Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time
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Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time
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Overview
It was H. G. Wells who coined the term “time machine”—but the concept of time travel, both forward and backward, has always provoked fascination and yearning. It has mostly been dismissed as an impossibility in the world of physics; yet theories posited by Einstein, and advanced by scientists including Stephen Hawking and Kip Thorne, suggest that the phenomenon could actually occur.
Building on these ideas, J. Richard Gott, a professor who has written on the subject for Scientific American, Time, and other publications, describes how travel to the future is not only possible but has already happened—and contemplates whether travel to the past is also conceivable. This look at the surprising facts behind the science fiction of time travel “deserves the attention of anyone wanting wider intellectual horizons” (Booklist).
“Impressively clear language. Practical tips for chrononauts on their options for travel and the contingencies to prepare for make everything sound bizarrely plausible. Gott clearly enjoys his subject and his excitement and humor are contagious; this book is a delight to read.” —Publishers Weekly
Product Details
| ISBN-13: | 9780547526577 |
|---|---|
| Publisher: | Houghton Mifflin Harcourt |
| Publication date: | 08/01/2017 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 304 |
| Sales rank: | 53,189 |
| File size: | 8 MB |
About the Author
Read an Excerpt
CHAPTER 1
DREAMING OF TIME TRAVEL
Man ... can go up against gravitation in a balloon, and why should he not hope that ultimately he may be able to stop or accelerate his drift along the Time-Dimension, or even turn about and travel the other way.
— H. G. WELLS, THE TIME MACHINE, 1895
What Would You Do With a Time Machine?
No idea from science fiction has captured the human imagination as much as time travel. What would you do if you had a time machine? You might go to the future and take a vacation in the twenty-third century. You might bring back a cure for cancer.
Then again, you might return to the past to rescue a lost loved one. You could kill Hitler and prevent World War II or book passage on the Titanic to warn the captain about the iceberg. But what if the captain ignored your warning, as he ignored all the other warnings about icebergs that he received, so that the great ship sank after all? In other words, would time travel let you change the past? The notion of time travel to the past can suggest paradoxes. What if, on a trip to the past, you accidentally killed your grandmother before she gave birth to your mother?
Even if changing the past is impossible, going there might still be very interesting. Even if you could not change history from the course we know it took, you still could participate in shaping that history. For example, you might go back in time to help the Allies win the Battle of the Bulge in World War II. People love to reenact Civil War battles — what if it were possible to participate in the real thing? Selecting a battle won by your side would give you the thrill of joining in the experience as well as the secure feeling of knowing the outcome. In fact, it might turn out that, in the end, the tide of battle was turned by tourists from the future. Indeed, people who have been far ahead of their time in their thinking, such as Jules Verne and Leonardo da Vinci, have sometimes been accused of being time travelers.
If you chose to embark on time travel, you could put together a stunning itinerary. You might meet historical figures such as Buddha, Muhammad, or Moses. You could see what Cleopatra really looked like or attend Shakespeare's first production of Hamlet. You might position yourself on that grassy knoll in Dallas to see for yourself whether Oswald was the lone assassin. You might take in Jesus' Sermon on the Mount and even film it. You could enjoy an evening walk through the Hanging Gardens of Babylon. The possibilities are unlimited.
We seem free to move around in space at will, but in time we are like helpless rafters in a mighty stream, propelled into the future at the rate of one second per second. One wishes one could sometimes paddle ahead to investigate the shores of the future, or perhaps turn around and go against the current to visit the past. The hope that such freedom will one day be ours is bolstered when we observe that many feats formerly thought impossible have now been realized and are even taken for granted. When Wells wrote The Time Machine in 1895, many people thought that heavier-than-air flying machines were impossible. Eventually the Wright brothers proved the skeptics wrong. Then people said that we could never break the sound barrier. But Chuck Yeager ultimately proved that the seemingly impossible was possible. Flights to the Moon were confined to the realm of fantasy — until the Apollo program achieved it. Might time travel be similar?
Today the subject of time travel has jumped from the pages of science fiction to the pages of physics journals as physicists explore whether it might be allowed by physical laws and even if it holds the key to how the universe began. In Isaac Newton's universe time travel was inconceivable. But in Einstein's universe it has become a real possibility. Time travel to the future is already known to be permitted, and physicists are investigating time travel to the past as well. To appreciate what scientists are studying now, an excellent first step is to explore major time-travel themes in science fiction, where many ideas in this arena were first advanced.
