by Carl Sagan
Yet you cannot carry this projection too far, because sooner or later you bump your nose. For example, when we get to relativity or quantum mechanics, we discover realms that are alien to our everyday experience, and suddenly the laws of nature turn out to be astonishingly different. The idea that as I walk in this direction my watch goes slightly slower and I am contracted in the direction of motion and my mass has increased slightly does not correspond to everyday experience. Nevertheless, that is an absolutely certain consequence of special relativity, and the reason it does not conform to common sense is that we are not in the habit of traveling close to the speed of light. We may one day be in that habit, and then the Lorentz transformations [1] will be natural, intuitive. But they aren't yet.
The idea that there is a cosmic speed limit, the speed of light, beyond which no material object can travel, again seems counterintuitive, even though it can be demonstrated, as Einstein did, from an astonishingly simple and basic analysis of what we mean by space, time, simultaneity, and so on.
Or if I were to propose to you that my arm could be in this position or in that position but it would be forbidden by the laws of nature to be in some intermediate position, that would likely strike you as absurd, as contrary to experience. And yet on the subatomic level, there is quantization of energy and position and momentum. The reason it seems counterintuitive is that we are not ordinarily down at the level of the very small, where quantum effects dominate.
So the history of science-especially physics-has in part been the tension between the natural tendency to project our everyday experience on the universe and the universe's noncompliance with this human tendency.
Now, there is another tendency from the psychological or social sphere projected upon the natural world. And that is the idea of privilege. Ever since the invention of civilization, there have been privileged classes in societies. There have been some groups that oppress others and that work to maintain these hierarchies of power. The children of the privileged grow up expecting that, through no particular effort of their own, they will retain a privileged position. At birth all of us imagine that we are the universe, and we don't distinguish the boundaries between ourselves and those around us. This is well established in infants. As we grow up, we discover that there are others who are apparently autonomous and that we're only one among many other people. And then, at least in some social situations, there is the sense that we are central, important. Other social groups, of course, don't have that view. But it is generally those with privilege and status, especially in ancient times, who became the scientists, and there was a natural projection of those attitudes upon the universe.
So, for example, Aristotle provided powerful arguments, none of them instantly dismissible, that the heavens moved and not the Earth, that the Earth is stationary and that the Sun, the Moon, the planets, the stars, rise and set by physically moving once around the Earth every day. With the exception of this kind of motion, the heavens were thought to be changeless. The Earth, while stationary, had all the corruption of the universe localized here.
Up there was matter, which was perfect, unchanging, a special kind of celestial matter that is, by the way, the origin of our word "quintessential." There were four essences down here, the imagined four elements of earth, water, fire, and air, and then there was that fifth element, that fifth essence out of which the heaven stuff was made. And that's why the word "quintessential"-"fifth essence"-comes about. It's interesting to see a kind of linguistic artifact of the previous worldview still present in the Oxford Unabridged. But it's amazing what's in the Oxford Unabridged
Now, in the fifteenth century, Nicolaus Copernicus suggested a different view. He proposed that it was the Earth that rotated and that the stars were in effect motionless. He proposed moreover that in order to explain these apparent movements of the planets against the background of more distant stars, the planets and the Earth, in addition to rotating, revolved around the Sun. That is, the Earth was demoted. You know the phrase- another linguistic artifact-the world, or the Earth. What is the definite article saying? It's saying there is only one. And that also goes straight back to pre-Copernican times, as does the phrase, natural as it is, of the Sun rising and the Sun setting.
Copernicus, by the way, felt his idea to be so dangerous that it was not published until he was on his deathbed, and even then it had an outrageous introduction by a man named Osiander, who was worried that it was too incendiary, too radical. Osiander wrote, in effect, "Copernicus doesn't really believe this. This is just a means of calculating. And don't anybody think he's saying anything contrary to doctrine." This was an important issue.
Aristotle's views had been accepted fully by the medieval church-Thomas Aquinas played a major role in that-and therefore by the time of Copernicus a serious objection to a geocentric universe was a theological offense. And you can see why, because if Copernicus were right, then the Earth would be demoted, no longer the Earth, the world, but just a world, an earth, one of many.
And then came the still more unsettling possibility, the idea that the stars were distant suns and that they also had planets going around them and that, after all, you can see thousands of stars with the naked eye. Suddenly the Earth is not only not central to this solar system but no longer central to any solar system. Well, there was a period in which we hoped that we were at the center of the Milky Way Galaxy. If we weren't at the center of our solar system, at least our solar system was at the center of the Milky Way Galaxy. And the definitive disproof of that occurred only in the 1920s, to give you an idea of how long it took for Copernican ideas to reach galactic astronomy.
