Tides on earth are described with newton's theory of gravitation. Relativistic effects on tides theoretically become measurable on very strong gravitational fields, possibly becoming twice as strong as tides predicted by newtonian gravity: http://adsabs.harvard.edu/abs/1983ApJ...264..620N Tides are presumably outcomes of gravitational forces. Einstein ditched forces and the concept of inertia in GR (http://archive.org/stream/TheBornEinsteinLetters/Born-TheBornEinsteinLetters_djvu.txt). So how is GR used to calculate tidal forces? If different parts of the body travel different geodesics, this would cause the body to tear apart over time. How can tides be described with the geometry of space-time?