21. A four-velocity is thus the normalized future-directed timelike tangent vector to a world line , and is a contravariant vector. 22. In the short story " Life-Line ", author Robert A . Heinlein describes the world line of a person: 23. Two world lines that start out separately and then intersect, signify a " collision " or " encounter ". 24. The slope of the world line ( deviation from being vertical ) gives the relative velocity to the observer. 25. The basic mathematics is as follows : The theory of special relativity puts some constraints on possible world lines . 26. Analogously, the world lines of test particles in free fall are spacetime geodesics, the straightest possible lines in spacetime. 27. Gamow was also writing " My World Line : An Informal Autobiography ", which was published posthumously in 1970. 28. Importantly, the world line of a particle free from all external, non-gravitational force, is a particular type of geodesic. 29. In curved spacetime, the " shortest " world line is that geodesic that minimizes the curvature along the geodesic. 30. The orbit of the Earth is an ellipse in space alone, but its world line is a helix in spacetime.
