Lecture Notes on General RelativitySean M. CarrollInstitutefor Theoretical Physics26600University of CaliforniaSanta Barbara, CA 93106carroll@itp.ucsb.edu3December1997[4610216/0b-18:A1XIeAbstractThese notes represent approximately one semester's worth of lectures on intro-ductory general relativityforbeginning graduate students in physics.Topics includemanifolds, Riemannian geometry, Einstein's equations, and three applications: grav-itational radiation, black holes, and cosmology.Individual chapters, and potentiallyupdatedversions,canbefound athttp://itp.ucsb.edu/"carroll/notes/.NSF-ITP/97-147gr-qc/9712019
arXiv:gr-qc/9712019v1 3 Dec 1997 Lecture Notes on General Relativity Sean M. Carroll Institute for Theoretical Physics University of California Santa Barbara, CA 93106 carroll@itp.ucsb.edu December 1997 Abstract These notes represent approximately one semester’s worth of lectures on introductory general relativity for beginning graduate students in physics. Topics include manifolds, Riemannian geometry, Einstein’s equations, and three applications: gravitational radiation, black holes, and cosmology. Individual chapters, and potentially updated versions, can be found at http://itp.ucsb.edu/~carroll/notes/. NSF-ITP/97-147 gr-qc/9712019
iTable of Contents0.Introductiontable of contents preface - bibliography1.SpecialRelativityandFlatSpacetimethe spacetime interval - the metric — Lorentz transformations - spacetime diagrams-vectors- the tangent space dual vectorstensors-tensor productstheLevi-Civita tensor-index manipulationelectromagnetismdifferential forms-Hodge duality - worldlines - proper time - energy-momentum vector - energy-momentum tensor perfect fluids energy-momentum conservation2.Manifoldsexamples non-examples maps continuity the chain rule open sets -chartsandatlasesmanifoldsexamplesof chartsdifferentiationvectors asderivatives coordinate bases the tensor transformation lawpartial derivativesare not tensors the metric again canonical form of the metric -Riemann normalcoordinates tensor densities volume forms and integration3. Curvaturecovariant derivatives and connections connection coefficientstransformationproperties the Christoffel connection structures on manifolds parallel trans-port-the parallel propagatorgeodesicsaffineparameters-the exponentialmap the Riemann curvature tensorsymmetries of the Riemann tensortheBianchi identity Ricci and Einstein tensors- Weyl tensor simple examplesgeodesic deviation tetrads and non-coordinate basesthe spin connectionMaurer-Cartan structure equations-fiber bundles and gauge transformations4.Gravitationthe Principle of Equivalence -gravitational redshift gravitation as spacetime cur-vature - the Newtonian limit - physics in curved spacetime - Einstein's equations the Hilbert action - the energy-momentum tensor again - the Weak Energy Con-dition - alternative theories - the initial value problem - gauge invariance andharmonic gauge-domains of dependencecausality5.MoreGeometrypullbacks and pushforwards diffeomorphisms integral curves Lie derivativesthe energy-momentum tensor one more timeisometries and Killing vectors
i Table of Contents 0. Introduction table of contents — preface — bibliography 1. Special Relativity and Flat Spacetime the spacetime interval — the metric — Lorentz transformations — spacetime diagrams — vectors — the tangent space — dual vectors — tensors — tensor products — the Levi-Civita tensor — index manipulation — electromagnetism — differential forms — Hodge duality — worldlines — proper time — energy-momentum vector — energymomentum tensor — perfect fluids — energy-momentum conservation 2. Manifolds examples — non-examples — maps — continuity — the chain rule — open sets — charts and atlases — manifolds — examples of charts — differentiation — vectors as derivatives — coordinate bases — the tensor transformation law — partial derivatives are not tensors — the metric again — canonical form of the metric — Riemann normal coordinates — tensor densities — volume forms and integration 3. Curvature covariant derivatives and connections — connection coefficients — transformation properties — the Christoffel connection — structures on manifolds — parallel transport — the parallel propagator — geodesics — affine parameters — the exponential map — the Riemann curvature tensor — symmetries of the Riemann tensor — the Bianchi identity — Ricci and Einstein tensors — Weyl tensor — simple examples — geodesic deviation — tetrads and non-coordinate bases — the spin connection — Maurer-Cartan structure equations — fiber bundles and gauge transformations 4. Gravitation the Principle of Equivalence — gravitational redshift — gravitation as spacetime curvature — the Newtonian limit — physics in curved spacetime — Einstein’s equations — the Hilbert action — the energy-momentum tensor again — the Weak Energy Condition — alternative theories — the initial value problem — gauge invariance and harmonic gauge — domains of dependence — causality 5. More Geometry pullbacks and pushforwards — diffeomorphisms — integral curves — Lie derivatives — the energy-momentum tensor one more time — isometries and Killing vectors