The Time Machine and Time as the Fourth Dimension
The idea of time travel gained prominence through Wells's wonderful novel. Most remarkable is his treatment of time as a fourth dimension, which anticipates Einstein's use of the concept ten years later.
The novel begins as the Time Traveler invites his friends to inspect his new invention — a time machine. He explains the idea to them:
"You know of course that a mathematical line, a line of thickness nil, has no real existence. ... Neither has a mathematical plane. These things are mere abstractions."
"That's all right," said the Psychologist.
"Nor, having only length, breadth, and thickness, can a cube have a real existence."
"There I object," said Filby. "Of course a solid body may exist. All real things —"
"... But wait a moment. Can an instantaneous cube exist?"
"Don't follow you," said Filby.
"Can a cube that does not last for any time at all, have a real existence?"
Filby became pensive. "Clearly," the Time Traveler proceeded, "any real body must have extension in four directions: it must have Length, Breadth, Thickness, and — Duration. ... There are really four dimensions, three ... of Space, and a fourth, Time. There is, however, a tendency to draw an unreal distinction between the former three dimensions and the latter because ... our consciousness moves intermittently ... along the latter from the beginning to the end of our lives."
The Time Traveler then shows his friends a small model of his invention — a metallic frame with ivory and quartz parts. One lever can propel it toward the future, and another can reverse the direction. He helps one of his friends push the future lever, and the model promptly disappears. Where did it go? It didn't move in space at all; it simply went to another time, the Time Traveler explains. His friends can't decide whether to believe him.
Next, the Time Traveler takes his friends to his home laboratory, to see his nearly complete, full-scale model. A week later he finishes the time machine, climbs aboard, and begins a remarkable journey to the future.
First he presses the future lever gently forward. Then he presses the one for stopping. He looks at his lab. Everything is the same. Then he notices the clock: "A moment before, as it seemed, it had stood at a minute or so past ten; now it was nearly half-past three!" He pushes the lever ahead again, and he can see his housekeeper flit across the room at high speed. Then he pushes the lever far forward. "The night came like the turning out of a light, and in another moment came tomorrow. ... As I put on a pace, night followed day like the flapping of a black wing. ... Presently, as I went on, still gaining velocity, the palpitation of night and day merged into one continuous grayness. ... I saw huge buildings rise up faint and fair, and pass like dreams."
Eventually, the Time Traveler brings his vehicle to a stop. The machine's dials show that he has arrived in the year 802,701. What does he find? The human race has split into two species: one, brutish and mean, living below ground — the Morlocks; the other, childlike and gentle, living above ground — the Eloi. Among the aboveground dwellers he finds a lovely young woman named Weena, whom he befriends. He discovers, to his horror, that the troglodytes living below breed and harvest the gentle people above like cattle — to eat. To make matters worse, the Morlocks manage to steal his time machine. When he finds it, he jumps aboard, and to escape the Morlocks, he pushes the lever into the extreme forward position. By the time he is able to bring the machine under control, he has moved into the far future. Mammals have become extinct, and only some crablike creatures and butterflies remain on Earth. He explores as far as 30 million years into the future, where he discovers a dull red Sun and lichen-like vegetation; the only animal life in evidence is a football-shaped creature with tentacles.
The Time Traveler then returns to his own time and to his friends. As proof of his experience in the future, he produces a couple of flowers Weena had given him, of a type unknown to his friends. After talking to his friends, the Time Traveler departs on his time machine and never returns. One friend muses about his fate. Where did he go? Did he return to the future or go instead to some prehistoric realm?
H. G. Wells's book was extraordinarily prescient in interpreting time as a fourth dimension. Einstein would use the idea in his 1905 theory of special relativity, which describes how time is measured differently by stationary and moving observers. Einstein's work, expanded by his mathematics professor Hermann Minkowski, shows that time can indeed be treated mathematically as a fourth dimension. Our universe is thus four-dimensional. By comparison, we say that the surface of Earth is two-dimensional because every point on Earth's surface can be specified by two coordinates — longitude and latitude. The universe, however, is four-dimensional. Locating an event in the universe requires four coordinates.
