Solving Fermi's Paradox Duncan H

Cambridge University Press 978-1-107-16365-2 — Solving Fermi's Paradox Duncan H. Forgan Frontmatter More Information

SOLVING FERMI’S PARADOX

The Search for Extraterrestrial Intelligence (SETI) has for 60 years attempted to solve Fermi’s paradox: if intelligent life is relatively common in the universe, where is everybody? Examining SETI through this lens, this volume summarises current thinking on the prevalence of intelligent life in the universe and discusses 66 distinct solutions to the so-called paradox. It describes the methodology of SETI and how many disciplines feed into the debate, from physics and biology, to phi- losophy and anthropology. The presented solutions are organised into three key groups: rare-Earth solutions, suggesting that planetary habitability, life and intelligence are uncommon; catastrophist solutions, arguing that civilisations do not survive long enough to make contact; and non-empirical solutions, which take theoretical approaches, such as that our methodology is awed. This comprehensive introduction to SETI concludes by looking at the future of the eld and speculating on humanity’s potential fate.

DUNCAN H. FORGAN is Associate Lecturer at the Centre for Exoplanet Sci- ence at the University of St Andrews. He is a founding member of the UK SETI research network and leads UK research efforts into the Search for Extraterrestrial Intelligence. His work includes simulations of civilisation evolution, developing observables of intelligence and policy for post-detection scenarios.

CAMBRIDGE ASTROBIOLOGY

Series Editors Bruce Jakosky, Alan Boss, Frances Westall, Daniel Prieur, and Charles Cockell

Books in the Series:

1. Planet Formation: Theory, Observations, and Experiments Edited by Hubert Klahr and Wolfgang Brandner ISBN 978-0-521-18074-0 2. Fitness of the Cosmos for Life: Biochemistry and Fine-Tuning Edited by John D. Barrow, Simon Conway Morris, Stephen J. Freeland, and Charles L. Harper, Jr. ISBN 978-0-521-87102-0 3. Planetary Systems and the Origin of Life Edited by Ralph Pudritz, Paul Higgs, and Jonathan Stone ISBN 978-0-521-87548-6 4. Exploring the Origin, Extent, and Future of Life: Philosophical, Ethical, and Theological Perspectives Edited by Constance M. Bertka ISBN 978-0-521-86363-6 5. Life in Antarctic Deserts and Other Cold Dry Environments Edited by Peter T. Doran, W. Berry Lyons, and Diane M. McKnight ISBN 978-0521-88919-3 6. Origins and Evolution of Life: An Astrobiological Perspective Edited by Muriel Gargaud, Puricación Lopez-Garcia, and Hervé Martin ISBN 978 0521-76131-4 7. The Astrobiogical Landscape: Philosophical Foundations of the Study of Cosmic Life Milan M. Cirkovi´ c´ ISBN 978 0521-19775-5 8. The Drake Equation: Estimating the Prevalence of Extraterrestrial Life through the Ages Edited by Douglas A. Vakoch and Matthew F. Dowd ISBN 978-1-107-07365-4 9. Astrobiology, Discovery, and Societal Impact Steven J. Dick ISBN 978-1-108-42676-3 10. Solving Fermi’s Paradox Duncan H. Forgan ISBN 978-1-107-16365-2

SOLVING FERMI’S PARADOX

DUNCAN H. FORGAN University of St Andrews, Scotland

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www.cambridge.org Information on this title: www.cambridge.org/9781107163652 DOI: 10.1017/9781316681510 c Duncan Forgan 2019 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 2019 Printed in the United Kingdom by TJ International Ltd. Padstow Cornwall A catalogue record for this publication is available from the British Library. Library of Congress Cataloging-in-Publication Data Names: Forgan, Duncan, 1984– author. Title: Solving Fermi’s paradox / Duncan Forgan (University of St Andrews, Scotland). Description: Cambridge ; New York, NY : Cambridge University Press, 2019. | Includes bibliographical references and index. Identiers: LCCN 2018052247 | ISBN 9781107163652 (alk. paper) Subjects: LCSH: Fermi’s paradox. | Life on other planets. Classication: LCC QB54 .F77945 2018 | DDC 999–dc23 LC record available at https://lccn.loc.gov/2018052247 ISBN 978-1-107-16365-2 Hardback Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

