Time: A Traveler's Guide

Clifford A. Pickover
Oxford University Press
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"Time is a relationship that we have with the rest of the universe; or more accurately, we are one of the clocks, measuring one kind of time. Animals and aliens may measure it differently. We may even be able to change our way of marking time one day, and open up new realms of experience, in which a day today will be a million years."
- George Zebrowski, OMNI, 1994

"'It's against reason,' said Filby. 'What reason?' said the Time Traveller."
- H. G. Wells, The Time Machine

"The job of science is to enable the inquiring mind to feel at home in a mysterious universe." - Lewis Carroll Epstein, Relativity Visualized

"The brain is a three-pound mass you can hold in your hand that can conceive of a universe a hundred-billion light-years across". - Marian Diamond

"Does anybody really know what time it is?" - Chicago

The Quest for Eternity

   What is time? Is time travel possible? For centuries, these questions have intrigued mystics, philosophers, and scientists. Much of ancient Greek philosophy was concerned with understanding the concept of eternity, and the subject of time is central to all the worlds' religions and cultures. Can the flow of time be stopped? Certainly some mystics thought so. Angelus Silesius, a sixth-century philosopher and poet, thought the flow of time could be suspended by mental powers:

Time is of your own making;
its clock ticks in your head.
The moment you stop thought
time too stops dead.

   This book is mostly about the science of time travel and only touches briefly on mysticism. However, the line between science and mysticism sometimes grows thin. Today, physicists would agree that time is one of the strangest properties of our universe. In fact, there is a story circulating among scientists of an immigrant to America who has lost his watch. He walks up to a man on a New York street and asks, "Please, Sir, what is time?" The scientist replies, "I'm sorry, you'll have to ask a philosopher. I'm just a physicist."

   Most cultures have a grammar with past and future tenses, and also demarcations like seconds and minutes, and yesterday and tomorrow. Yet we cannot say exactly what time is. Although the study of time became scientific during the time of Galileo and Newton, a comprehensive explanation was given only in this century by Einstein who declared, in effect, time is simply what a clock reads. The clock can be the rotation of a planet, sand falling in an hour glass, a heartbeat, or vibrations of a cesium atom. A typical grandfather clock follows the simple Newtonian law that states that the velocity of a body not subject to external forces remains constant. This means that clock hands travel equal distances in equal times. While this kind of clock is useful for everyday life, modern science finds that time can be warped in various ways, like clay in the hands of a cosmic sculptor.

   Science-fiction authors have had various uses for time machines including: dinosaur hunting, tourism, visits to one's ancestors, and animal collecting. Ever since the time of H. G. Wells's famous novel The Time Machine (1895), people have grown increasingly intrigued by the idea of traveling through time. (I was lucky enough to have chats with H. G. Wells's grandson who told me that his grandfather's book The Time Machine has never been out of print, something rare for a book a century old.) In the book, the protagonist uses a "black and polished brass" time machine to gain mechanical control over time as well as return to the present to bring back his story and assess the consequences of the present on the future. Wells was a graduate of the Imperial College of Science and Technology, and scientific language permeates his discussions. Many believe Wells's book to be the first story about a time machine, but seven years before 22-year-old Wells wrote the first version of The Time Machine, Edward Page Mitchell, an editor of the New York Sun, published "The Clock That Went Backward." One of the earliest methods for fictional time travel didn't even involve a machine; the main character in Washington Irving's "Rip Van Winkle" (1819) simply fell asleep for decades. King Arthur's daughter Gweneth slept for 500 years under Merlin's spell.

   Ancient legends of time distortion are, in fact, quite common. One of the most poetic descriptions of time travel occurs in a popular medieval legend describing a monk entranced for a minute by the song of a magical bird. When the bird stops singing, the monk discovers that several hundred years have passed. Another example is the Moslem legend of Mohamad carried by a mare into heaven. After a long visit, the prophet returns to Earth just in time to catch a jar of water the horse had kicked over before starting its ascent.