i6.WeakFields andGravitationalRadiationthe weak-field limit defined - gauge transformations -linearized Einstein equations gravitational plane waves - transverse traceless gauge - polarizations gravita-tional radiation by sourcesenergy loss7.The Schwarzschild Solution and Black Holesspherical symmetrythe Schwarzschild metricBirkhoff's theoremgeodesicsof Schwarzschild Newtonian vs.relativistic orbitsperihelion precession-theeventhorizon-black holes-Kruskal coordinates—formation of black holesPenrose diagrams conformal infinity no hair charged black holes cosmiccensorship extremal black holes rotating black holes Killing tensors thePenrose process- irreducible massblack hole thermodynamics8.Cosmologyhomogeneity and isotropy the Robertson-Walker metric -forms of energy andmomentumFriedmann equations-cosmological parametersevolution of thescale factorredshift -Hubble's law
ii 6. Weak Fields and Gravitational Radiation the weak-field limit defined — gauge transformations — linearized Einstein equations — gravitational plane waves — transverse traceless gauge — polarizations — gravitational radiation by sources — energy loss 7. The Schwarzschild Solution and Black Holes spherical symmetry — the Schwarzschild metric — Birkhoff’s theorem — geodesics of Schwarzschild — Newtonian vs. relativistic orbits — perihelion precession — the event horizon — black holes — Kruskal coordinates — formation of black holes — Penrose diagrams — conformal infinity — no hair — charged black holes — cosmic censorship — extremal black holes — rotating black holes — Killing tensors — the Penrose process — irreducible mass — black hole thermodynamics 8. Cosmology homogeneity and isotropy — the Robertson-Walker metric — forms of energy and momentum — Friedmann equations — cosmological parameters — evolution of the scale factor — redshift — Hubble’s law
iiPrefaceThese lectures represent an introductory graduate course in general relativity, both its foun-dations and applications.They are a lightly edited version of notes I handed out whileteaching Physics 8.962, the graduate course in GR at MIT, during the Spring of 1996.Al-though they are appropriately called "lecture notes", the level of detail is fairly high, eitherincluding all necessary steps or leaving gaps that can readily be filled in by the reader. Never-theless, there are various ways in which these notes differ from a textbook; most importantly,they are not organized into short sections that can be approached in various orders, but aremeant tobegonethroughfrom start to finish.A special effort hasbeenmadetomaintaina conversational tone, inan attempt togo slightly beyond the bare results themselves andintothecontext inwhich theybelong.The primary question facing any introductory treatment of general relativity is the levelof mathematical rigor at which to operate. There is no uniquely proper solution, as differentstudents will respond with different levels of understanding and enthusiasm to differentapproaches.Recognizingthis,Ihavetried toprovidesomething foreveryone.Thelecturesdo not shy away from detailed formalism (as for example in the introduction to manifolds),but also attempt to include concrete examples and informal discussion of the concepts underconsideration.As these are advertised as lecture notes rather than an original text, at times I haveshamelessly stolen from various existing books on the subject (especially those by Schutz.Wald, Weinberg, and Misner, Thorne and Wheeler). My philosophy was never to try to seekoriginality for its own sake; however, originality sometimes crept in just because I thoughtI could be more clear than existing treatments. None of the substance of the material inthese notes is new:the only reason for reading them is if an individual reader finds theexplanations here easier to understand than thoseelsewhere.Time constraints during the actual semester prevented me from covering some topics inthe depth which they deserved, an obvious example being the treatment of cosmology.Ifthe time and motivation come to pass, I may expand and revise the existing notes; updatedversions will be available at http://itp.ucsb.edu/~carroll/notes/.Of course I willappreciate having my attention drawn to any typographical or scientific errors, as well assuggestions for improvement of all sorts.Numerous people have contributed greatly both to my own understanding of generalrelativity and to these notes in particulartoomanytoacknowledgewithanyhopeofcompleteness. Special thanks are due to Ted Pyne, who learned the subject along with me,taught me a great deal, and collaborated on a predecessor to this course which we taughtas a seminar in the astronomy department at Harvard. Nick Warner taught the graduatecourse at MIT which I took before ever teaching it, and his notes were (as comparison will
