time, because they are in the front. Now, I'm going to need one of you to turn the star and the planet, and another person to pull the star and planet along. And what you will see is that the star you can make out will be moving in a funny wiggly pattern, which will be the clue, the evidence, for the existence of the dark planet. Okay, let's have a spin. Good. And a pull. And you see this funny motion that the star makes because of the planet. Thank you very much. So that's another way of finding out the existence of a planet that you couldn't see directly.

Well, both of these methods are being used, and by the time that you people are as old as I am, we should know for all the nearest stars whether they have planets going around them or not. We might know dozens or even hundreds of other planetary systems and see if they are like our own, or very different, or no other planets going around other stars at all. That will happen in your lifetime, and it will be the first time in the history of the world that anybody found out really if there are planets around other stars.

Now the nearby stars, the ones you can see with the naked eyes, those are all in what's called "the solar neighborhood." That's really what astronomers call it, "the neighborhood." But it's a very tiny place in the Milky Way galaxy. The Milky Way is that band of light that you see across the sky on a clear night. I can't tell if there are any more clear nights in Brooklyn, but you must have seen the Milky Way, right, that big band of light? Well, that's just a hundred billion stars all seen together, edge-on, as in this picture. If you could get out of the Milky Way galaxy, and look down on it, it would look like that picture. And if we did look down in the Milky Way galaxy, where would the sun and nearby stars be? Would it be in the center, where things look important, or at least well-lit? No. We would be way out here in the suburbs, in the countryside of the galaxy. We're not in any important place. All of the stars that you could see would be a little little place like that, and the Milky Way would be this band of light a hundred billion stars all together. The fact that we live in the outskirts of the galaxy was discovered a long time ago, toward the end of the First World War, by a man named Harlow Shapley, who was mapping the position of these clusters of stars. See, everyone of these is a bunch of maybe 10,000 stars all together. It's called a globular cluster. And you can see that they are centered around the middle, around the center of the galaxy. People used to think that the sun was at the center of the galaxy, something important about our position. It turns out to be wrong. We live in the outskirts. The globular clusters are centered around the marvelous middle of the Milky Way galaxy.

And then it turned out that this isn't the only galaxy. We live in this one, but there are many others. And as this picture shows, there are many different kinds of galaxies of which ours might just be this one. There are in fact a hundred billion other galaxies, each of which contains something like a hundred billion stars. Think of how many stars and planets and kinds of life there might be in this vast and awesome universe.

As long as there have been humans, we have searched for our place in the cosmos. Where are we? Who are we? We find that we live on an insignificant planet of a humdrum star lost in a galaxy tucked away in some forgotten corner of a universe in which there are far more galaxies than people. We make the world significant by the courage of our questions, and by the depth of our answers.

We embarked on our journey to the stars 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 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.

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