Lecture Notes Physik LN General Relativity MATTHIAS BARTELMANN HEIDELBERG UNIVERSITY PUBLISHING GENERAL RELATIVITY Lecture Notes Physik LN GENERAL RELATIVITY Matthias Bartelmann Institut für Theoretische Astrophysik Universität Heidelberg About the Author Matthias Bartelmann is professor for theoretical astrophysics at Heidelberg University. He mostly addresses cosmological questions, concerning in particular the formation and evolution of cosmic structures. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. This book is published under the Creative Commons Attribution 4.0 License (CC BY-SA 4.0). The cover is subject to the Creative Commons License CC BY-ND 4.0. The electronic open access version of this work is permanently available on Heidelberg University Publishing’s website: https://heiup.uni-heidelberg.de urn: urn:nbn:de:bsz:16-heiup-book-534-9 doi: https://doi.org/10.17885/heiup.534 Text © 2019, Matthias Bartelmann ISSN 2566-4816 (PDF) ISSN 2512-4455 (Print) ISBN 978-3-947732-59-3 (PDF) ISBN 978-3-947732-60-9 (Softcover) v Instead of a preface Before we begin, some words are in order on the purpose and the limits of these notes, on the notation used, and on some of the many people I am indebted to. Each of the chapters of these notes is meant for a week of two lectures of two hours each. Much more could be said about all of their topics in all directions, in terms of mathematics, physics, and experimental tests of general relativity. These notes are meant as an introduction which can in no way be considered complete. They may serve as a first guide through the subject, not a comprehensive one. These lectures are part of a curriculum in which cosmology, gravitational lensing, and theoretical astrophysics are regularly taught separately. In these areas, they are thus only meant to lay the foundation. We use index-free notation where possible and convenient. Then, the curvature, the curvature tensor, the Ricci tensor and the Ricci scalar, often denoted with an R with different numbers of indices, need different symbols. We denote the curvature and the curvature tensor, closely related as they are, with R¯, the Ricci tensor with R and the Ricci scalar with R. Since the symbol G is then reserved for the Einstein tensor, we write Newton’s gravitational constant as G. Indices refering to coordinates on general, d-dimensional manifolds are written as Latin characters. On 4-dimensional, spacetime manifolds, Greek indices run from 0 to 3, while Latin indices refer to spatial coordi- nates and run from 1 to 3. These lecture notes grew over several years. Many students were ex- posed to this lecture and contributed corrections and suggestions that greatly helped improving it. In particular, Dr. Christian Angrick and Dr. Francesco Pace were kind and patient enough to meticulously work through the entire notes and point out many mistakes. Thank you all very much! Particular thanks are due to the wonderful and inspiring teachers I myself had on general relativity. Jürgen Ehlers always impressed me with his depth and clarity of thinking, and Norbert Straumann introduced me to the elegance and beauty of the theory. Contents 1 Introduction 1 1.1 The idea behind general relativity ........... 1 1.2 Fundamental properties of gravity ........... 4 1.3 Consequences of the equivalence principle ....... 7 1.4 Futile attempts ..................... 9 2Differential Geometry I 15 2.1 Differentiable manifolds ................ 15 2.2 The tangent space .................... 18 2.3 Dual vectors and tensors ................ 24 2.4 The metric ........................ 27 3Differential Geometry II 31 3.1 Connections and covariant derivatives ......... 31 3.2 Geodesics ........................ 34 3.3 Curvature ........................ 38 3.4 Riemannian connections ................ 42 4 Physics in Gravitational Fields 47 4.1 Motion of particles ................... 47 4.2 Motion of light ..................... 49 4.3 Energy-momentum (non-)conservation ......... 54 4.4 The Newtonian limit .................. 58 5Differential Geometry III 63 vii viii CONTENTS 5.1 The Lie derivative .................... 63 5.2 Killing vector fields ................... 68 5.3 Differential forms .................... 69 5.4 Integration ........................ 73 6 Einstein’s Field Equations 77 6.1 The physical meaning of curvature ........... 77 6.2 Einstein’s field equations ................ 82 6.3 Lagrangian formulation ................. 84 6.4 The energy-momentum tensor ............. 86 7 Weak Gravitational Fields 91 7.1 Linearised theory of gravity ............... 91 7.2 Gauge transformations ................. 93 7.3 Nearly Newtonian gravity ................ 