iii Preface These lectures represent an introductory graduate course in general relativity, both its foundations and applications. They are a lightly edited version of notes I handed out while teaching Physics 8.962, the graduate course in GR at MIT, during the Spring of 1996. Although they are appropriately called “lecture notes”, the level of detail is fairly high, either including all necessary steps or leaving gaps that can readily be filled in by the reader. Nevertheless, there are various ways in which these notes differ from a textbook; most importantly, they are not organized into short sections that can be approached in various orders, but are meant to be gone through from start to finish. A special effort has been made to maintain a conversational tone, in an attempt to go slightly beyond the bare results themselves and into the context in which they belong. The primary question facing any introductory treatment of general relativity is the level of mathematical rigor at which to operate. There is no uniquely proper solution, as different students will respond with different levels of understanding and enthusiasm to different approaches. Recognizing this, I have tried to provide something for everyone. The lectures do not shy away from detailed formalism (as for example in the introduction to manifolds), but also attempt to include concrete examples and informal discussion of the concepts under consideration. As these are advertised as lecture notes rather than an original text, at times I have shamelessly stolen from various existing books on the subject (especially those by Schutz, Wald, Weinberg, and Misner, Thorne and Wheeler). My philosophy was never to try to seek originality for its own sake; however, originality sometimes crept in just because I thought I could be more clear than existing treatments. None of the substance of the material in these notes is new; the only reason for reading them is if an individual reader finds the explanations here easier to understand than those elsewhere. Time constraints during the actual semester prevented me from covering some topics in the depth which they deserved, an obvious example being the treatment of cosmology. If the time and motivation come to pass, I may expand and revise the existing notes; updated versions will be available at http://itp.ucsb.edu/~carroll/notes/. Of course I will appreciate having my attention drawn to any typographical or scientific errors, as well as suggestions for improvement of all sorts. Numerous people have contributed greatly both to my own understanding of general relativity and to these notes in particular — too many to acknowledge with any hope of completeness. Special thanks are due to Ted Pyne, who learned the subject along with me, taught me a great deal, and collaborated on a predecessor to this course which we taught as a seminar in the astronomy department at Harvard. Nick Warner taught the graduate course at MIT which I took before ever teaching it, and his notes were (as comparison will
ivreveal) an important inffuence on these. George Field offered a great deal of advice andencouragement as I learned the subject and struggled to teach it. Tamas Hauer struggledalong with me as the teaching assistant for 8.962, and was an invaluable help.All of thestudents in 8.962 deserve thanks for tolerating my idiosyncrasies and prodding me to everhigherlevels of precision.During the course of writing these notes I was supported by U.S. Dept. of Energy contract no. DE-AC02-76ER03069 and National Science Foundation grants PHY/92-06867 andPHY/94-07195
iv reveal) an important influence on these. George Field offered a great deal of advice and encouragement as I learned the subject and struggled to teach it. Tam´as Hauer struggled along with me as the teaching assistant for 8.962, and was an invaluable help. All of the students in 8.962 deserve thanks for tolerating my idiosyncrasies and prodding me to ever higher levels of precision. During the course of writing these notes I was supported by U.S. Dept. of Energy contract no. DE-AC02-76ER03069 and National Science Foundation grants PHY/92-06867 and PHY/94-07195
