# Gravitation

## Book Preface

This is a textbook on gravitation physics (Einstein’s “general relativity” or “geometrodynamics”). It supplies two tracks through the subject. The first track is focused on the key physical ideas. It assumes, as mathematical prerequisite, only vector analysis and simple partial-differential equations. It is suitable for a one-semester course at the junior or senior level or in graduate school; and it constitutes-in the opinion of the authors-the indispensable core of gravitation theory that every advanced student of physics should learn. The Track-l material is contained in those pages of the book that have a 1 outlined in gray in the upper outside corner, by which the eye of the reader can quickly pick out the Track-l sections. In the contents, the same purpose is served by a gray bar beside the section, box, or figure number.

The rest of the text builds up Track 1 into Track 2. Readers and teachers are invited to select, as enrichment material, those portions of Track 2 that interest them most. With a few exceptions, any Track-2 chapter can be understood by readers who have studied only the earlier Track-l material. The exceptions are spelled out explicitly in “dependency statements” located at the beginning of each Track-2 chapter, or at each transition within a chapter from Track 1 to Track 2. The entire book (all of Track 1 plus all of Track 2) is designed for a rigorous, full-year course at the graduate level, though many teachers of a full-year course may prefer a more leisurely pace that omits some of the Track-2 material. The full book is intended to give a competence in gravitation physics comparable to that which the average Ph.D. has in electromagnetism. When the student achieves this competence, he knows the laws of physics in flat spacetime (Chapters 1-7). He can predict orders of magnitude. He can also calculate using the principal tools ofmodern differential geometry (Chapters 8-15), and he can predict at all relevant levels of precision. He understands Einstein’s geometric framework for physics (Chapters 16-22). He knows the applications of greatest present-day interest: pulsars and neutron stars (Chapters 23-26); cosmology (Chapters 27-30); the Schwarzschild geometry and gravitational collapse (Chapters 31-34); and gravitational waves (Chapters 35-37). He has probed the experimental tests of Einstein’s theory (Chapters 38-40). He will be able to read the modern mathematical literature on differential geometry, and also the latest papers in the physics and astrophysics journals about geometrodynamics and its applications. Ifhe wishes to go beyond the field equations, the four major applications, and the tests, he will find at the end of the book (Chapters 41-44) a brief survey of several advanced topics in general relativity. Among the topics touched on here, superspace and quantum geometrodynamics receive special attention. These chapters identify some of the outstanding physical issues and lines of investigation being pursued today.

Whether the department is physics or astrophysics or mathematics, more students than ever ask for more about general relativity than mere conversation. They want to hear its principal theses clearly stated. They want to know how to “work the handles of its information pump” themselves. More universities than ever respond with a serious course in Einstein’s standard 1915 geometrodynamics. What a contrast to Maxwell’s standard 1864 electrodynamics! In 1897, when Einstein was a student at Zurich, this subject was not on the instructional calendar of even half the universities of Europe.1 “We waited in vain for an exposition of Maxwell’s theory,” says one of Einstein’s classmates. “Above all it was Einstein who was disappointed,” 2 for he rated electrodynamics as “the most fascinating subject at the time” 3_as many students rate Einstein’s theory today!

Maxwell’s theory recalls Einstein’s theory in the time it took to win acceptance. Even as.late as 1904 a book could appear by so great an investigator as William Thomson, Lord Kelvin, with the words, “The so-called ‘electromagnetic theory of light’ has not helped us hitherto … it seems to me that it is rather a backward step … the one thing about it that seems intelligible to me, I do not think is admissible … that there should be an electric displacement perpendicular to the line of propagation.” 4 Did the pioneer of the Atlantic cable in the end contribute so richly to Maxwell electrodynamics-from units, and principles of measurement, to the theory of waves guided by wires-because of his own early difficulties with the subject? Then there is hope for many who study Einstein’s geometrodynamics today! By the 1920’s the weight of developments, from Kelvin’s cable to Marconi’s wireless, from the atom of Rutherford and Bohr to the new technology of highfrequency circuits, had produced general conviction that Maxwell was right. Doubt dwindled. Confidence led to applications, and applications led to confidence. Many were slow to take up general relativity in the beginning because it seemed to be poor in applications. Einstein’s theory attracts the interest of many today because it is rich in applications. No longer is attention confined to three famous but meager tests: the gravitational red shift, the bending of light by the sun, and the precession of the perihelion of Mercury around the sun.