And then there was the hope that, well, at least maybe our galaxy was at the center of all the other galaxies, all those many billions of other galaxies. But modern views have it that there is no such thing as a center of the universe, at least not in ordinary three-dimensional space, and we are certainly not at it.
So those who wished for some central cosmic purpose for us, or at least our world, or at least our solar system, or at least our galaxy, have been disappointed, progressively disappointed. The universe is not responsive to our ambitious expectations. A grinding of heels can be heard screeching across the last five centuries as scientists have revealed the noncentrality of our position and as many others have fought to resist that insight to the bitter end. The Catholic Church threatened Galileo with torture if he persisted in the heresy that it was the Earth that moved and not the Sun and the rest of the celestial bodies. It was serious business.
Now, at the same time, another of the Aristotelian precepts was challenged. That was the idea that except for the moving of crystal spheres into which the planets were embedded, nothing changes up in the heavens. In 1572 there was a supernova explosion in the constellation Cassiopeia. A star that had previously been invisible suddenly became so bright that it could be seen by the naked eye. The Danish astronomer Tycho Brahe noticed it. Well, if nothing changes up there, how is it that suddenly a star appeared-I mean suddenly, in a period of a week or less, from invisibility to something easily seen-and then stayed for some months before fading away? Something was wrong.
Just a few years later, there was an impressive comet, the Comet of 1577, and Tycho Brahe-decades after Copernicus- had the presence of mind to organize an international set of observations of that comet. The idea was to see if it was down here in the Earth's atmosphere, as Aristotle had insisted it must be, or up there among the planets. Part of the reason that Aristotle had insisted that the comets were meteorological phenomena was his belief in an unchanging heaven.
So Brahe thought, if the comet is close to the Earth, then two observers far from each other will see it against different background stars. This is called parallax, which you easily can demonstrate by simply winking your eye, first the left and then the right, with a finger propped up about a foot in front of your nose. The finger seems to move as you blink.
Brahe reasoned that if the comet was very far away, then the
two observers who were far apart would see it in almost exactly the same part of the sky. You could determine how far away it was by how much it moved between those two different vantage points, how much the parallax was. And Brahe determined it was surely farther away than the Moon and, therefore, up there, in the planetary realm, and not down here, where the weather is. That was another upsetting discovery for the institutionalized Aristotelian wisdom.
Now, as science has progressed, there have been-one after another-a series of assaults on human vainglory. One of them, for example, is the discovery that the Earth is much older than anyone had expected. Human history goes back only a few thousand years. Many people believed that the world was not much older than human history. And there was no sense of evolution, no sense of vast vistas of time. And then the geological and pa-leontological evidence began to accumulate, making it very difficult to see how the geological forms and the fossils of now-extinct plants and animals could have come into being, unless the Earth were enormously older than the few thousand years that had been projected. That is a battle still being fought. In the United States, for example, there are people who are called "creationists," the more radical of whom insist that the Earth is less than ten thousand years old. The shorter the age of the Earth, the greater the relative role of humans in the history of the Earth is. If the Earth is, as we certainly know it to be, 4,500 million years old and the human species at most a few million years old, probably less than that, then we have been here for only an instant of geological time, for less than one one-thousandth of the history of the Earth, and therefore in time, as in space, we have been demoted from the central to an incidental aspect.
And then evolution itself was still a further disquieting discovery, because at least it had been hoped that humans were separate from the rest of the natural world, that we had been specifically put here in a way different from petunias, let's say. And yet Darwin's historic work showed that we were very likely related in an evolutionary sense with all the other beasts and vegetables on the planet. And there remain many people who are enormously offended by this idea.
This sense of offense has-I'm only speculating-deep psychological roots. Part of it is, I believe, an unwillingness to come to grips with the more instinctive aspects of human nature. But I believe it is essential to understand this if we wish to survive. I think ignoring that, imagining all humans are rational actors in the present phase, is enormously dangerous in an age of nuclear weapons. I think the discomfort that some people feel in going to the monkey cages at the zoo is a warning sign.
Then, in the early part of this century, there was still another such assault, which came with special relativity. Because one of the central points of special relativity is that there are no privileged frames of reference, that we are not in an important position or state of motion. There is nothing privileged about the velocity that we have or the acceleration that we have; the universe can be understood precisely if it is true that we do not have a special frame of reference.
Now, it's certainly true that there is something special about our position in time. The universe has changed. A microsecond after the Big Bang, it was quite different from how it is right now. So no one maintains these days that there is not something special about our epoch in the sense that the universe itself evolves. But in terms of position, velocity, and acceleration, there is nothing privileged about where we are. This insight was obtained by a young man who was opposed to privilege in the social sphere. If you look at Einstein's autobiographical writings, I think it is quite clear that his opposition to privilege in the social world was connected with his opposition to privilege in fundamental physics.