VBibliographyThe typical level of difficulty (especiallymathematical)of thebooks is indicated by a numberof asterisks, one meaning mostly introductory and three being advanced. The asterisks arenormalized to these lecture notes, which would be given [**].The first four books werefrequently consulted in the preparation of these notes, the next seven are other relativity textswhich I have found to be useful, and the last four are mathematical background references.·B.F. Schutz, A First Course in General Relativity (Cambridge, 1985) [*].This is avery nice introductory text. Especially useful if, for example, you aren't quite clear onwhat the energy-momentum tensor really means. S. Weinberg, Gravitation and Cosmology (Wiley, 1972) [**].A really good book atwhat it does, especially strong on astrophysics, cosmology, and experimental tests.However, it takes an unusual non-geometric approach to the material, and doesn'tdiscuss black holes.. C. Misner, K. Thorne and J. Wheeler, Gravitation (Freeman, 1973) [**j. A heavy book,in various senses. Most things you want to know are in here, although you might haveto work hard to get to them (perhaps learning something unexpected in the process).. R. Wald, General Relativity (Chicago, 1984) [***]. Thorough discussions of a numberof advanced topics, including black holes, global structure, and spinors. The approachis more mathematically demanding than the previous books, and the basics are coveredpretty quickly.:E.Taylor and J.Wheeler, SpacetimePhysics (Freeman, 1992)[*].Agood introductionto special relativity.·R. D'Inverno, Introducing Einstein's Relativity (Oxford, 1992) [**].A book I haven'tlooked at very carefully, but it seems as if all the right topics are covered withoutnoticeable ideological distortion.: A.P. Lightman, W.H. Press, R.H. Price, and S.A. Teukolsky, Problem Book in Rela-tivity and Gravitation (Princeton, 1975) [**]. A sizeable collection of problems in allareas of GR, with fully worked solutions, making it all the more difficult for instructorsto invent problems the students can't easilyfind theanswers to.N.Straumann, General Relativity and RelativisticAstrophysics (Springer-Verlag, 1984)[***].Afairly high-level book, which starts out with a good deal of abstract geometryand goes on to detailed discussions of stellar structure and other astrophysical topics
v Bibliography The typical level of difficulty (especially mathematical) of the books is indicated by a number of asterisks, one meaning mostly introductory and three being advanced. The asterisks are normalized to these lecture notes, which would be given [**]. The first four books were frequently consulted in the preparation of these notes, the next seven are other relativity texts which I have found to be useful, and the last four are mathematical background references. • B.F. Schutz, A First Course in General Relativity (Cambridge, 1985) [*]. This is a very nice introductory text. Especially useful if, for example, you aren’t quite clear on what the energy-momentum tensor really means. • S. Weinberg, Gravitation and Cosmology (Wiley, 1972) [**]. A really good book at what it does, especially strong on astrophysics, cosmology, and experimental tests. However, it takes an unusual non-geometric approach to the material, and doesn’t discuss black holes. • C. Misner, K. Thorne and J. Wheeler, Gravitation (Freeman, 1973) [**]. A heavy book, in various senses. Most things you want to know are in here, although you might have to work hard to get to them (perhaps learning something unexpected in the process). • R. Wald, General Relativity (Chicago, 1984) [***]. Thorough discussions of a number of advanced topics, including black holes, global structure, and spinors. The approach is more mathematically demanding than the previous books, and the basics are covered pretty quickly. • E. Taylor and J. Wheeler, Spacetime Physics (Freeman, 1992) [*]. A good introduction to special relativity. • R. D’Inverno, Introducing Einstein’s Relativity (Oxford, 1992) [**]. A book I haven’t looked at very carefully, but it seems as if all the right topics are covered without noticeable ideological distortion. • A.P. Lightman, W.H. Press, R.H. Price, and S.A. Teukolsky, Problem Book in Relativity and Gravitation (Princeton, 1975) [**]. A sizeable collection of problems in all areas of GR, with fully worked solutions, making it all the more difficult for instructors to invent problems the students can’t easily find the answers to. • N. Straumann, General Relativity and Relativistic Astrophysics (Springer-Verlag, 1984) [***]. A fairly high-level book, which starts out with a good deal of abstract geometry and goes on to detailed discussions of stellar structure and other astrophysical topics
