Rilegato

ISBN 10: 0713995831 ISBN 13: 9780713995831

Casa editrice: Allen Lane, 2001

Is time travel possible? According to internationally acclaimed physicist and science writer Paul Davies, the answer is definitely yes. But if we can travel in time, surely we can glimpse the future and act to change it? Or alter the past, creating all sorts of bizarre paradoxes? This hugely entertaining brain-twisting book reveals how it might be done. With his remarkable gift for easy explanation, Paul Davies shows how to use gravity to visit the future and how to warp space to reach the past; and, in a "tour de force", how to build a time machine from a traversable wormhole. This is the ultimate time traveller's companion. Paul Davies expertly and effortlessly explains the mind-boggling physics that makes time travel possible, and more importantly shows how we can make sense of this very possiblity. Might the inconceivable be achievable? Can we break the last cosmic taboo?

About the Author:

Paul Davies is an internationally acclaimed physicist, writer and broadcaster, now based in South Australia. He works in the fields of cosmology, gravitation, and quantum field theory, with particular emphasis on black holes and the origin of the universe. He is the author of some twenty award-winning books, including The Mind of God and The Fifth Miracle: The Search for the Origin of Life.

Excerpt. © Reprinted by permission. All rights reserved.:

Prologue

What if it were possible to build a machine that could transport a human being through time?

Is that credible?

A hundred years ago, few people believed it possible for humans to travel through outer space. Time travel, like space travel, was merely science fiction. Today, spaceflight is almost commonplace. Might time travel one day become commonplace too?

Traveling in time is certainly easy to envisage. You step into the time machine, press a few buttons, and step out again, not just somewhere else, but somewhen else-another time altogether. Writers of science fiction have exploited this theme again and again since H. G. Wells blazed the trail with his famous 1895 story The Time Machine. British audiences (the author included) thrilled to the adventures of the time lord Doctor Who and his attractive lady accomplices. Hollywood movies such as Back to the Future and books such as Timeline make it all seem so easy.

So can it really be done? Is time travel a scientific possibility?

A moment's thought uncovers some tricky questions. Where exactly are the past and future? Surely the past has disappeared and cannot be re-trieved, while the future hasn't yet come into being. How can a person go to a world that doesn't exist? Sidestepping that, what about the inevitable paradoxes that come from visiting the past and changing it? What does that do to the present? And if time travel were feasible, where are all the time tourists from the future, coming back to peer curiously at twenty-first-century society?

There is no doubt that time travel poses some serious problems, even for physicists used to thinking about outlandish concepts like antimatter and black holes. But maybe that is because we are looking at time in the wrong way. After all, our view of time has changed dramatically over the years. In ancient cultures it was associated with process and change, and rooted in the cycles and rhythms of nature. Later, Sir Isaac Newton took a more abstract and mechanistic view. "Absolute, true and mathematical time, flowing equably without relation to anything external" was the way he expressed it, and this became the accepted notion among scientists for two hundred years.

Everyone assumed without question that, whatever one's preferred definition, time is the same everywhere and for everybody. In other words, it is absolute and universal. True, we might feel time passing differently according to our moods, but time itself is simply time. The purpose of a clock is to circumvent mental distortions and record, objectively, the time. Implicit in this view is that time can be chopped up into three parts: past, present, and future. The present-now-is supposed to be the fleeting moment of true reality, with the past banished to history-mere shadowy memory-and the future still hazy and unformed. And that all-important now is taken to be the same moment throughout the universe: your now and my now are identical wherever we are and whatever we are doing.

Such is the commonsense picture of time, the one we use in daily life. Few people think about time any differently. But it's wrong-deeply and seriously wrong.

That it couldn't be right became apparent about the turn of the twentieth century. The credit for exposing the flaws in the everyday notion of time is largely associated with the name of Albert Einstein and the theory of relativity. At a stroke, Einstein's work demolished Newton's view of both space and time, rendered meaningless the universal division of time into past, present, and future, and paved the way for time travel. The theory of relativity is nearly a century old. Following publication of the so-called special theory of relativity in 1905, it was accepted by physicists almost immediately. Over the decades it has been exhaustively tested in many experiments. Today, the scientific community is unanimous that "time is relative" and the commonsense notion of an absolute time with a universal "now" is a fiction. Yet among the general public, the relativity of time still comes as something of a shock. Many people seem not to have heard about it at all. Some of them refuse flatly to believe it when told, in spite of the clear experimental evidence.

