Sean Carroll - From Eternity to Here: The Quest for the Ultimate Theory of Time  

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Copyright 2010 by Sean Carroll

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Photograph on page 37 by Martin Rll, 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 TRADEMARKMARCA 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
530.11dc22 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. Were not thinking small here. The questions were 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 dayyet, 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.

Oh, its 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 doesalthough well 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 thats going to continue for the foreseeable future, and possibly forever.

A century ago, we didnt know any of thatscientists 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 timenot 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

Its clear that the universe evolves as time passesthe 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. Thats 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 processesthings 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 cant 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 passesthats the famous Second Law of Thermodynamics. 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. Its 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.

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