viF.de Felice and C.Clarke, Relativity on Curved Manifolds (Cambridge, 1990)[***]A mathematical approach, but with an excellent emphasis on physically measurablequantities.·S.Hawking and G.Ellis, The Large-Scale Structure of Space-Time (Cambridge, 1973)[***].An advanced book which emphasizes global techniques and singularity theorems..R. Sachs and H. Wu, General Relativity for Mathematicians (Springer-Verlag, 1977)[***].Just what the title says, although the typically dry mathematics prose styleis here enlivened by frequent opinionated asides about both physics and mathematics(and the state oftheworld).:B. Schutz, Geometrical Methods of Mathematical Physics (Cambridge, 1980)[**]Another good book by Schutz, this one covering some mathematical points that areleft out of the GR book (but at a very accessible level). Included are discussions of Liederivatives,differentialforms,and applicationstophysicsotherthanGR..V.Guillemin and A. Pollack, Differential Topology (Prentice-Hall, 1974) [**j.Anentertaining survey of manifolds, topology, differential forms, and integration theory.. C. Nash and S. Sen, Topology and Geometry for Physicists (Academic Press, 1983)[***]. Includes homotopy, homology, fiber bundles and Morse theory, with applicationsto physics; somewhat concise..F.W.Warner,Foundations of Differentiable Manifolds and Lie Groups (Springer-Verlag, 1983)[***.The standard text in the field, includes basic topics such asmanifolds and tensor fields as well as more advanced subjects
vi • F. de Felice and C. Clarke, Relativity on Curved Manifolds (Cambridge, 1990) [***]. A mathematical approach, but with an excellent emphasis on physically measurable quantities. • S. Hawking and G. Ellis, The Large-Scale Structure of Space-Time (Cambridge, 1973) [***]. An advanced book which emphasizes global techniques and singularity theorems. • R. Sachs and H. Wu, General Relativity for Mathematicians (Springer-Verlag, 1977) [***]. Just what the title says, although the typically dry mathematics prose style is here enlivened by frequent opinionated asides about both physics and mathematics (and the state of the world). • B. Schutz, Geometrical Methods of Mathematical Physics (Cambridge, 1980) [**]. Another good book by Schutz, this one covering some mathematical points that are left out of the GR book (but at a very accessible level). Included are discussions of Lie derivatives, differential forms, and applications to physics other than GR. • V. Guillemin and A. Pollack, Differential Topology (Prentice-Hall, 1974) [**]. An entertaining survey of manifolds, topology, differential forms, and integration theory. • C. Nash and S. Sen, Topology and Geometry for Physicists (Academic Press, 1983) [***]. Includes homotopy, homology, fiber bundles and Morse theory, with applications to physics; somewhat concise. • F.W. Warner, Foundations of Differentiable Manifolds and Lie Groups (SpringerVerlag, 1983) [***]. The standard text in the field, includes basic topics such as manifolds and tensor fields as well as more advanced subjects
December1997LectureNotesonGeneralRelativitySean M. Carroll1Special Relativity and Flat SpacetimeWe will begin with a whirlwind tour of special relativity(SR)and life in flat spacetime.The point will be both to recall what SR is all about, and to introduce tensors and relatedconcepts that will be crucial later on, without the extra complications of curvature on topof everything else. Therefore, for this section we will always be working in fat spacetime,and furthermore we will only use orthonormal (Cartesian-like) coordinates. Needless to sayit is possible to do SR in any coordinate system you like, but it turns out that introducingthe necessary tools for doing so would take us halfway to curved spaces anyway, so we willputthatoff forawhile.It is often said that special relativity is a theory of 4-dimensional spacetime: three ofspace, one of time.But of course, the pre-SR world of Newtonian mechanics featured threespatial dimensions and a time parameter. Nevertheless, there was not much temptation toconsider these as different aspects of a single 4-dimensional spacetime.Why not?space at afixedtimex, y,zConsider a garden-variety 2-dimensional plane. It is typically convenient to label thepoints on such a plane by introducing coordinates, for example by defining orthogonal andy axes and projecting each point onto these axes in the usual way.However,it is clear thatmost of the interesting geometrical facts about the plane are independent of our choice ofcoordinates. As a simple example, we can consider the distance between two points, given1
