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PRINCETON U N I V E R S I T Y PRESS P R I N C E T O N & OXFORD AN ASTROPHYSICAL TOUR WELCOME J · TO THE RICHARD UNIVERSE GOTT MICHAEL NEIL A · STRAUSS DEG RASSE TYSON Copyright © 2016 by Neil deGrasse Tyson, Michael A. Strauss, and J. Richard Gott Requests for permission to reproduce material from this work should be sent to Permissions, Princeton University Press Published by Princeton University Press 41 William Street, Princeton, New Jersey 08540 In the United Kingdom: Princeton University Press 6 Oxford Street, Woodstock, Oxfordshire OX20 1TR press.princeton.edu Jacket and book design by Chris Ferrante All Rights Reserved Library of Congress Cataloging-in-Publication Data Names: Tyson, Neil deGrasse. | Strauss, Michael Abram. | Gott, J. Richard. Title: Welcome to the universe : an astrophysical tour / Neil deGrasse Tyson, Michael A. Strauss, and J. Richard Gott. Description: Princeton : Princeton University Press, 2016. | Includes bibliographical references and index. Identifiers: LCCN 2016013487 | ISBN 9780691157245 (hardback : alk. paper) Subjects: LCSH: Cosmology—Popular works. | Stars—Popular works. | Relativity (Physics)—Popular works. Classification: LCC QB982 .T974 2016 | DDC 523.1—dc23 LC record available at https://lccn.loc.gov/2016013487 British Library Cataloging- in- Publication Data is available This book has been composed in Adobe Text Pro and Trade Gothic LT Std Printed on acid- free paper. ∞ Printed in the United States of America 10 9 8 7 6 5 4 3 2 1 To the memories of Lyman Spitzer, Jr., Martin Schwarzschild, Bohdan Paczyński, and John Bahcall, indelible influences on the three of us in astrophysics research and education. C ONTENTS PREFACE 11 PART I. STARS, PLANETS, AND LIFE 15 1 T HE SIZE AND SCALE OF THE UNIVERSE 17 Neil deGrasse Tyson 2 F ROM THE DAY AND NIGHT SKY TO PLANETARY ORBITS 26 Neil deGrasse Tyson 3 N EWTON’S LAWS 42 Michael A. Strauss 4 H OW STARS RADIATE ENERGY (I) 54 Neil deGrasse Tyson 5 H OW STARS RADIATE ENERGY (II) 71 Neil deGrasse Tyson 6 S TELLAR SPECTRA 81 Neil deGrasse Tyson 7 T HE LIVES AND DEATHS OF STARS (I) 93 Neil deGrasse Tyson 7 8 T HE LIVES AND DEATHS OF STARS (II) 111 Michael A. Strauss 9 W HY PLUTO IS NOT A PLANET 126 Neil deGrasse Tyson 10 T HE SEARCH FOR LIFE IN THE GALAXY 146 Neil deGrasse Tyson PART II. GALAXIES 171 11 T HE INTERSTELLAR MEDIUM 173 Michael A. Strauss 12 OUR MILKY WAY 183 Michael A. Strauss 13 T HE UNIVERSE OF GALAXIES 197 Michael A. Strauss 14 T HE EXPANSION OF THE UNIVERSE 207 Michael A. Strauss 15 T HE EARLY UNIVERSE 222 Michael A. Strauss 16 Q UASARS AND SUPERMASSIVE BLACK HOLES 241 Michael A. Strauss PART III. EINSTEIN AND THE UNIVERSE 255 17 E INSTEIN’S ROAD TO RELATIVITY 257 J. Richard Gott 18 I MPLICATIONS OF SPECIAL RELATIVITY 270 J. Richard Gott 19 E INSTEIN’S GENERAL THEORY OF RELATIVITY 289 J. Richard Gott 8 C ONTENTS 20 B LACK HOLES 300 J. Richard Gott 21 C OSMIC STRINGS, WORMHOLES, AND TIME TRAVEL 321 J. Richard Gott 22 T AHE SH PE OF THE UNIVERSE AND THE BIG BANG 347 J. Richard Gott 23 I NFLATION AND RECENT DEVELOPMENTS IN COSMOLOGY 374 J. Richard Gott 24 OU R FUTURE IN THE UNIVERSE 400 J. Richard Gott ACKNOWLEDGMENTS 425 APPENDIX 1 Derivation of E = mc 2 427 APPENDIX 2 Bekenstein, Entropy of Black Holes, and Information 431 NOTES 433 SUGGESTED READING 439 INDEX 441 C ONTENTS 9 PREFACE When my granddaughter Allison was born, one of the first things I said to her was “Welcome to the universe!” It’s something my coauthor Neil Tyson has said many times on radio and TV. Indeed it is one of Neil’s signature sayings. When you are born, you become a citizen of the universe. It behooves you to look around and get curious about your surroundings. Neil felt a call from the universe on a first visit to the Hayden Planetarium in New York City when he was 9 years old. As a city kid, he saw the glories of the nighttime sky for the first time displayed on the planetarium dome and decided at that moment to become an astronomer. Today he is the director of that institution. In fact, we are all touched by the universe. The hydrogen in your body was forged in the birth of the universe itself, while the other elements in your body were made in distant, long- dead stars. When you call a friend on your mobile phone, you should thank astronomers. Mobile phone technology depends on Maxwell’s equations, whose verification depended on the fact that astronomers had already measured the speed of light. The GPS that tells your phone where you are and helps you navigate relies on Einstein’s theory of general relativity, which was verified by astronomers measuring the