In the coming chapters we shall see how the theory of relativity implies that a limited form of time travel is certainly possible, while unrestricted time travel-to any epoch, past or future-might just be possible too. If this seems hard to swallow, remind yourself of J. B. S. Haldane's famous dictum: "The universe is not only queerer than we think, it is queerer than we can think."

1. How to Visit the Future

In an obvious sense we are all time travelers. Do nothing, and you will be conveyed inexorably into the future at the stately pace of one second per second. But this is of limited interest. A true time traveler needs to leap forward dramatically in time and reach the future sooner than everyone else.

Can it be done?

Indeed it can. Scientists have no doubt whatever that it is possible to build a time machine to visit the future. And they've known the formula for nearly a century.

[ Time and Motion

It was in 1905 that Albert Einstein first demonstrated the possibility of time travel. He did this by first demolishing the commonsense picture of time dating back to Newton and replacing it with his own concept of relative time.

Einstein was twenty-six when he published his special theory of relativity. He was then not the pipe-smoking disheveled sage with tousled gray hair who provided the role model for many a fictional nutty professor, but a dapper young man in a suit working at the Swiss patent office. In his spare time, the young Einstein was studying the way light moves. In doing so, he noticed an inconsistency between the motion of light and that of material objects. Using only high-school mathematics, he demonstrated that if light behaves the way that physicists supposed, Newton's straightforward idea of time must be flawed.

The trail of reasoning that leads from the motion of light to this startling conclusion about time has been discussed thoroughly elsewhere and need not concern us here. What matters for our purposes is the central claim of the special theory of relativity, which is that Time is elastic. It can be stretched and shrunk. How? Simply by moving very fast.

What precisely do I mean by "stretching time"? Let me state it more carefully. According to the special theory of relativity, the exact duration of time between two specified events will depend on how the observer is moving. The interval between successive chimes on my clock might be one hour when I am sitting still in my living room, but it will be less than one hour if I spend that time moving about.

To express the same thing in a more practical manner, suppose I board an airplane in New York and fly to Rio and back while you stay at Kennedy Airport. Then the duration of the journey according to me isn't the same as the duration according to you. In fact, it is a bit less for me.

Two points need to be made at the outset. First, I'm not talking about the apparent duration of the journey. Your experience of being bored at the airport with the hours seeming to drag by, while I am happily occupied watching airline movies, is not the effect being discussed here. Mental time is a fascinating topic in psychology, but my concern is with physical time, the sort measured by mindless clocks. The second point is that the time discrepancy for the example given is minuscule-only a few hundred-millionths of a second-far too small to be noticed by a human being; however, it is measurable by modern clocks.

That is pretty much what the physicists Joe Hafele and Richard Keating did in 1971. They put highly accurate atomic clocks into airplanes, flew them around the world, and compared their readings with identical clocks left on the ground. The results were unmistakable: time ran more slowly in the airplane than in the laboratory, so that when the experiment was over the airborne clocks were fifty-nine nanoseconds slow relative to the grounded clocks-exactly the amount predicted in Einstein's theory.

Because your time and my time get out of step if we move differently, there can obviously be no universal, absolute time, as Newton assumed. Talk of the time is meaningless. The physicist is bound to ask: Whose time?

Significant though the Hafele-Keating experiment may be historically, it is hardly the stuff of science fiction: a timewarp of fifty-nine nanoseconds doesn't make for an adventure. To get a really big effect you have to move very fast. The benchmark here is the speed of light, a dizzying 300,000 kilometers per second. The closer to the speed of light you travel, the bigger the timewarp gets.

Physicists call the slowing of time by motion the time dilation effect. Think of a speed. Divide by the speed of light. Square it. Subtract from 1. Take the square root. The answer is . . . Einstein's time dilation factor! This is a graph of the "slowdown factor." Notice how the graph shows the dilation factor as a function of speed and starts out fairly flat, but plummets as light speed is approached. At half the speed of light, time is about 13 percent slowed; at 99 percent, it is seven times slower-1 minute is reduced to about 8.5 seconds.

Technically, the timewarp becomes infinite when the speed of light is reached. This is a sign of trouble. In fact, it tells us that a normal material body can't reach the speed of light. There is a "light barrier" that can never be breached. The no-faster-than-light rule is a key result of the theory of relativity:

Nothing can break the light barrier.