December 1997 Lecture Notes on General Relativity Sean M. Carroll 1 Special Relativity and Flat Spacetime We will begin with a whirlwind tour of special relativity (SR) and life in flat spacetime. The point will be both to recall what SR is all about, and to introduce tensors and related concepts that will be crucial later on, without the extra complications of curvature on top of everything else. Therefore, for this section we will always be working in flat spacetime, and furthermore we will only use orthonormal (Cartesian-like) coordinates. Needless to say it is possible to do SR in any coordinate system you like, but it turns out that introducing the necessary tools for doing so would take us halfway to curved spaces anyway, so we will put that off for a while. It is often said that special relativity is a theory of 4-dimensional spacetime: three of space, one of time. But of course, the pre-SR world of Newtonian mechanics featured three spatial dimensions and a time parameter. Nevertheless, there was not much temptation to consider these as different aspects of a single 4-dimensional spacetime. Why not? space at a fixed time t x, y, z Consider a garden-variety 2-dimensional plane. It is typically convenient to label the points on such a plane by introducing coordinates, for example by defining orthogonal x and y axes and projecting each point onto these axes in the usual way. However, it is clear that most of the interesting geometrical facts about the plane are independent of our choice of coordinates. As a simple example, we can consider the distance between two points, given 1
21 SPECIALRELATIVITYANDFLATSPACETIMEby2 = (Ar)2 + (Ay)?.(1.1)In a different Cartesian coordinate system, defined by r' and y' axes which are rotated withrespect to the originals, the formula for the distance is unaltered:s2= (△r)2 +(△y)?.(1.2)We therefore say that the distance is invariant under such changes of coordinates.1yAAsAyAyAxAxThis is why it is useful to think of the plane as 2-dimensional: although we use two distinctnumbers to label each point, the numbers are not the essence of the geometry, since we canrotate axes into each other while leaving distances and so forth unchanged.In Newtonianphysics this is not the case with space and time; there is no useful notion of rotating spaceand time into each other. Rather, the notion of "all of space at a single moment in time'has a meaning independent of coordinates.Such is not the case in SR. Let us consider coordinates (t, r, y, z) on spacetime, set up inthefollowing way. The spatial coordinates (,y,z)comprise a standard Cartesian system.constructed for example by welding together rigid rods which meet at right angles. The rodsmustbemovingfreely,unaccelerated.Thetimecoordinateisdefinedbyasetofclockswhicharenotmovingwithrespecttothespatialcoordinates.(Sincethisisathoughtexperimentwe imagine that the rods are infinitely long and there is one clock at every point in space.)The clocks are synchronized in the following sense:if you travel from one point in space toany other in a straight line at constant speed, the time difference between the clocks at the
1 SPECIAL RELATIVITY AND FLAT SPACETIME 2 by s 2 = (∆x) 2 + (∆y) 2 . (1.1) In a different Cartesian coordinate system, defined by x ′ and y ′ axes which are rotated with respect to the originals, the formula for the distance is unaltered: s 2 = (∆x ′ ) 2 + (∆y ′ ) 2 . (1.2) We therefore say that the distance is invariant under such changes of coordinates. ∆ ∆ ∆ y x’ x y y’ x x’ s y’ ∆ ∆ This is why it is useful to think of the plane as 2-dimensional: although we use two distinct numbers to label each point, the numbers are not the essence of the geometry, since we can rotate axes into each other while leaving distances and so forth unchanged. In Newtonian physics this is not the case with space and time; there is no useful notion of rotating space and time into each other. Rather, the notion of “all of space at a single moment in time” has a meaning independent of coordinates. Such is not the case in SR. Let us consider coordinates (t, x, y, z) on spacetime, set up in the following way. The spatial coordinates (x, y, z) comprise a standard Cartesian system, constructed for example by welding together rigid rods which meet at right angles. The rods must be moving freely, unaccelerated. The time coordinate is defined by a set of clocks which are not moving with respect to the spatial coordinates. (Since this is a thought experiment, we imagine that the rods are infinitely long and there is one clock at every point in space.) The clocks are synchronized in the following sense: if you travel from one point in space to any other in a straight line at constant speed, the time difference between the clocks at the
