(Lecture: 10 minutes)

See§13.2 of the text.

Some students may not yet have seen an explanation of the tides; a simple analysis reveals both that there are two high and low tides per day. In general relativity, this effect is due to geodesic deviation, which cases “straight” lines to approach each other — just as lines of longitude do. This is in fact the modern view of gravity, as seen through general relativity: Gravity curves space, and curvature causes geodesic deviation. It's all about tidal effects!