This example adapted from Russian physicist George Gamow further illustrates the point. If I want to invite you to a party, I must give you four coordinates. I may say the party will be at 43rd Street and 3rd Avenue on the 51st floor next New Year's Eve. The first three coordinates (43rd Street, 3rd Avenue, 51st floor) locate its position in space. Then I must tell you the time. The first two coordinates tell you where to go on the surface of the Earth, the third tells you how high to go, and the fourth tells you when to arrive. Four coordinates — four dimensions.
We may visualize our four-dimensional universe by using a three-dimensional model. Figure 1 shows such a model of the solar system. The two horizontal dimensions represent two dimensions of space (for simplicity, the third dimension of space is left out), and the vertical dimension represents the dimension of time. Up is toward the future; down is toward the past.
The first time I saw a model like this was in George Gamow's delightful book One, Two, Three ... Infinity, which I read when I was about 12 years old. It changes one's perspective. Typically, textbooks present a two-dimensional diagram of the solar system. The Sun is shown as a circular disk, and Earth a smaller disk near it. Earth's orbit is presented as a dashed circle on the flat page. This two-dimensional model captures only one instant of time. But suppose we had a movie of the solar system, showing how Earth orbits the Sun. Each frame of the movie would be a two-dimensional picture of the solar system — a snapshot at a particular time. By cutting the film into individual frames and stacking these on top of one another, you can get a clear picture of spacetime. The ascending frames show later and later events. The time of an individual frame is given by its vertical position in the stack. The Sun appears in the center of each frame as a yellow disk that does not move. Thus, within the stack, the Sun becomes a vertical yellow rod, extending from the bottom of the stack to the top — showing the Sun's progress from the past to the future. In each frame, Earth is a small blue dot, and in each ascending frame it is farther along on its orbit. So in the stack Earth becomes a blue helix winding around the yellow rod at the center. The radius of the helix is equal to the radius of Earth's orbit, 93 million miles, or, as we astronomers like to say, 8 light-minutes (because it takes light, traveling at 186,000 miles per second, about 8 minutes to cross that distance). The distance in time for the helix to complete a turn is, of course, 1 year (see Figure 1). This helix is Earth's world line, its path through spacetime. If we were to think four-dimensionally, we would see that Earth is not just a sphere — it is really a helix, a long piece of spaghetti spiraling around the Sun's world line through time.
As the Time Traveler said, all real objects have four dimensions — width, breadth, height, and duration. Real objects have an extension in time. Your dimensions are perhaps 6 feet tall, 1 foot thick, 2 feet wide, and 80 years in duration. You have a world line too. Your world line starts with your birth, snakes through space and forward in time, threading through all the events of your life, and ends at your death.
A time traveler who visits the past is just someone whose world line somehow loops back in time, where it could even intersect itself. This would allow the time traveler to shake hands with himself. The older man could meet up with his younger self and say, "Hi! I'm your future self! I've traveled back in time to say hello!" (see Figure 2). The surprised younger man would reply, "Really?" He would then continue his life, becoming old and eventually looping back to that same event — where he would recognize his younger self, shake hands, and say, "Hi! I'm your future self! I've traveled back in time to say hello!"
Back to the Future and the Grandmother Paradox
But what if, as an older man, the time traveler refuses to say hello and instead simply kills his younger self? Time travel to the past suggests such a paradox. When I do television interviews about time travel, the first question I am always asked is this: "what if you went back in time and killed your grandmother before she gave birth to your mother?" The problem is obvious: if you kill your grandmother, then your mother would have never been born, and you would never have been born; if you were never born, you could never go back in time, and so you could not kill your grandmother. This conundrum, known as the Grandmother Paradox, is often thought sufficiently potent to rule out time travel to the past.
A famous example from science-fiction stories that have explored this idea is the 1985 movie Back to the Future. The hero, played by Michael J. Fox, goes back in time to 1955 and accidentally interferes with the courtship of his parents. This creates a problem: if his parents don't fall in love, he will never be born, so his own existence is imperiled. He realizes he must act to ensure that his parents fall in love. Things don't go well at first — his mother begins to fall in love with him, the mysterious stranger, instead of his father. (Freud, take note.) To bring his parents together, he hatches an elaborate plan. He realizes it is failing when the images of himself and his brother and sister vanish from the family picture he carries in his wallet — a bad sign. Later he sees his own hand fading away. He can look right through it. He is disappearing. He begins to feel faint. Because he has interrupted his parents' romance, he is slipping out of existence. Later, when his plan finally succeeds and his parents are united, he suddenly feels better and his hand returns to normal. He looks in his wallet; the pictures of himself and his brother and sister have reappeared.
