iAPPLICATIONS OFCLASSICAL PHYSICSRoger D. Blandford and Kip S. ThorneCalifornia Institute of Technology2004—2005version 0400.2.K.pdf, September 26, 2004iiPrefacePlease send comments, suggestions, and erratavia email to [email protected], or on paper toKip Thorne,130-33 Caltech, Pasadena CA 91125 Sec. 3.7 and Ex. 3.6 and 3.7This book is an introduction to the fundamentals and 21st-century applications ofall the major branches of classical physics except classic al mechanics, electromagnetictheory, and elementary thermodynamics (which we assume the reader has already learnedelsewhere).Classical physics and this book deal with physical phenomena on macroscopic scales:scales where the particulate natures of matter and radiation are secondary to the be haviorof particles in bulk; scales where particles’ statistical as opp osed to individual prope rtiesare important, and where matter’s inherent graininess can be smoothed over. In thisbook, we shall take a journey through spacetime and phase space, through statisticaland continuum mechanics (including solids, fluids, and plasmas), and through optics andrelativity, both special and general. In our journey, we shall seek to comprehend thefundamental laws of classical physics in their own terms, and in relation to quantumphysics. Using carefully chosen examples, we shall show how the classical laws are appliedto important, contemporary, 21st-century problems and to everyday phenomena, and weshall uncover some deep connections among the various fundamental laws, and connectionsamong the practical techniques that are use d in different subfields of physics.Many of the most important recent developments in physics—and more generally inscience and engineering—involve classical subjects such as optics, fluids, plasmas, randomprocesses, and curved spacetime. Unfortunately, many young physicists today have littleunderstanding these subjects and their applications. Our goal, in writing this book, is torectify that. More specifically:• We believe that every masters- level or PhD physicist should be familiar with thebasic concepts of all the major branches of classical physics, and should have hadsome experience in applying them to real-world phenomena; this book is designed tofacilitate that.• A large fraction of physics, astronomy and engineering graduate students in the UnitediiiStates and around the world use classical physics extensively in their research, andeven more of them go on to careers in which classical physics is an essential component;this book is designed to facilitate that research and those careers.In pursuit of these goals, we seek, in this book, to give the reader a clear understandingof the basic concepts an d principles of classical physics. We present these principles in thelanguage of modern physics (not nineteenth century applied mathematics), and presentthem for physicists as distinct from mathematicians or engineers — though we hope thatmathematicians and engineers will also find our presentation useful. As far as possible, weemphasize theory that involves general principles which extend well beyond the particularsubjects we study.In this book, we also seek to teach the reader how to apply classical physics ideas. Wedo so by presenting contemporary applications from a variety of fields, such as• fundamental physics, experimental physics and applied physics;• astrophysics and cosmology;• geophysics, oceanography and meteorology;• engineering, optical science & technology, radio science & technology, and informationscience & technology.Why is the range of applications so wide? Because we believe that physicists should haveat their disposal enough understanding of general principles to attack problems that arisein unfamiliar environments. In the modern era, a large fraction of physics students willgo on to careers away from the core of fundamental physics. For such students, a broadexposure to non-core applications will be of great value; for those who wind up in the core,such an exposure is of value culturally, and also because ideas from other fields often turnout to have impact back in the core of physics. Our examples will illustrate how basicconcepts and problem solving techniques are freely interchanged between disciplines.Classical physics is defined as the physics where Planck’s constant can be approxi-mated as zero. To a large extent, it is the body of physics for which the fundamental equa-tions were established prior to the development of quantum mechanics in the 1920’s. Doesthis imply that it should be studied in isolation from quantum mechanics? Our answer is,most emphatically, “No!”. The reasons are simple. First, quantum mechanics has primacyover clas sical physics: classical physics is an approximation, often excellent, sometimespoor, to quantum mechanics. Second, in recent decades many concepts and mathematicaltechniques developed for quantum mechanics have been imported into classical physicsand used to enlarge our classical understanding and enhance our computational capability.An example that we shall discuss occurs in plasma physics, where nonlinearly interact-ing waves are treated as quanta (“plasmons”), despite the fact that they are solutions ofclassical field equations. Third, ideas developed initially for “classical” problems are fre-quently adapted for application to avowedly quantum mechanical subjects; examples (notdiscussed in this book) are found in supersymmetric string theory and in the liquid dropmodel of the atomic nucleus. Because of these intimate connections between quantum andclassical physics, quantum physics will appear frequently in this book, in a variety of ways.The amount and variety of material covered in this book may seem overwhelming.ivIf so, please keep in mind the key goals of the book: to teach the fundamental concepts,which are not so extensive that they should overwhelm, and to illustrate those concepts.Our goal is not to provide a mastery the many illustrative applications contained in thebook, but rather to convey the spirit of how to apply the basic concepts of classical physics.This book will also seem much more manageable and less overwhelming when onerealizes that the same concepts and problem solving techniques appear over and over again,in a variety of different subjects and applications. These unifying concepts and techniquesare listed in Appendix B, in outline form, along with the specific applications and
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