Here is a question that requires you to have a copy of the book (A First Course in GR) to answer it. On page 226, near the bottom of the page is this sentence: The second assumption is called the slow-motion assumption, since it implies that the typical velocity inside the source region, WHICH is [itex]\Omega[/itex] times the size of THAT REGION, should be much less than 1. Emphasis mine. Can someone tell me what THAT REGION is? If you tell me that THAT REGION is the source region, then I will ask what WHICH refers to.