Well, if we don't have a distinctive position or velocity or acceleration, or a separate origin from the other plants and animals, then at least, maybe, we are the smartest beings in the entire universe. And that is our uniqueness. So today the battle, the Copernican battle, is, in somewhat covert form, being waged on the issue of extraterrestrial intelligence. Now, this doesn't guarantee that there is extraterrestrial intelligence. It may be that the Copernican insights-the principle of mediocrity, if you wish to call it that-worked for all these other things, but on extraterrestrial life it doesn't, and we are unique. I will come back to that later, but I believe that the ongoing Copernican revolution is relevant to this debate as well.
There is today another battlefield on which the Copernican insights are being attacked. It is connected with one of the classic arguments for the existence of God, that is, the Western kind of God, namely, the argument from design.
The idea of the argument from design goes like this: Suppose you know nothing about watches and you come upon an elegantly tooled pocketwatch. And you open it and everything is going tick-tick-tick-tick, and there are all those gears and levers and burnished brass, and such things are not made in nature. Therefore the existence of such a complex mechanism, the existence of the watch, implies a watchmaker. Now we go and look at an organism. Let's take a very modest organism, a bacterium. Well, you look in there and you find a much more complex mechanism than a pocketwatch. A bacterium has many more moving parts, much more information than what you would have to write down in order to describe how to make a pocket-watch. And yet the world is full of bacteria. They're everywhere, enormous quantities of them. And is it possible that this being, far more complex than a watch, arose spontaneously out of who knows what sort of collisions of atoms? Isn't it more likely that this "watch" also implies a watchmaker? That is one example of the argument from design, and you can imagine that every part of nature might be vulnerable to such an interpretation. Everything, that is, except utter chaos.
Well, Darwin showed, through natural selection, that there was a way other than the existence of a Watchmaker, a way in which it was possible for enormous order to emerge from a more disordered natural world without the interposition of any capital-W Watchmaker. That was natural selection.
The ideas behind natural selection were that there was such a thing as a hereditary material, that there were spontaneous changes in the hereditary material, that those changes were expressed in the external form and function of the organism, that organisms made many more copies of themselves than the environment could support, and therefore that some selection among various natural experiments was made by the environment for reproductive success, that some organisms, by pure accident, were better suited to leaving offspring than others.
Now, an essential aspect of this idea is that you need to have enough time. If the universe is only a few thousand years old, then Darwinian evolution is nonsense. There isn't time. On the other hand, if the Earth is a few thousand million years old, then there is enormous time, and we can at least contemplate that this is the source, as certainly all of modern biology suggests, of the complexity and beauty of the biological world.
This sort of argument from design we can find in other aspects of nature. And I'd like to discuss two of them. One is Isaac Newton's understanding of the order within the solar system, and the other is a most interesting although, I believe, flawed approach to the laws of nature, put forth in recent times, called the "anthropic principle."
One of Newton's many extraordinary accomplishments was to show that, granting a few simple and highly nonarbitrary laws of nature, he could deduce to high precision the motion of the planets in the solar system. The Newtonian method has remained valid from that time to this. It is precisely Newtonian physics that is used routinely in my line of work, sending spacecraft to the planets, something that you might be tempted to say was far beyond Newton's expectations. But he in fact did envision at least the launching of objects into Earth orbit.
What Newton found is that there is a distinctive plane to the solar system. Copernicus had essentially proposed this, but Newton showed in detail how it worked. The orbits of the planets circle the Sun, all of them very close to the ecliptic plane, also called the zodiacal plane (because the constellations of the zodiac are arrayed around this plane). And th
at's why the planets and the Sun and the Moon apparently move through the zodiac. "Why is everything so regular?" Newton asked. "Why are all the planets in the same plane? Why do they all go around the Sun in the same direction?" It's not that Mercury goes around one way while Venus goes around in another way. All of the planets go around in the same sense. And, as far as he knew then, they all rotated in the same sense. The planets had something astonishingly regular about them. On the other hand, the comets that were known in his day were helter-skelter. Their orbits were at every possible angle to the ecliptic plane. Some went around in the direct sense; some went around in the retrograde sense. And they were tilted in all sorts of directions.
Newton believed that the distribution of cometary orbits was the state of nature and that is how the planets would have moved had there not been an intervening hand. He believed that God established the initial conditions for the planets that made them all go around the Sun in the same direction, in the same plane, and rotating in a compatible sense.
Now, in fact, this is not a strong conclusion. And Newton, who was extraordinarily perceptive in so many areas, was clearly not here.