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.