95 7.4 Gravitational waves ................... 100 8 The Schwarzschild Solution 111 8.1 Cartan’s structure equations ............... 111 8.2 Stationary and static spacetimes ............ 115 8.3 The Schwarzschild solution ............... 116 8.4 Solution of the field equations ............. 120 9 The Schwarzschild Spacetime 127 9.1 Orbits in the Schwarzschild spacetime ......... 127 9.2 Comparison to the Kepler problem ........... 130 9.3 Perihelion shift and light deflection ........... 133 9.4 Spins in the Schwarzschild spacetime ......... 137 10 Schwarzschild Black Holes 143 10.1 The singularity at r = 2m ................ 143 10.2 The Kruskal continuation ................ 147 CONTENTS ix 10.3 Physical meaning of the Kruskal continuation ..... 152 10.4 Redshift approaching the Schwarzschild radius .... 156 11 Charged, Rotating Black Holes 161 11.1 The Reissner-Nordström solution ............ 161 11.2 The Kerr-Newman solution ............... 164 11.3 Motion near a Kerr black hole ............. 172 11.4 Entropy and temperature of a black hole ........ 177 12 Homogeneous, Isotropic Cosmology 183 12.1 Spherically-symmetric spacetimes ........... 183 12.2 Homogeneous and isotropic spacetimes ........ 187 12.3 Friedmann’s equations ................. 191 12.4 Density evolution and redshift ............. 194 13 Relativistic Astrophysics 199 13.1 Light bundles ...................... 199 13.2 Gravitational lensing .................. 201 13.3 The Tolman-Oppenheimer-Volkoff solution ...... 206 13.4 The mass of non-rotating neutron stars ......... 209 A Electrodynamics 213 A.1 Electromagnetic field tensor .............. 213 A.2 Maxwell’s equations .................. 213 A.3 Lagrange density and energy-momentum tensor .... 214 BDifferential Geometry 217 B.1 Manifold ........................ 217 B.2 Tangent and dual spaces ................ 217 B.3 Tensors ......................... 218 B.4 Covariant derivative ................... 219 B.5 Parallel transport and geodesics ............. 220 x CONTENTS B.6 Torsion and curvature .................. 220 B.7 Pull-back, Lie derivative, Killing fields ......... 221 B.8 Differential forms .................... 223 B.9 Cartan’s structure equations ............... 224 B.10 Differential operators and integration .......... 225 C Penrose-Carter diagrams 227 Chapter 1 Introduction 1.1 The idea behind general relativity There was no need for general relativity when Einstein started working on it. There was no experimental data signalling any failure of the Newtonian theory of gravity, except perhaps for the minute advance of the perihelion of Mercury’s orbit by 43 per century, which researchers at the time tried to explain by perturbations not included yet into the calculations of celestial mechanics in the Solar System. Essentially, Einstein found general relativity because he was deeply dissatisfied with some of the concepts of the Newtonian theory, in par- ticular the concept of an inertial system, for which no experimental demonstration could be given. After special relativity, he was convinced quite quickly that trying to build a relativistic theory of gravitation led to conclusions which were in conflict with experiments. Action at a distance is impossible in special relativity because the absolute meaning of space and time had to be given up. The most straightforward way to combine special relativity with Newtonian gravity seemed to start from Poisson’s equation for the gravitational potential and to add time derivatives to it so as to make it relativistically invariant. However, it was then unclear how the law of motion should be modified because, according to special relativity, energy and mass are equiva- lent and thus the mass of a body should depend on its position in a gravitational field. This led Einstein to a result which raised his suspicion. In Newtonian theory, the vertical acceleration of a body in a vertical gravitational field is independent of its horizontal motion. In a special-relativistic extension of Newton’s theory, this would no longer be the case: the 1 2 1 Introduction vertical gravitational acceleration would depend on the kinetic energy of a body, and thus not be independent of its horizontal motion. Figure 1.1 Albert Einstein (1879–1955) during a lecture in Vienna, 1921. Source: Wikipedia This was in striking conflict with experiment, which says that all bodies experience the same gravitational acceleration. At this point, the equiva- lence of inertial and gravitational mass struck Einstein as a law of deep significance. It became the heuristic guiding principle in the construction of general relativity. Freely falling frames of reference This line of thought leads to the fundamental concept of general