If the world is to be understood, if we are to avoid such logical paradoxes when traveling at high speeds, there are some rules, commandments of Nature, that must be obeyed. Einstein codified these rules in the special theory of relativity. Light (reflected or emitted) from an object travels at the same velocity whether the object is moving or stationary: Thou shalt not add thy speed to the speed of light. Also, no material object may move faster than light: Thou shalt not travel at or beyond the speed of light. Nothing in physics prevents you from traveling as close to the speed of light as you like; 99.9 percent of the speed of light would be 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. Otherwise, you could get to any speed you wanted by adding velocities on a moving platform.
Europeans around the turn of the century generally believed in privileged frames of reference: that German, or French, or British culture and political organization were better than those of other countries; that Europeans were superior to other peoples who were fortunate enough to be colonized. The social and political application of the ideas of Aristarchus and Copernicus was rejected or ignored. The young Einstein rebelled against the notion of privileged frames of reference in physics as much as he did in politics. In a universe filled with stars rushing helter-skelter in all directions, there was no place that was “at rest,” no framework from which to view the universe that was superior to any other framework. This is what the word relativity means. The idea is very simple, despite its magical trappings: in viewing the universe, every place is as good as every other place. The laws of Nature must be identical no matter who is describing them. If this is to be true—and it would be stunning if there were something special about our insigificant location in the Cosmos—then it follows that no one may travel faster than light.
We hear the crack of a bullwhip because its tip is moving faster than the speed of sound, creating a shock wave, a small sonic boom. A thunderclap has a similar origin. It was once thought that airplanes could not travel faster than sound. Today supersonic flight is commonplace. But the light barrier is different from the sound barrier. It is not merely an engineering problem like the one the supersonic airplane solves. It is a fundamental law of Nature, as basic as gravity. And there are no phenomena in our experience—like the crack of the bullwhip or the clap of thunder for sound—to suggest the possibility of traveling in a vacuum faster than light. On the contrary, there is an extremely wide range of experience—with nuclear accelerators and atomic clocks, for example—in precise quantitative agreement with special relativity.
The problems of simultaneity do not apply to sound as they do to light because sound is propagated through some material medium, usually air. The sound wave that reaches you when a friend is talking is the motion of molecules in the air. Light, however, travels in a vacuum. There are restrictions on how molecules of air can move which do not apply to a vacuum. Light from the Sun reaches us across the intervening empty space, but no matter how carefully we listen, we do not hear the crackle of sunspots or the thunder of the solar flares. It was once thought, in the days before relativity, that light did propagate through a special medium that permeated all of space, called “the luminiferous aether.” But the famous Michelson-Morley experiment demonstrated that such an aether does not exist.
We sometimes hear of things that can travel faster than light. Something called “the speed of thought” is occasionally proffered. This is an exceptionally silly notion—especially since the speed of impulses through the neurons in our brains is about the same as the speed of a donkey cart. That human beings have been clever enough to devise relativity shows that we think well, but I do not think we can boast about thinking fast. The electrical impulses in modern computers do, however, travel nearly at the speed of light.
Special relativity, fully worked out by Einstein in his middle twenties, is supported by every experiment performed to check it. Perhaps tomorrow someone will invent a theory consistent with everything else we know that circumvents paradoxes on such matters as simultaneity, avoids privileged reference frames and still permits travel faster than light. But I doubt it very much. Einstein’s prohibition against traveling faster than light may clash with our common sense. But on this question, why should we trust common sense? Why should our experience at 10 kilometers an hour constrain the laws of nature at 300,000 kilometers per second? Relativity does set limits on what humans can ultimately do. But the universe is not required to be in perfect harmony with human ambition. Special relativity removes from our grasp one way of reaching the stars, the ship that can go faster than light. Tantalizingly, it suggests another and quite unexpected method.