   Today, we know that time travel need not be confined to myths, science fiction, Hollywood movies, or even speculation by theoretical physicists. Time travel is possible. For example, an object traveling at high speeds ages more slowly than a stationary object.:sup.1:esup. This means that if you were to travel into outer space and return, moving close to light-speed, you could travel thousands of years into the Earth's future. In addition to high-speed travel, researchers have proposed numerous ways in which time machines can be built that do not seem to violate any known laws of physics. These methods allow you to travel to any point in the world's past or future and are discussed towards the end of this book.

   Newton's most important contribution to science was his mathematical definition of how motion changes with time (Figure 1). He showed the force causing apples to fall is same as the force that drives planetary motions and produces tides. However, Newton was puzzled by the fact that gravity seemed to operate instantaneously at a distance. He admitted he could only describe it without understanding how it worked. Not until Einstein's general theory of relativity was gravity changed from a "force" to the movement of matter along the shortest path in a curved spacetime. The Sun bends spacetime, and spacetime tells the planets how to move. For Newton, both space and time were absolute. Space was a fixed, infinite, unmoving metric against which absolute motions could be measured. Newton also believed the universe was pervaded by a single absolute time that could be symbolized by an imaginary clock off somewhere in space. Einstein changed all this with his relativity theories, and once wrote "Newton, forgive me."

   Einstein's first major contribution to the study of time occurred when he revolutionized physics with his "special theory of relativity" by showing how time changes with motion. Today, scientists do not see problems of time or motion as "absolute" with a single correct answer. Because time is relative to the speed one is traveling at, there can never be a clock at the center of the Universe to which everyone can set their watches. Your entire life is the blink of an eye to an alien traveling close to the speed of light. Today, Newton's mechanics have become a special case within Einstein's theory of relativity. Einstein's relativity will eventually become a subset of a new science more comprehensive in its description of the fabric of our universe. (The word "relativity" derives from the fact that the appearance of the world depends on our state of motion; it is "relative.")

   We are a moment in astronomic time, a transient guest of the Earth. Our wet, wrinkled brains do not allow us to comprehend many mysteries of time and space. Our brains evolved to make us run from sabre-toothed tigers on the African savanna, to hunt deer, and to efficiently scavenge from the kills of large carnivores. Despite our mental limitations, we have come remarkably far. We have managed to pull back the cosmic curtains a crack to let in the light. Questions raised by physicists, from Newton to Goedel to Einstein to Hawking, are among the most profound we can ask...

   Is time real? Does it flow in one direction only? Does it have a beginning or end? What is eternity? None of these questions can be answered to scientists' satisfaction. Yet the mere asking of these questions stretches our minds, and the continual search for answers provides useful insights along the way.

Who this Book is For

   This book will allow you to travel through time and space, and you needn't be an expert in physics. Some information is repeated so that each chapter contains sufficient background information, but I suggest you read the chapters in order as you gradually build your knowledge. To facilitate your journey, I start most chapters with a dialog between quirky explorers who experiment with time from within the (usually) safe confines of a Museum of Music in New York City. This simple science fiction is not only good fun but it also serves a serious purpose, that of expanding your imagination. We might not yet be able to easily travel in time like the characters in the story, but at least time travel is not forbidden by the current laws of physics. As you read the story, think about how humans might respond to future developments in science that could lead to time travel.

   When writing this book, I did not set out to write a systematic and comprehensive study of time. Instead, I have chosen a selection of topics relating to time travel which I think will enlighten a wide range of readers. Although Einstein's theory of relativity is nearly a century old, its strange consequences are still not widely known. People still often learn of them with a sense of awe, mystery and bewilderment. Even armed with Einstein's theories, humans have only a vague understanding of time, and various problems and paradoxes still need to be solved.

   By the time you've finished this book, you will be able to

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