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2.1.3 Problems in Elementary Particle Physics D-Brane • Koji Hashimoto D-Brane Superstrings and New Perspective of Our World 123 Dr. Koji Hashimoto RIKEN Mathematical Physics Lab. Hirosawa 2-1 351-0198 Saitama Wako Japan [email protected] The Original Japanese version was published by University of Tokyo Press, Japan, 2006. Translated by Haruko Hashimoto and Koji Hashimoto. ISBN 978-3-642-23573-3 e-ISBN 978-3-642-23574-0 DOI 10.1007/978-3-642-23574-0 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2011944016 c Springer-Verlag Berlin Heidelberg 2012 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) To Haruko and to my parents • Preface This book is dedicated for explaining and introducing “D-branes,” a new concept on which theoretical developments in superstring theory in the past decade are all based on, with brief words and equations. The original version of this book was written in Japanese, and published by Tokyo University Press in 2006. I hope you enjoy surprising and intriguing novel perspective of our world which is shown by frontier of researches on elementary particle physics, cosmology and superstring theory which may unify all matters and forces of our world. D-branes, or in general “branes,” are membranes in higher-dimensional spaces, in mathematical physics. Once we apply this notion of “branes” to various physics, such as elementary particle physics and cosmology, we reach surprising change of our perspective on our world: we may live on 3-dimensional membranes in higher-dimensional space. This scenario is called “braneworld,” and not a scientific fantasy. In fact, every day and night, quite a number of particle physicists and cosmologists all over the world investigate this fascinating possibility of “living on hypersurfaces.” Why is this fascinating? It is not because the hypothesis is novel as a sci-fi, but because it is a logical consequence of superstring theory, a candidate theory unifying all forces and matters. And the hypothesis has a scientific possibility of resolving long-standing problems in particle physics, cosmology and gravity. This book is written for undergraduates who are interested in particle physics, superstring theory and cosmology, and also for researchers and graduate students whose major is not particle physics. In writing this book, I tried to minimize the assumed knowledge for readers, and I skip subjects which are necessary to really study elementary particle physics. I suppose readers have basic knowledge only on mechanics and electromagnetism, and a short path to the frontier research results is provided in this book. As a whole, I describe, in plain language, mechanism of elementary particle physics and superstring theory, and bring a “flavor” of frontier science research on D-branes. For instance, many conceptual figures are used in this book, for you to grasp easily and intuitively various physical concepts. Indeed, researchers everyday draw these kind of figures on blackboards for research. vii viii Preface Footnotes are for advanced/supporting explanations so that most of readers can just skip them on a first reading. Some of the important concepts which are indispensable in undergraduate/graduate courses in physics, such as quantum mechanics and quantum field theories, statistical mechanics and thermodynamics, are not described in detail but briefly outlined in this book, so readers who like to learn particle physics on more solid basis are advised to use some other textbooks. But I hope that this book is an entrance to view easily what is actually happening now at the frontier of theoretical physics. This book is not a textbook, so readers can just skip whatever they feel difficult or not interesting. I had to use some equations to show the fun of logic which leads to tremendously interesting results of the physics researches and how researchers are excited with them. But equations are only for advanced readers, so it is not necessary to follow all of them. It is important to “feel logically” that D-branes are necessary and indispensable in superstring theory, and how they provide us with such an interesting outcome. Even if in some parts of the book you may feel that the content is just mathematics, it is important to notice that all physics are based on mathematical equations, and the “brane paradigm” is completely based on such a logical reasoning. D-branes are a kind of “solitons.” Solitons are one of the important concepts in physics, so the first part of this book is for explaining basics of solitons. You will find that the importance of the D-branes originates in solitons. In case you are bored by the mathematical structure of solitons, you may start reading from Chap. 6. Braneworlds, a hypothesis that we live on some hyper-surfaces in higher- dimensional space, and other applications of D-branes are treated topic-wise in Chap. 6. The topics covered there are • Braneworld scenario • Black hole production in experiments • Braneworld cosmology and inflation • Counting of states on black holes by superstring theory • Holographic principle: gravitational description of quarks (also known as AdS/CFT correspondence) and readers can start from anywhere. These are just a part of the main achievements of superstring theory using D-branes; there are other important developments which could not be covered in this book. Some explanations in this book are not precise in theoretical sense, which is for readers to understand physics and mathematics in an easier manner. At the end of this book, I include some web links to research papers, with which readers can just feel more flavors of frontier researches. In 1995, I entered a graduate course in Kyoto University and eventually started reading the famous original research paper by J. Polchinski on D-branes, written in 1995. This paper initiated the revolution of superstring theory. Now, after a decade, this book is complete, largely owing to my friends and collaborators with whom I really enjoyed physics discussions, at Kyoto University, University of Tokyo, and Preface ix all over the world. I would like to thank them all. I thank my parents who allowed me to pursue my interest in physics. And I am indebted to my wife Haruko for encouraging me and supporting my life as a physicist. Tokyo Koji Hashimoto August 2011 • Contents 1 From Elementary Particles to D-Branes and Strings .................... 1 1.1 String Theory and Problems in Elementary Particle Physics ......... 2 1.2 What is D-Brane? ....................................................... 6 1.3 Organization of This Book ............................................. 10 2 Solitons and Elementary Particle Physics ................................. 13 2.1 Elementary Particle Physics and Field Theory ........................ 15 2.1.1 Fields ............................................................ 15 2.1.2 Equation of Motion, Quantization and Elementary Particles ......................................................... 16 2.1.3 Problems in Elementary Particle Physics ...................... 18 2.2 What is Soliton?......................................................... 21 2.2.1 Field Theory and Soliton ....................................... 22 2.2.2 Elementary Particles Versus Solitons .......................... 24 2.3 Symmetries in Field Theories and Solitons ............................ 25 2.3.1 Symmetry Breaking, Tachyons, Vacuum Condensation ................................................... 26 2.3.2 Solitons Emerging from Spontaneous Symmetry Breaking ............................................ 30 2.3.3 Vortex: Soliton in Two Dimensions ............................ 33 2.3.4 Monopole: Soliton in Three Dimensions ...................... 37 2.4 Importance of Soliton: Observation and Duality ...................... 40 2.4.1 Observing Soliton: Monopole in Grand Unified Theory...... 40 2.4.2 Duality and Soliton: Solving Difficulties in Field Theory .... 43 3 Dimensions of Solitons, Dimensions of String Theory ................... 49 3.1 Dimensions of Solitons and Braneworld ............................... 50 3.1.1 Solitons and Dimensions ....................................... 50 3.1.2 Cosmic String: Vortex in Three Dimensions .................. 52 3.1.3 Braneworld ...................................................... 53 3.1.4 How to Live on Solitons?....................................... 54 xi xii Contents 3.2 Higher Dimensions and Compactification in String Theory .......... 56 3.2.1 Species of String Theories...................................... 57 3.2.2 Spectrum of String Theory: Relation Between Particles and Strings ............................................ 59 3.2.3 Compactified Spacetime.......................................
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