deflection of starlight passing near the Sun. Did you know there is an ultimate limit to how much informa- tion can ever be stored in a 6-in ch- diameter hard drive and that it depends on black hole physics? At a more mundane level, the seasons you experience every year depend directly on the tilt of Earth’s axis relative to the plane of its orbit around the Sun. 11 This book aims to better acquaint you with the universe in which you live. The idea for this book started when the three of us taught a new undergraduate course on the universe for nonscience majors at Princeton University—f or stu- dents who perhaps had never taken a science course before. For this purpose, Neta Bahcall, our colleague and director of undergraduate studies, selected Neil deGrasse Tyson, Michael Strauss, and me. Neil’s genius at explaining science to nonscientists was apparent, Michael had just discovered the most distant quasar yet found in the universe, and I had just received the university’s President’s Award for Distinguished Teaching. The course was launched with great fanfare and attracted so many students that we couldn’t hold it in our own building and had to move it to the biggest lecture hall in the Physics Department. Neil talked about “Stars and Planets,” Michael talked about “Galaxies and Quasars,” and I talked about “Einstein, Relativity, and Cosmology.” The course was mentioned in Time magazine, when Time honored Neil as one of the 100 most influential people in the world in 2007. Among other features of this book, you will get to know Neil as a professor, telling you things he tells his students. After we had taught the course for a number of years, we decided to put its ideas down in the form of a book for readers who hungered for a deeper understanding of the universe. We give you a tour of the universe from an astrophys- ical point of view, from the point of view of trying to FIGURE 0.1. The three authors, left to right: Strauss, Gott, and Tyson. understand what is going on. Photo credit: Princeton, Denise Applewhite We tell you how Newton and Einstein got their greatest ideas. You know Stephen Hawking is famous. But we tell you what made him famous. The great movie of his life story,The The- ory of Everything, won Eddie Redmayne a best actor Oscar for his compelling portrayal of Hawking. It shows Hawking having his greatest idea by simply staring into the fireplace and having it suddenly come to him. We tell you what the movie left out: how Hawking didn’t believe the work of Jacob Bekenstein, but he ended up reaffirming it and taking it to an entirely new conclusion. 12 PREFACE And that’s the same Jacob Bekenstein who found the ultimate limit for how much information could be stored on your 6- inch- diameter hard drive. It’s all connected. In this book, of all the topics in the universe, we focus particularly on those we are most passionate about, and we hope our excitement will be contagious. Much has been added to astronomical knowledge since we began, and this book reflects that. Neil’s views on the status of Pluto have been ratified by the International Astronomical Union, in a historic vote in 2006. Thousands of new planets have been discovered circling other stars. We discuss them. The standard cosmological model, including normal atomic nuclei, dark matter, and dark energy, is now known with exquisite accuracy, thanks to results from the Hubble Space Telescope, the Sloan Digital Sky Survey, and the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck satellites. Physicists have discovered the Higgs Boson at the Large Hadron Collider in Europe, bringing us one step closer to the hoped for theory of everything. The Laser Interferom- eter Gravitational- Wave Observatory (LIGO) experiment has made a direct detection of gravitational waves from two inspiraling black holes. We explain how astronomers have determined how much dark matter there is, and how we know that it is not made of ordinary matter (with atomic nuclei containing protons and neutrons). We explain how we know the density of dark energy, and how we know that it has a negative pressure. We cover cur- rent speculations on the origin of the universe and on its future evolution. These questions bring us to the frontiers of physics knowledge today. We have included spectacular images from the Hubble Space Telescope, the WMAP sat- ellite, and the New Horizons spacecraft—show ing Pluto and its moon Charon. The universe is awesome. Neil shows you that in the very first chapter. This leaves many people thrilled, but feeling tiny and insignificant at the same time.

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