Following George Gamow, let us imagine a place where the speed of light is not its true value of 300,000 kilometers per second, but something very modest: 40 kilometers per hour, say—and strictly enforced. (There are no penalties for breaking laws of Nature, because there are no crimes: Nature is self-regulating and merely arranges things so that its prohibitions are impossible to transgress.) Imagine that you are approaching the speed of light on a motor scooter. (Relativity is rich in sentences beginning “Imagine …” Einstein called such an exercise a Gedankenexperiment, a thought experiment.) As your speed increases, you begin to see around the corners of passing objects. While you are rigidly facing forward, things that are behind you appear within your forward field of vision. Close to the speed of light, from your point of view, the world looks very odd—ultimately everything is squeezed into a tiny circular window, which stays just ahead of you. From the standpoint of a stationary observer, light reflected off you is reddened as you depart and blued as you return. If you travel toward the observer at almost the speed of light, you will become enveloped in an eerie chromatic radiance: your usually invisible infrared emission will be shifted to the shorter visible wavelengths. You become compressed in the direction of motion, your mass increases, and time, as you experience it, slows down, a breathtaking consequence of traveling close to the speed of light called time dilation. But from the standpoint of an observer moving with you—perhaps the scooter has a second seat—none of these effects occur.
These peculiar and at first perplexing predictions of special relativity are true in the deepest sense that anything in science is true. They depend on your relative motion. But they are real, not optical illusions. They can be demonstrated by simple mathematics, mainly first-year algebra and therefore understandable to any educated person. They are also consistent with many experiments. Very accurate clocks carried in airplanes slow down a little compared to stationary clocks. Nuclear accelerators are designed to allow for the increase of mass with increasing speed; if they were not designed in this way, accelerated particles would all smash into the walls of the apparatus, and there would be little to do in experimental nuclear physics. A speed is a distance divided by a time. Since near the velocity of light we cannot simply add speeds, as we are used to doing in the workaday world, the familiar notions of absolute space and absolute time—independent of your relative motion—must give way. That is why you shrink. That is the reason for time dilation.
Traveling close to the speed of light you would hardly age at all, but your friends and your relatives back home would be aging at the usual rate. When you returned from your relativistic journey, what a difference there would be between your friends and you, they having aged decades, say, and you having aged hardly at all! 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. But is it possible, in terms of practical engineering, to travel close to the speed of light? Is a starship feasible?
Tuscany was not only the caldron of some of the thinking of the young Albert Einstein; it was also the home of another great genius who lived 400 years earlier, Leonardo da Vinci, who delighted in climbing the Tuscan hills and viewing the ground from a gr
eat height, as if he were soaring like a bird. He drew the first aerial perspectives of landscapes, towns and fortifications. Among Leonardo’s many interests and accomplishments—in painting, sculpture, anatomy, geology, natural history, military and civil engineering—he had a great passion: to devise and fabricate a machine that could fly. He drew pictures, constructed models, built full-size prototypes—and not one of them worked. No sufficiently powerful and lightweight engine then existed. The designs, however, were brilliant and encouraged the engineers of future times. Leonardo himself was depressed by these failures. But it was hardly his fault. He was trapped in the fifteenth century.
A similar case occurred in 1939 when a group of engineers calling themselves the British Interplanetary Society designed a ship to take people to the Moon—using 1939 technology. It was by no means identical to the design of the Apollo spacecraft, which accomplished exactly this mission three decades later, but it suggested that a mission to the Moon might one day be a practical engineering possibility.
Today we have preliminary designs for ships to take people to the stars. None of these spacecraft is imagined to leave the Earth directly. Rather, they are constructed in Earth orbit from where they are launched on their long interstellar journeys. One of them was called Project Orion after the constellation, a reminder that the ship’s ultimate objective was the stars. Orion was designed to utilize explosions of hydrogen bombs, nuclear weapons, against an inertial plate, each explosion providing a kind of “putt-putt,” a vast nuclear motorboat in space. Orion seems entirely practical from an engineering point of view. By its very nature it would have produced vast quantities of radioactive debris, but for conscientious mission profiles only in the emptiness of interplanetary or interstellar space. Orion was under serious development in the United States until the signing of the international treaty that forbids the detonation of nuclear weapons in space. This seems to me a great pity. The Orion starship is the best use of nuclear weapons I can think of.
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 existing fission power plants. We do not have fusion reactors yet, but they are confidently expected in the new few decades. Orion and Daedalus might travel at 10 percent the speed of light. A trip to Alpha Centauri, 4.3 light-years away, would then take forty-three years, less than a human lifetime. Such ships could not travel close enough to the speed of light for special relativistic time dilation to become important. Even with optimistic projections on the development of our technology, it does not seem likely that Orion, Daedalus or their ilk will be built before the middle of the twenty-first century, although if we wished we could build Orion now.
