This explains why moving along the geodesics in spacetime is considered inertial. Using the geodesic equation, the motion of masses falling along the geodesics can be calculated. In special relativity, parallel geodesics remain parallel. In other geometries, length may be measured along possibly curved paths, called geodesics. This definition makes minimal surfaces a 2-dimensional analogue to geodesics. The differential geometry of surfaces revolves around the study of geodesics. The Cauchy horizon is generated by closed null geodesics. Its most important application is in the theory of ellipsoid geodesics, (Vincenty, Karney). Morse's lemma on stability of geodesics. The space between objects shrinks or grows as the various geodesics converge or diverge. Torsion is particularly useful in the study of the geometry of geodesics. In general, such shortest-distance paths are called "geodesics". Note that the square root will be imaginary for tachyonic geodesics. Such boundaries are generated by null geodesics. These are the hexagonal geodesics described above.