A hand can fade in a fictional story, but in the physical realm, atoms just don't dematerialize that way. Besides, according to the parameters of the story, the boy is dematerializing because, as a time traveler, he prevented his parents from falling in love, thereby circumventing his own birth. But if he was never born, his entire world line, from the point of his birth to his adventures as time traveler, should vanish, leaving no one to interfere with his parents — so his birth would have happened after all. Clearly, this fictional story has not resolved the Grandmother Paradox. Physically possible solutions to such time-travel paradoxes exist, but physicists are divided on which of two approaches is correct.
Timescape and the Many-Worlds Theory
First, the radical alternative. It involves quantum mechanics, that field of physics developed in the early twentieth century to explain the behavior of atoms and molecules. Quantum mechanics shows how particles have a wave nature, and waves have a particle nature. A key feature is Heisenberg's uncertainty principle, which tells us that we cannot establish a particle's position and velocity with arbitrary accuracy. Such quantum fuzziness, although usually negligible in the macroscopic world, is important on atomic scales. Quantum mechanics explains how atoms emit or absorb light at specific wavelengths when electrons jump from one energy level to another. The wave nature of particles leads to unusual effects such as quantum tunneling, in which a helium nucleus may suddenly jump out of a uranium nucleus, causing its radioactive decay. Solving quantum wave equations allows you to predict the probability of finding a particle at various places. This in turn leads, in one interpretation, to the many-worlds theory of quantum mechanics, which posits different parallel worlds where the particle is detected at those various places. Many physicists think this interpretation is an unnecessary addition to the theory, but a number of physicists working on the frontiers of our understanding of quantum theory do take this many-worlds interpretation and its refinements and extensions seriously.
(Continues…)
Excerpted from "Time Travel in Einstein's Universe"
by .
Copyright © 2001 J. Richard Gott III.
Excerpted by permission of Houghton Mifflin Harcourt Publishing Company.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Table of Contents
Title Page,
Contents,
Copyright,
Dedication,
Acknowledgments,
Preface,
Dreaming of Time Travel,
Time Travel to the Future,
Time Travel to the Past,
Time Travel and the Beginning of the Universe,
Report from the Future,
Notes,
Annotated References,
Index,
About the Author,
What People are Saying About This
J. Richard Gott is one of the most original thinkers of our time. In this intriguing exposition on time travel, based in part on his newly discovered solution to Einstein's equations, Professor Gott empowers the reader to explore this fascinating frontier where science fiction becomes science fact.
— (Neil Degrasse Tyson, astrophysicist and director, Hayden Planetarium, New York City)
Time Travel in Einstein's Universe is engrossing, mind-bending, and original. It put me through a time warp for several nights running (while I was reading it), so I ended up losing time (and sleep) over it. But I really couldn't put it down. Richard Gott is one of the cleverest cosmologists of our time, and the voyage he takes us on is well worthwhile.
— (Richard Preston, author of The Hot Zone and The Cobra Event)
In this book Gott offers an excellent exploration of the concept of time travel. Most interesting is the 'universe creating itself' idea. He drives the point back to a very deep level of physical philosophy without losing the popular reader. Gott is impressive both because of his insights and because of his ability to articulate them so far as to make the interested layperson follow him.
Interviews
An Exclusive Interview with J. Richard Gott
Barnes & Noble.com: Like many people, I'm fascinated by the idea of time travel. You give scenarios where time travel would be possible, but only by super-advanced civilizations, using cosmic strings, for example. If time travel is possible at all, is there some hope of achieving it before reaching such exalted heights?
Richard Gott: Time travel to the future is possible, and we are doing it in a small way even today. Einstein showed that rapidly moving clocks tick more slowly than ones on Earth. If you were to travel to a star 500 light-years away and return at 99.995 percent of the speed of light, you would age only 10 years during the trip -- but you would find Earth 1,000 years older when you returned. Our greatest time traveler so far is cosmonaut Sergei Avdeyev, who orbited the earth for 748 days in 3 space flights, with the result that he is about 1/50th of a second younger than he would have been if he had stayed home. Put another way, he has journeyed about 1/50th of a second into the future, because when he returned he found earth about 1/50th of a second further into the future than he expected. If, in the upcoming century, an astronaut were to go to the planet Mercury and spend 30 years living there before returning home, he would be about 22 seconds younger than if he had stayed on Earth. Clocks on Mercury tick more slowly than those on Earth, both because Mercury orbits the sun at higher speed and because it is deeper in the sun's gravitational field -- which Einstein showed also causes clocks to slow. How far we travel into the future is basically a matter of how much money we are willing to spend on the project. In the 21st century I think we will continue to see time travel to the future, but only in small jumps.