This includes not just material bodies but waves, field disturbances-physical influences of any sort. It spoils a lot of science fiction because, fast though it goes, light still takes a long time to cover interstellar distances. The nearest star, for example, is over four light-years away, which means it takes light over four years to get there from Earth. The Milky Way galaxy is about 100,000 light-years across. Administering a galactic empire would be a slow process.

However, there is some compensation. Because time is stretched by speed, interstellar journeys would seem quicker for the astronauts than for those left on Earth at mission control. In a spaceship traveling at 99 percent of the speed of light, a trip across the galaxy would be completed in only 14,000 years. At 99.99 percent of the speed of light, the gain is even more spectacular: the trip lasts a mere 1,400 years. If you could reach 99.999999 percent of the speed of light, the trip could be completed in a human lifetime. Speeds like this are far beyond current spacecraft technology. (Our best spacecraft reach a paltry 0.01 percent of the speed of light.) But there are objects that travel very close to the speed of light. These are subatomic particles, such as cosmic rays and atomic fragments emitted in radioactive decays, or purposely accelerated in giant "atom smashers." It's possible to observe very large time dilations by using these particles as simple clocks. The particle accelerator known as the Large Electron Positron (LEP) collider at the Centre Européenne pour la Recherche Nucléaire (CERN) laboratory near Geneva could propel electrons to 99.999999999 percent of the speed of light. This is so fast it falls short of the speed of light by a literal snail's pace. At this speed, timewarp factors approaching a million were achieved. Even this pales into insignificance conpared to timewarp factors of billions experienced by some cosmic ray particles.

In a series of careful experiments carried out at CERN in 1966, particles called muons were circulated inside a small accelerator to test Einstein's time dilation equation to high precision. Muons are unstable and decay with a known half-life. A muon sitting on your desktop would decay on average in about two microseconds. But when muons were moving inside the accelerator at 99.7 percent of the speed of light, their average lifetime was extended by a factor of twelve.

The twins effect
The effect of motion on time is often discussed using the parable of the twins. It goes something like this. Sally and Sam decide to test Einstein's theory, so Sally boards a rocket ship in 2001 and zooms off at 99 percent of the speed of light to a nearby star situated ten light-years away. Sam stays at home. On reaching her destination, Sally immediately turns around and heads home at the same speed. Sam observes the duration of her journey to be just over twenty Earth years. But Sally experiences time differently. For her, the journey has taken less than three years, so when she gets back to Earth she finds that the date there is 2021 and Sam is now seventeen years older than she is. Sally and Sam are no longer twins of the same age. In effect, Sally has been transported seventeen years into Sam's future. With a high enough speed, you could "jump" to any date in the future you like. The year 3000 could be reached in less than six months by traveling at 99.99999 percent of the speed of light.

Traveling through time works the opposite way from traveling through space. The shortest distance between two points is a straight line, so in daily life you get from A to B most quickly by following a direct route. But when it comes to time travel, it is stay-at-home Sam who ages more; that is, he takes longer to reach year 2021. By zooming about, Sally dramatically shortens the time difference between the two events "Earth year 2001" and "Earth year 2021." In fact, the more she zooms this way and that, the shorter the time difference between these two events becomes.

Some people find the twins effect paradoxical, because from Sally's point of view, she is at rest in the rocket ship while the Earth flies away. However, there is no paradox, because the situation for Sally and Sam is not symmetrical. Sally is the one who accelerates away by firing the rocket motors, then maneuvers around the star, and finally decelerates to land on Earth. These changes in motion single her out as the one to age less.

Note that Sally cannot "get back" to Earth year 2007 (there being six years' round-trip travel time after departure) this way, in order to reequalize her age with Sam's. If she reverses her trajectory, she will succeed only in leaping another seventeen years into Sam's future. High-speed motion is a one-way journey into the future.

[ How to Use Gravity to Travel into the Future Speed is only one method of warping time. Another is gravity. As early as 1908 Einstein began extending his special theory of relativity to include the effects of gravity. Using another ingenious argument concerning light, he came to the remarkable conclusion that Gravity slows time.

He didn't clinch the argument until 1915, when he presented his so-called general theory of relativity. This work extended the special theory published in 1905 to include the effects of gravitational fields on time, and on space too.

Putting the numbers into Einstein's equation shows that the Earth's gravity causes clocks to lose one micro-second every three hundred years. This leads to the curious prediction that
Time runs faster
in space.

But not so much that...