41. From the standpoint of physical measurement, a more natural notion of distance between two world lines is the " radar distance ". 42. One usually takes the proper time of an object or an observer as the curve parameter \ tau along the world line . 43. It does this by sending a signal along the world line of a person and detecting the echo from the far end. 44. I was wondering, has anyone followed all the world lines and checked whether they make sense, according to the rules of the film? 45. For six decades, travelers from around the world lined up at the United States passport office in the International Building at Rockefeller Center. 46. Following the same argument the range below the origin and between the photon world lines is the absolute past relative to the origin. 47. Men and women from around the world lined up for hours Monday to pass by the rebbe's walled-in grave in a Queens cemetery. 48. The globe was divided between America and the Soviet Union, with most of the world lined up in predictable, if occasionally shifting, alliances. 49. By integration of the equations of motion one obtains the world lines of accelerated bodies corresponding to a sequence of momentary inertial frames. 50. So far a world line ( and the concept of tangent vectors ) has been described without a means of quantifying the interval between events.