For Rosa

Contents

Preface page xvii Part I Introduction 1

1 Introducing the Paradox 3 1.1 Fermi and His Paradox 3 1.1.1 The Paradox, in Brief 3 1.1.2 Fermi’s Paradox Is Not His, and It Isn’t a Paradox 4 1.2 Drake and His Equation 7 1.3 The Great Filter 11

2 Fact A – The Great Silence 15 2.1 Radio SETI 21 2.1.1 The Water Hole 21 2.1.2 Doppler Drift 24 2.1.3 Radio Frequency Interference 24 2.1.4 Single-Dish Radio Astronomy 25 2.1.5 Multi-Dish Radio Interferometry 27 2.1.6 A History of Radio SETI observations 30 2.2 Optical SETI 30 2.2.1 Artefact SETI 35 2.2.2 A History of Optical SETI Observations 37

3 Classifying Scenarios and Solutions to the Paradox 38 3.1 Contact Scenarios and the Rio Scale 38 3.2 The Three Classes of Civilisation – the Kardashev Scale 40 3.3 The Three Classes of Solutions to Fermi’s Paradox 43 3.3.1 Rare Earth (R) Solutions 43 3.3.2 Uncommunicative (U) Solutions 44

x Contents 3.3.3 Catastrophic (C) Solutions 44 3.3.4 Hard and Soft Solutions 44

Part II Rare Earth Solutions 45

4 Habitable Worlds Are Rare 47 4.1 Worlds Are Not Rare 47 4.1.1 The First Exoplanets – Pulsar Timing 47 4.1.2 The Radial Velocity Method 48 4.1.3 Transit Detection 53 4.1.4 Gravitational Microlensing 56 4.1.5 Direct Imaging 61 4.1.6 Detecting Life on Exoplanets 64 4.1.7 The Exoplanet Population 68 4.2 Habitable Zones 71 4.2.1 Circumstellar Habitable Zones 71 4.2.2 Galactic Habitable Zones 75 4.3 The Earth as a Habitable Planet 84 4.4 Good and Bad Planetary Neighbours 87 4.4.1 Milankovitch Cycles 87 4.4.2 Planetary Neighbours as Asteroid Shields 89 4.5 Does the Earth Need the Moon To Be Habitable? 91 4.5.1 What Inuence Does the Moon Have on Earth’s Habitability? 91 4.5.2 What If the Moon Never Existed? 93 4.5.3 Are Large Moons Common in Terrestrial Planet Formation? 95

5 Life Is Rare 98 5.1 What Is Life? 98 5.2 The Origins of Life 100 5.2.1 Oparin-Haldane Theory and the Miller-Urey Experiments 101 5.2.2 ‘Vents’ Theories 103 5.2.3 ‘Pools’ Theories 104 5.2.4 Multiple Origins of Life? 104 5.3 The Major Evolutionary Transitions in Terrestrial Organisms 104 5.3.1 Free-Floating Self-Replicating Molecules to Molecules in Compartments 105 5.3.2 Independent Replicators to Chromosomes 105

Contents xi 5.3.3 Cyanobacteria to Photosynthesis 109 5.3.4 Prokaryotes to Eukaryotes 111 5.3.5 Single-Cell to Multicellular Organisms 112 5.3.6 Asexual Reproduction to Sexual Reproduction 115 5.3.7 The Brain 119 5.3.8 Solitary Individuals to Social Units 121 5.3.9 Social Units to Civilisation 125 5.3.10 Biology to Post-Biology? 125

