From Eternity to Here
Table of Contents
PART ONE - TIME, EXPERIENCE, AND THE UNIVERSE
Chapter 1 - THE PAST IS PRESENT MEMORY
Chapter 2 - THE HEAVY HAND OF ENTROPY
Chapter 3 - THE BEGINNING AND END OF TIME
PART TWO - TIME IN EINSTEIN’S UNIVERSE
Chapter 6 - LOOPING THROUGH TIME
PART THREE - ENTROPY AND TIME’S ARROW
Chapter 7 - RUNNING TIME BACKWARD
Chapter 8 - ENTROPY AND DISORDER
Chapter 9 - INFORMATION AND LIFE
Chapter 10 - RECURRENT NIGHTMARES
PART FOUR - FROM THE KITCHEN TO THE MULTIVERSE
Chapter 12 - BLACK HOLES: THE ENDS OF TIME
Chapter 13 - THE LIFE OF THE UNIVERSE
Chapter 14 - INFLATION AND THE MULTIVERSE
Chapter 15 - THE PAST THROUGH TOMORROW
DUTTON
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Copyright © 2010 by Sean Carroll
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Photograph on page 37 by Martin Röll, licensed under the Creative Commons Attribution ShareAlike 2.0 License, from Wikimedia Commons. Photograph on page 47 courtesy of the Huntington Library. Image on page 53 by the NASA/WMAP Science Team. Photograph on page 67 courtesy of Corbis Images. Image on page 119 courtesy of Getty Images. Figures on pages 147, 153, 177, 213, 270, 379, and 382 by Sean Carroll. Photograph on page 204 courtesy of the Smithsonian Institution. Photograph on page 259 courtesy of Professor Stephen Hawking. Photograph on page 267 courtesy of Professor Jacob Bekenstein. Photograph on page 295 by Jerry Bauer, from Wikimedia Commons. Photograph on page 315 courtesy of the Massachusetts Institute of Technology. All other images courtesy of Jason Torchinsky.
REGISTERED TRADEMARK—MARCA REGISTRADA
LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA
Carroll, Sean M., 1966-
From eternity to here : the quest for the ultimate theory of time / Sean Carroll.
p. cm.
Includes bibliographical references and index.
eISBN : 978-1-101-15215-7
1. Space and time. I. Title.
QC173.59.S65C37 2009
530.11—dc22 2009023828
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To Jennifer
For all time
PROLOGUE
Does anybody really know what time it is?
—Chicago, “Does Anybody Really Know What Time It Is?”
This book is about the nature of time, the beginning of the universe, and the underlying structure of physical reality. We’re not thinking small here. The questions we’re tackling are ancient and honorable ones: Where did time and space come from? Is the universe we see all there is, or are there other “universes” beyond what we can observe? How is the future different from the past?
According to researchers at the
Oxford English Dictionary
,
time
is the most used noun in the English language. We live through time, keep track of it obsessively, and race against it every day—yet, surprisingly, few people would be able to give a simple explanation of what time actually
is
.
In the age of the Internet, we might turn to Wikipedia for guidance. As of this writing, the entry on “Time” begins as follows:
Time is a component of a measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects. Time has been a major subject of religion, philosophy, and science, but defining time in a non-controversial manner applicable to all fields of study has consistently eluded the greatest scholars.
1
Oh, it’s on. By the end of this book, we will have defined
time
very precisely, in ways applicable to all fields. Less clear, unfortunately, will be
why
time has the properties that it does—although we’ll examine some intriguing ideas.
Cosmology, the study of the whole universe, has made extraordinary strides over the past hundred years. Fourteen billion years ago, our universe (or at least the part of it we can observe) was in an unimaginably hot, dense state that we call “the Big Bang.” Ever since, it has been expanding and cooling, and it looks like that’s going to continue for the foreseeable future, and possibly forever.
A century ago, we didn’t know any of that—scientists understood basically nothing about the structure of the universe beyond the Milky Way galaxy. Now we have taken the measure of the observable universe and are able to describe in detail its size and shape, as well as its constituents and the outline of its history. But there are important questions we cannot answer, especially concerning the early moments of the Big Bang. As we will see, those questions play a crucial role in our understanding of time—not just in the far-flung reaches of the cosmos, but in our laboratories on Earth and even in our everyday lives.
TIME SINCE THE BIG BANG
It’s clear that the universe evolves as time passes—the early universe was hot and dense; the current universe is cold and dilute. But I am going to be drawing a much deeper connection. The most mysterious thing about time is that it has a direction: the past is different from the future. That’s the
arrow of time
—unlike directions in space, all of which are created pretty much equal, the universe indisputably has a preferred orientation in time. A major theme of this book is that the arrow of time exists because the universe evolves in a certain way.
The reason why time has a direction is because the universe is full of irreversible processes—things that happen in one direction of time, but never the other. You can turn an egg into an omelet, as the classic example goes, but you can’t turn an omelet into an egg. Milk disperses into coffee; fuels undergo combustion and turn into exhaust; people are born, grow older, and die. Everywhere in Nature we find sequences of events where one kind of event always happens before, and another kind after; together, these define the arrow of time.
Remarkably, a single concept underlies our understanding of irreversible processes: something called
entropy
, which measures the “disorderliness” of an object or conglomeration of objects. Entropy has a stubborn tendency to increase, or at least stay constant, as time passes—that’s the famous Second Law of Thermodynamics.
2
And the reason why entropy wants to increase is deceptively simple: There are more ways to be disorderly than to be orderly, so (all else being equal) an orderly arrangement will naturally tend toward increasing disorder. It’s not that hard to scramble the egg molecules into the form of an omelet, but delicately putting them back into the arrangement of an egg is beyond our capabilities.
The traditional story that physicists tell themselves usually stops there. But there is one absolutely crucial ingredient that hasn’t received enough attention: If everything in the universe evolves toward increasing disorder, it must have started out in an exquisitely ordered arrangement. This whole chain of logic, purporting to explain why you can’t turn an omelet into an egg, apparently rests on a deep assumption about the very beginning of the universe: It was in a state of very low entropy, very high order.
The arrow of time connects the early universe to something we experience literally every moment of our lives. It’s not just breaking eggs, or other irreversible processes like mixing milk into coffee or how an untended room tends to get messier over time. The arrow of time is the reason why time seems to flow around us, or why (if you prefer) we seem to move through time. It’s why we remember the past, but not the future. It’s why we evolve and metabolize and eventually die. It’s why we believe in cause and effect, and is crucial to our notions of free will.
And it’s all because of the Big Bang.
WHAT WE SEE ISN’T ALL THERE IS
The mystery of the arrow of time comes down to this: Why were conditions in the early universe set up in a very particular way, in a configuration of low entropy that enabled all of the interesting and irreversible processes to come? That’s the question this book sets out to address. Unfortunately, no one yet knows the right answer. But we’ve reached a point in the development of modern science where we have the tools to tackle the question in a serious way.