The combination of radar ranging and general relativity is, step by step, transforming the solar-system celestial mechanics of an older generation to a new subject, with a new level of precision, new kinds of effects, and a new outlook. Pulsars, discovered in 1968, find no acceptable explanation except as the neutron stars predicted in 1934, objects with a central density so high (~1014g/cm3) that the Einstein predictions of mass differ from the Newtonian predictions by 10 to 100 per cent. About further density increase and a final continued gravitational collapse, Newtonian theory is silent. In contrast, Einstein’s standard 1915 geometrodynamics predicted in 1939 the properties of a completely collapsed object, a “frozen star” or “black hole.” By 1966 detailed digital calculations were available describing the formation of such an object in the collapse of a star with a white-dwarf core. Today hope to discover the first black hole is not least among the forces propelling more than one research: How does rotation influence the properties of a black hole? What kind of pulse of gravitational radiation comes off when such an object is formed? What spectrum of x-rays emerges when gas from a companion star piles up on its way into a black hole? 5 All such investigations and more base themselves on Schwarzschild’s standard 1916 static and spherically symmetric solution of Einstein’s field equations, first really understood in the modern sense in 1960, and in 1963 generalized to a black hole endowed with angular momentum.

Beyond solar-system tests and applications of relativity, beyond pulsars, neutron stars, and black holes, beyond geometrostatics (compare electrostatics!) and stationary geometries (compare the magnetic field set up by a steady current!) lies geometrodynamics in the full sense of the word (compare electrodynamics!). Nowhere does Einstein’s great conception stand out more clearly than here, that the geometry of space is a new physical entity, with degrees of freedom and a dynamics of its own. Deformations in the geometry of space, he predicted in 1918, can transport energy from place to place. Today, thanks to the initiative of Joseph Weber, detectors of such gravitational radiation have been constructed and exploited to give upper limits to the flux of energy streaming past the earth at selected frequencies. Never before has one realized from how many kinds of processes significant gravitational radiation can be anticipated. Never before has there been more interest in picking up this new kind of signal and using it to diagnose faraway events. Never before has there been such a drive in more than one laboratory to raise instrumental sensitivity until gravitational radiation becomes a workaday new window on the universe.

The expansion of the universe is the greatest of all tests of Einstein’s geometrodynamics, and cosmology the greatest of all applications. Making a prediction too fantastic for its author to credit, the theory forecast the expansion years before it was observed (1929). Violating the short time-scale that Hubble gave for the expansion, and in the face of “theories” (“steady state”; “continuous creation”) manufactured to welcome and utilize this short time-scale, standard general relativity resolutely persisted in the prediction of a long time-scale, decades before the astro- physical discovery (1952) that the Hubble scale of distances and times was wrong, and had to be stretched by a factor of more than five. Disagreeing by a factor of the order of thirty with the average density of mass-energy in the universe deduced from astrophysical evidence as recently as 1958, Einstein’s theory now as in the past argues for the higher density, proclaims “the mystery of the missing matter,” and encourages astrophysics in a continuing search that year by year turns up new indications of matter in the space between the galaxies. General relativity forecast the primordial cosmic fireball radiation, and even an approximate value for its present temperature, seventeen years before the radiation was discovered. This radiation brings information about the universe when it had a thousand times smaller linear dimensions, and a billion times smaller volume, than it does today. Quasistellar objects, discovered in 1963, supply more detailed information from a more recent era, when the universe had a quarter to half its present linear dimensions. Telling

about a stage in the evolution of galaxies and the universe reachable in no other way, these objects are more than beacons to light up the far away and long ago. They put out energy at a rate unparalleled anywhere else in the universe. They eject matter with a surprising directivity. They show a puzzling variation with time, different between the microwave and the visible part of the spectrum. Quasistellar objects on a great scale, and galactic nuclei nearer at hand on a smaller scale, voice a challenge to general relativity: help clear up these mysteries! If its wealth of applications attracts many young astrophysicists to the study of Einstein’s geometrodynamics, the same attraction draws those in the world of physics who are concerned with physical cosmology, experimental general relativity, gravitational radiation, and the properties of objects made out of superdense matter. Of quite another motive for study of the subject, to contemplate Einstein’s inspiring vision of geometry as the machinery of physics, we shall say nothing here because it speaks out, we hope, in every chapter of this book.

Why a new book? The new applications of general relativity, with their extraordinary physical interest, outdate excellent textbooks of an earlier era, among them even that great treatise on the subject written by Wolfgang Pauli at the age of twenty-one. In addition, differential geometry has undergone a transformation of outlook that isolates the student who is confined in his training to the traditional tensor calculus of the earlier texts. For him it is difficult or impossible either to read the writings of his up-to-date mathematical colleague or to explain the mathematical content of his physical problem to that friendly source of help. We have not seen any way to meet our responsibilities to our students at our three institutions except by a new exposition, aimed at establishing a solid competence in the subject, contemporary in its mathematics, oriented to the physical and astrophysical applications of greatest present-day interest, and animated by belief in the beauty and simplicity High Island

South Bristol, Maine

September 4, 1972

Charles W Misner

Kip S. Thorne

John Archibald Wheeler

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