6 Intelligence Is Rare 127 6.1 The Evolutionist Argument against Intelligence 127 6.2 What Is Intelligence? 129 6.3 How Did Humans Become Intelligent? 130 6.3.1 Why Be Intelligent at All? 135 6.4 What Is the Role of Language? 136 6.4.1 The Origin of Human Language 136 6.4.2 Possible Biological Origins of Language 139 6.4.3 What Can We Say About the Language of ETIs? 141 6.4.4 Non-Human Communication 141 6.5 What Is the Role of Art? 145 6.6 What Is the Role of Science and Mathematics? 149 6.7 What Is the Role of Political Structures? 150

Part III Catastrophist Solutions 153

7 Doomsday Arguments 155 7.1 The Carter-Leslie Argument 155 7.2 Gott’s Argument 157

8 Death by Impact 159 8.1 Asteroids 159 8.2 Comets 160 8.3 The Risk of Impact on Earth 161 8.4 The Consequences of Impact 164 8.5 Impact as an Existential Risk to ETIs 166

9 Death by Terrestrial Disaster 169 9.1 Supervolcanism 169 9.2 Magnetic Field Collapse 173

xii Contents 10 DeathbyStar 179 10.1 Stellar Activity 179 10.1.1 Solar Winds 181 10.1.2 Flares and Coronal Mass Ejections 181 10.1.3 Earth’s Protection from Stellar Activity 184 10.2 Stellar Evolution 184 10.3 Death by Stellar Evolution 190 10.4 Death by Red Giant 191 10.5 Death by Supernova 194 10.5.1 Type Ia (White Dwarf Nova) 194 10.5.2 Type II Supernovae (Core-Collapse) 196 10.6 Close Stellar Encounters 198

11 Death on a Galactic Scale? 201 11.1 Active Galactic Nuclei 201 11.2 Gamma Ray Bursts 204 11.3 Terrestrial Atmospheric Responses to Very High-Energy Events 206 11.3.1 UV Radiation 206 11.3.2 Gamma and X-Rays 207 11.3.3 Cosmic Rays 207 11.3.4 Muons and Other Secondary Particles 208 11.3.5 The Role of Heliospheric Screening 208 11.3.6 Mitigation against High-Energy Radiation Events 209

12 Death by Unsustainable Growth 212

13 Death by Self-Induced Environmental Change 218 13.1 Atmospheric Damage 218 13.2 Destruction of the Land Environment 221 13.3 Destruction of the Ocean Environment 222 13.4 Destruction of the Biosphere 223 13.5 Destruction of the Space Environment (Kessler Syndrome) 224

14 Self-Destruction at the Nanoscale 228 14.1 Genetic Engineering 228 14.2 Nanotechnology, Nanomachines and ‘Grey Goo’ 230 14.3 Estimates of Existential Risk from Nanoscale Manipulation 231

15 Articial Intelligence and the Singularity 234 15.1 The Turing Machine 235 15.2 A Brief History of AI Techniques 236

Contents xiii 15.2.1 Logic-Driven AI: Symbolic Approaches 236 15.2.2 Biologically Motivated Approaches 239 15.2.3 Data-Driven Approaches 241 15.3 The Turing Test 242 15.4 Is the Singularity Inevitable? 243 15.5 Controls on AI 245 15.6 Is Strong AI Likely? 246

16 War 248 16.1 The Origins of Human Warfare 249 16.2 Is Violence Innate to Humans? 250 16.3 Nuclear War and Weapons of Mass Destruction 251 16.4 Is War Inevitable amongst Intelligent Civilisations? 253

17 Societal Collapse 254 17.1 Common Causes of Societal Collapse 255 17.2 Intellectual Collapse 258

Part IV Uncommunicative Solutions 259

18 Intelligent Life Is New 261 18.1 Carter’s Argument against SETI 261 18.2 Global Regulation Mechanisms and the Phase Transition Solution 262

19 Exploration Is Imperfect 265 19.1 Percolation Theory Models 265 19.2 Exploration of Realistic Star Fields 267 19.3 Human Designs for Interstellar Probes 270 19.3.1 Chemical Rockets 271 19.3.2 External Nuclear Pulse Propulsion 271 19.3.3 Internal Nuclear Pulse Propulsion 272 19.3.4 Antimatter Drive 273 19.3.5 The Interstellar Ramjet 274 19.3.6 Light Sails 275 19.3.7 Gravitational Assists 278 19.3.8 Relativistic Effects 279

