Cosmos - Carl Sagan [113]
But other physicists propose that two alternative histories, two equally valid realities, could exist side by side—the one you know and the one in which you were never born. Perhaps time itself has many potential dimensions, despite the fact that we are condemned to experience only one of them. Suppose you could go back into the past and change it—by persuading Queen Isabella not to support Christopher Columbus, for example. Then, it is argued, you would have set into motion a different sequence of historical events, which those you left behind in our time line would never know about. If that kind of time travel were possible, then every imaginable alternative history might in some sense really exist.
History consists for the most part of a complex bundle of deeply interwoven threads, social, cultural and economic forces that are not easily unraveled. The countless small, unpredictable and random events that flow on continually often have no long-range consequences. But some, those occurring at critical junctures or branch points, may change the pattern of history. There may be cases where profound changes can be made by relatively trivial adjustments. The farther in the past such an event is, the more powerful may be its influence—because the longer the lever arm of time becomes.
A polio virus is a tiny microorganism. We encounter many of them every day. But only rarely, fortunately, does one of them infect one of us and cause this dread disease. Franklin D. Roosevelt, the thirty-second President of the United States, had polio. Because the disease was crippling, it may have provided Roosevelt with a greater compassion for the underdog; or perhaps it improved his striving for success. If Roosevelt’s personality had been different, or if he had never had the ambition to be President of the United States, the great depression of the 1930’s, World War II and the development of nuclear weapons might just possibly have turned out differently. The future of the world might have been altered. But a virus is an insignificant thing, only a millionth of a centimeter across. It is hardly anything at all.
On the other hand, suppose our time traveler had persuaded Queen Isabella that Columbus’ geography was faulty, that from Eratosthenes’ estimate of the circumference of the Earth, Columbus could never reach Asia. Almost certainly some other European would have come along within a few decades and sailed west to the New World. Improvements in navigation, the lure of the spice trade and competition among rival European powers made the discovery of America around 1500 more or less inevitable. Of course, there would today be no nation of Colombia, or District of Columbia or Columbus, Ohio, or Columbia University in the Americas. But the overall course of history might have turned out more or less the same. In order to affect the future profoundly, a time traveler would probably have to intervene in a number of carefully chosen events, to change the weave of history.
It is a lovely fantasy, to explore those worlds that never were. By visiting them we could truly understand how history works; history could become an experimental science. If an apparently pivotal person had never lived—Plato, say, or Paul, or Peter the Great—how different would the world be? What if the scientific tradition of the ancient Ionian Greeks had survived and flourished? That would have required many of the social forces of the time to have been different—including the prevailing belief that slavery was natural and right. But what if that light that dawned in the eastern Mediterranean 2,500 years ago had not flickered out? What if science and the experimental method and the dignity of crafts and mechanical arts had been vigorously pursued 2,000 years before the Industrial Revolution? What if the power of this new mode of thought had been more generally appreciated? I sometimes think we might then have saved ten or twenty centuries. Perhaps the contributions of Leonardo