On The Trail of Blackbody Radiation On the Trail of Blackbody Radiation Max Planck and the Physics of His Era Don S. Lemons, William R. Shanahan, and Louis J. Buchholtz illustrated by Marta Gyeviki The MIT Press Cambridge, Massachusetts London, England © 2022 Massachusetts Institute of Technology All rights reserved. No part of this book may be reproduced in any form by any electronic or mechanical means (including photocopy- ing, recording, or information storage and retrieval) without permis- sion in writing from the publisher. The MIT Press would like to thank the anonymous peer reviewers who provided comments on drafts of this book. The generous work of aca- demic experts is essential for establishing the authority and quality of our publications. We acknowledge with gratitude the contributions of these otherwise uncredited readers. This book was set in ITC Stone Serif Std and ITC Stone Sans Std by New Best-set Typesetters Ltd. Library of Congress Cataloging- in- Publication Data Names: Lemons, Don S. (Don Stephen), 1949- author. | Shanahan, William R., author. | Buchholtz, Louis J., author. Title: On the trail of blackbody radiation : Max Planck and the physics of his era / Don S. Lemons, William R. Shanahan, and Louis J. Buchholtz ; Illustrated by Marta Gyeviki. Description: Cambridge, Massachusetts : The MIT Press, [2022] | Includes bibliographical references and index. Identifiers: LCCN 2021035122 | ISBN 9780262047043 (hardcover) Subjects: LCSH: Blackbody radiation. Classification: LCC QC484 .L46 2022 | DDC 530.12—dc23/ eng/20211014 LC record available at https://lccn.loc.gov/2021035122 10 9 8 7 6 5 4 3 2 1 Dedications Don S. Lemons: To his wife Allison and sons Nathan and Micah William R. Shanahan: To his wife Katie Louis J. Buchholtz: To his wife Barbara, daughter Clara, and son Will Contents Preface xi A Brief Guide to the Trail xv 1 The Prehistory of Blackbody Radiation 1 1.1 Pictet’s Experiment and Prevost’s Exchanges 1 1.2 Reflectors, Absorbers, and Emitters of Radiant Heat 4 1.3 Blackbodies and Blackbody Radiation 5 2 Classical Thermodynamics 7 2.1 Why Thermodynamics? 7 2.2 Equilibrium and the Zeroth Law of Thermodynamics 8 2.3 The First Law of Thermodynamics 10 2.4 Thermodynamic Temperature 12 2.5 The Second Law of Thermodynamics 14 2.6 The Fluid System 16 2.7 Example: The Ideal Gas 17 2.8 The Adiabatic Invariant of an Ideal Gas 19 2.9 The Entropy of an Ideal Gas 20 2.10 Relations among Different Forms of the Adiabatic Invariant 22 3 Kirchhoff’s Law, 1859 25 3.1 Blackbody Radiation and the Laws of Thermodynamics 25 viii Contents 3.2 The Energy Density of Blackbody Radiation 25 3.3 The Spectral Energy Density 29 3.4 Kirchhoff’s Law of Thermal Radiation 29 4 The Stefan-B oltzmann Law, 1884 33 4.1 Radiation Pressure 33 4.2 The Stefan-B oltzmann Law 36 4.3 The Adiabatic Invariant of Blackbody Radiation 38 4.4 An Alternate Derivation of the Stefan- Boltzmann Law 39 4.5 The Entropy of Blackbody Radiation 42 4.6 The Universality of Blackbody Radiation 43 4.7 Boltzmann’s 1884 Derivation 44 5 Wien’s Contributions, 1893–1 896 51 5.1 Spectral Energy Density 51 5.2 Cumulative Spectral Energy Density 52 5.3 Thermodynamic Adiabatic Invariants 53 5.4 Wien’s Electromagnetic Adiabatic Invariant 56 5.5 Wien’s Displacement Law 59 5.6 A Dimensional Consequence of Wien’s Displacement Law 62 5.7 A Practical Consequence of Wien’s Displacement Law 63 5.8 Wien’s 1896 Distribution 64 5.9 Wien’s 1893 Derivation 65 6 The Damped, Driven, Simple Harmonic Oscillator 69 6.1 Planck Resonator 69 6.2 Simple Harmonic Oscillator 70 6.3 The Damped, Simple Harmonic Oscillator 72 6.4 The Damped, Driven, Simple Harmonic Oscillator 73 6.5 Lorentzian Approximation for Weak Damping 75 7 The Fundamental Relation 79 7.1 The Fundamental Relation 79