Time travel to the past is more difficult. It is allowed by Einstein's theory of gravity but it requires an extreme twisting of space, as might occur in wormholes or around rapidly contracting cosmic string loops (where you would likely end up trapped inside a black hole). To understand whether you could complete such a time travel journey to the past before being killed, we may need to understand the laws of quantum gravity -- this is one reason the problem is so interesting. Time travel to the past is a project only supercivilizations could attempt. Kip Thorne and his colleagues have proposed a wormhole solution propped open by the quantum vacuum state between electrically charged plates weighing 200 million times as much as the sun, while a cosmic string loop that you could circle to go back a year in time would weigh about half the mass of our galaxy. These are construction projects on a grand scale. Physicists like myself working on this are not yet at the point of taking out patents on time machines, but we are interested in knowing if it is possible in principle under the laws of physics, because that may provide clues to how the universe works -- and even how it began.
B&N.com: Your research is not on human time travel, per se, but on the theory that the universe created itself through a kind of time loop, like a jinn.
RG: Time loops allow unusual phenomena. First, consider your own path through space and time. It is called your world-line. It starts at your birth, snakes through all the events of your life, and ends at your death. If space and time are sufficiently twisted to allow time travel to the past, there may be some particles (called jinn) with circular world-lines, having no beginning and no end. Some famous time-travel science fiction stories have included such jinn. The pocket watch in the movie "Somewhere in Time" is an example. A time traveler in the story receives a watch from an old woman. He takes the watch back in time to give to that same woman when she was young. She saves it her whole life and returns the watch to him when she has become an old woman. The watch has a circular world-line -- it never went anywhere near a watch factory. In a variation on this theme, Robert Heinlein wrote a famous story, "All You Zombies," where a time traveler's convoluted visits to the past (one including a sex-change operation) allowed him to become both his own mother and father! Jinn particles are not just fascinating devices, but must be considered in calculating quantum probabilities in time travel situations.
In trying to understand the origin of the universe, Andrei Linde proposed that universes can give birth to other universes, like branches budding off a tree. This eventually produces an infinite fractal tree with an infinite number of universes. But where did the trunk come from? Li-Xin Li and I have proposed that one of the branches simply loops back in time to become the trunk. This small time loop at the very beginning of the universe allows the universe to be its own mother!
B&N.com: Along with time travel, you also describe many of the new discoveries in physics and cosmology. Is this a particularly exciting time to be working in these fields?
RG: We live, as most people will, in an epoch of high population. Since it is people who make discoveries, it is not surprising to find ourselves living in an epoch in which many exciting discoveries of all sorts are being made. In cosmology, we have seen extraordinary progress in the last century -- from Hubble's discovery in 1929 of the expansion of the universe to Penzias and Wilson's 1965 discovery of the cosmic microwave background radiation left over from the Big Bang itself. Breakthrough theoretical ideas, such as Alan Guth's inflation and Linde's chaotic inflation have led us to a picture of universes spawning universes. Recent measurements of the cosmic microwave background and distant supernovae have been in dramatic accord with such theoretical ideas. Giant surveys (like the Sloan Digital Sky Survey) are allowing us to begin mapping the universe in unprecedented detail, while advances in computers enable us to model the formation and clustering of galaxies to test our theories. Superstring theory offers us the hope of one day finding a "theory of everything," explaining and unifying all the laws of physics. These are all extremely exciting developments.
B&N.com: Is there anything else you'd like to add?
RG: Time travel research is one of the most fun topics in physics, but it has a serious side as well. Time travel was inconceivable in Newton's universe, but Einstein's universe it has become a possibility. Einstein started the discussion by showing that time travel to the future was possible. Kurt Gödel, one of the 20th century's most distinguished mathematicians, found a solution to Einstein's equations of gravity allowing time travel to the past -- a rotating universe. Kip Thorne and his colleagues found a different solution involving wormholes, and I found one involving cosmic strings. Trying to understand whether these solutions can be realized may require us to understand quantum gravity. As Li-Xin Li and I have argued, time travel solutions may even hold the key to how the universe began.
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