20 Probe Exploration Is Dangerous 282 20.1 The Self-Replicating Probe 282 20.2 Can Probes Self-Replicate without Evolving? 284 20.3 Predators and Prey 285

xiv Contents 21 The Aliens Are Quiet 291 21.1 They Communicate at a Different Frequency 291 21.2 They Communicate Using a Different Method 296 21.2.1 Neutrinos 296 21.2.2 Gravitational Waves 298 21.3 We Do Not Understand Them 304 21.4 They Do Not Communicate 306 21.4.1 Arguments in Favour of METI 306 21.4.2 Arguments against METI 313 21.4.3 Can the METI Debate Be Resolved? 316

22 They Live Too Far Away 317 22.1 The Feasibility of Crewed Interstellar Travel 318 22.2 The Feasibility of Reliable Interstellar Communication Networks 322

23 The Zoo/Interdict Hypothesis 326

24 The Simulation Hypothesis 330 24.1 How Much Computing Power Is Needed? 332 24.2 Where Could We Get It From? 334 24.2.1 The Matrioshka Brain 335 24.3 Arguments against the Simulation Hypothesis 336

25 They Are Already Here 338 25.1 Bracewell or ‘Lurker’ Probes 338 25.2 Unidentied Flying Objects and Accounts of Alien Abduction 342

26 They Were Here Long Ago 346 26.1 The ‘Ancient Astronaut’ Hypothesis 347

Part V Conclusions 351

27 Solving Fermi’s Paradox 353 27.1 The Future of Fact A 353 27.1.1 Current and Future Radio/Optical SETI Missions 353 27.1.2 Gravitational Waves and SETI 355 27.1.3 Neutrino SETI 355 27.1.4 Transit SETI 356 27.2 Future Attempts at Interstellar Missions 358 27.3 Theoretical and Numerical Advances 360

Contents xv 27.3.1 Numerical Modelling of Civilisation Growth, Evolution and Interaction 361 27.3.2 Characterising Observables of Intelligence in Astrophysical Data 364 27.4 Can the Paradox Be Solved? 367

Appendix A A Database of Solutions to Fermi’s Paradox 370 A.1 Rare Earth Solutions 370 A.2 Catastrophist Solutions 372 A.3 Uncommunicative Solutions 373

References 375 Index 411

Preface

This textbook attempts to tackle the many academic concepts and theories that guide our thinking about the search for extraterrestrial intelligence (SETI), using Fermi’s Paradox as a lens. While writing it, I have aimed at two objectives: 1. Provide an exhaustive list of solutions to Fermi’s Paradox as of the time of publication, and 2. Provide context to this list of solutions, by describing and explaining their theoretical/numerical/observational underpinnings. This is no mean feat, and the lack of graduate-level textbooks which attempt to achieve both objectives is probably an indication. We will see that the question raised by Fermi’s Paradox requires an extremely broad swathe of academic disciplines to formulate a complete set of solutions. In fact, it could be argued that all academic disciplines have contributions to make, and that SETI is a study of the human condition, generalised to include all sentient beings. Obviously, a textbook that requires contributions from the entirety of academe will be necessarily abbreviated, and in the interests of focus I have been forced to edit out or ignore aspects that would otherwise merit more attention. As such, this book should be seen as an introduction to the most pertinent facts and concepts. For example, in the Introduction I give a brief account of the observational aspects of SETI, but the eld is worthy of far more discussion. This can be found in a range of excellent textbooks on observational SETI and radio astronomy in general (see References). This book is dominated by astronomy in its make-up, principally because SETI remains an astronomy-dominated discipline. What should hopefully be clear upon reading this textbook is that SETI’s accounting of other disciplines is quite uneven, and there are many topics highly deserving of further study. I will do my best to highlight areas where I believe that further work is necessary, to encourage the reader to help ll in these gaps, so that subsequent editions of this book can display our more developed understanding of life and intelligence in the Universe.

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