For voyages beyond the nearest stars, something else must be done. Perhaps Orion and Daedalus could be used as multigeneration ships, so those arriving at a planet of another star would be the remote descendants of those who had set out some centuries before. Or perhaps a safe means of hibernation for humans will be found, so that the space travelers could be frozen and then reawakened centuries later. These nonrelativistic starships, enormously expensive as they would be, look relatively easy to design and build and use compared to starships that travel close to the speed of light. Other star systems are accessible to the human species, but only after great effort.
Fast interstellar spaceflight—with the ship velocity approaching the speed of light—is an objective not for a hundred years but for a thousand or ten thousand. But it is in principle possible. A kind of interstellar ramjet has been proposed by R. W. Bussard which scoops up the diffuse matter, mostly hydrogen atoms, that floats between the stars, accelerates it into a fusion engine and ejects it out the back. The hydrogen would be used both as fuel and as reaction mass. But in deep space there is only about one atom in every ten cubic centimeters, a volume the size of a grape. For the ramjet to work, it needs a frontal scoop hundreds of kilometers across. When the ship reaches relativistic velocities, the hydrogen atoms will be moving with respect to the spaceship at close to the speed of light. If adequate precautions are not taken, the spaceship and its passengers will be fried by these induced cosmic rays. One proposed solution uses a laser to strip the electrons off the interstellar atoms and make them electrically charged while they are still some distance away, and an extremely strong magnetic field 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 Earth. We are talking of engines the size of small worlds.
But let us spend a moment thinking about such a ship. The Earth gravitationally attracts us with a certain force, which if we are falling we experience as an acceleration. Were we to fall out of a tree—and many of our proto-human ancestors must have done so—we would plummet faster and faster, increasing our fall speed by ten meters (or thirty-two feet) per second, every second. This acceleration, which characterizes the force of gravity holding us to the Earth’s surface, is called 1 g, g for Earth gravity. We are comfortable with accelerations of 1 g; we have grown up with 1 g. If we lived in an interstellar spacecraft that could accelerate at 1 g, we would find ourselves in a perfectly natural environment. In fact, the equivalence between gravitational forces and the forces we would feel in an accelerating spaceship is a major feature of Einstein’s later general theory of relativity. With a continuous 1 g acceleration, after one year in space we would be traveling very close to the speed of light [(0.01 km/sec2) × (3 × 107 sec) = 3 × 105 km/sec].
Suppose that such a spacecraft accelerates at 1 g, approaching more and more closely to the speed of light until the midpoint of the journey; and then is turned around and decelerates at 1 g until arriving at its destination. For most of the trip the velocity would be very close to the speed of light and time would slow down enormously. A nearby mission objective, a sun that may have planets, is Barnard’s Star, about six light-years away. It could be reached in about eight years as measured by clocks aboard the ship; the center of the Milky Way, in twenty-one years; M31, the Andromeda galaxy, in twenty-eight years. Of course, people left behind on Earth would see things differently. Instead of twenty-one years to the center of the Galaxy, they would measure an elapsed time of 30,000 years. When we got home, few of our friends would be left to greet us. In principle, such a journey, mounting the decimal points ever closer to the speed of light, would even permit us to circumnavigate the known universe in some fifty-six years ship time. We would return tens of billions of years in our future—to find the Earth a charred cinder and the Sun dead. Relativistic spaceflight makes the universe accessible to advanced civilizations, but only to those who go on the journey. There seems to be no way for information to travel back to those left behind any faster than the speed of light.
The designs for Orion, Daedalus and the Bussard Ramjet are probably farther from the actual interstellar spacecraft we will one day build than Leonardo’s models are from today’s supersonic transports. But if we do not destroy ourselves, I believe that we will one day venture to the stars. When our solar system is all explored, the planets of other stars will beckon.
Space travel and time travel are connected. We can travel fast into space only by traveling fast into the future. But what of the past? Could we return to the past and change it? Could we make events turn out differently from what the history books assert? We travel slowly into the future all the time, at the rate of one day every day. With relativistic spaceflight we could travel fast into the future. But many physicists believe that a voyage into the past is impossible. Even if you had a device that could travel backwards in time, they say, you would be unable to do anything that would make any difference. If you journeyed into the past and prevented your parents from meeting, then you would never have been born—which is something of a contradiction, since you clearly exist. Like the proof of the irrationality of √2, like the discussion of simultaneity in special relativity, this is an argument in which the premise is challe
nged because the conclusion seems absurd.
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.