31 SPECIALRELATIVITYANDFLATSPACETIMEends of your journey is the same as if you had made the same trip, at the same speed, in theother direction. The coordinate system thus constructed is an inertial frame.An event is defined as a single moment in space and time, characterized uniquely by(t, r,y, z). Then, without any motivation for the moment, let us introduce the spacetimeinterval betweentwo events:2 = -(c△t)? + (△r)? + (△y)? +(△z)?(1.3)(Notice that it can be positive, negative, or zero even for two nonidentical points.) Here, cis some fixed conversion factor between space and time; that is, a fixed velocity. Of courseit will turn out to be the speed of light; the important thing, however, is not that photonshappen to travel at that speed, but that there exists a c such that the spacetime intervalis invariant under changes of coordinates. In other words, if we set up a new inertial frame(t',r',y',z) by repeating our earlier procedure, but allowing for an offset in initial position,angle, and velocity between the new rods and the old, the interval is unchanged:s? = -(cAt)? + (Ar')? + (Ay)? + (Az).(1.4)This is why it makes sense to think of sR as a theory of 4-dimensional spacetime, knownas Minkowski space. (This is a special case of a 4-dimensional manifold, which we willdeal with in detail later.) As we shall see, the coordinate transformations which we haveimplicitly defined do, in a sense, rotate space and time into each other. There is no absolutenotion of "simultaneous events"; whether two things occur at the same time depends on thecoordinates used. Therefore the division of Minkowski space into space and time is a choicewe make for our own purposes, not something intrinsic to the situation.Almost all of the“paradoxes"associated with SR result from a stubborn persistence ofthe Newtonian notions of a unique time coordinate and the existence of “space at a singlemoment in time." By thinking in terms of spacetime rather than space and time together,theseparadoxes tend todisappear.Let's introduce some convenient notation. Coordinates on spacetime will be denoted byletterswithGreek superscript indices runningfrom0 to3,with0generallydenoting thetime coordinate. Thus,ro = ctl =a(1.5)rh :r?=y23=2(Don't start thinking of the superscripts as exponents.) Furthermore, for the sake of sim-plicity we will choose units in which(1.6)c=1;
1 SPECIAL RELATIVITY AND FLAT SPACETIME 3 ends of your journey is the same as if you had made the same trip, at the same speed, in the other direction. The coordinate system thus constructed is an inertial frame. An event is defined as a single moment in space and time, characterized uniquely by (t, x, y, z). Then, without any motivation for the moment, let us introduce the spacetime interval between two events: s 2 = −(c∆t) 2 + (∆x) 2 + (∆y) 2 + (∆z) 2 . (1.3) (Notice that it can be positive, negative, or zero even for two nonidentical points.) Here, c is some fixed conversion factor between space and time; that is, a fixed velocity. Of course it will turn out to be the speed of light; the important thing, however, is not that photons happen to travel at that speed, but that there exists a c such that the spacetime interval is invariant under changes of coordinates. In other words, if we set up a new inertial frame (t ′ , x′ , y′ , z′ ) by repeating our earlier procedure, but allowing for an offset in initial position, angle, and velocity between the new rods and the old, the interval is unchanged: s 2 = −(c∆t ′ ) 2 + (∆x ′ ) 2 + (∆y ′ ) 2 + (∆z ′ ) 2 . (1.4) This is why it makes sense to think of SR as a theory of 4-dimensional spacetime, known as Minkowski space. (This is a special case of a 4-dimensional manifold, which we will deal with in detail later.) As we shall see, the coordinate transformations which we have implicitly defined do, in a sense, rotate space and time into each other. There is no absolute notion of “simultaneous events”; whether two things occur at the same time depends on the coordinates used. Therefore the division of Minkowski space into space and time is a choice we make for our own purposes, not something intrinsic to the situation. Almost all of the “paradoxes” associated with SR result from a stubborn persistence of the Newtonian notions of a unique time coordinate and the existence of “space at a single moment in time.” By thinking in terms of spacetime rather than space and time together, these paradoxes tend to disappear. Let’s introduce some convenient notation. Coordinates on spacetime will be denoted by letters with Greek superscript indices running from 0 to 3, with 0 generally denoting the time coordinate. Thus, x µ : x 0 = ct x 1 = x x 2 = y x 3 = z (1.5) (Don’t start thinking of the superscripts as exponents.) Furthermore, for the sake of simplicity we will choose units in which c = 1 ; (1.6)