the concept of world lines is used widely in theoretical physics, for it shows some interesting facts about high-speed motion. for example, the time dilation equation presented by albert einstein is algebraically undefined when an object's velocity is the speed of light, but using world lines one can find that when the speed is the speed of light, time will stop. although einstein's equation (for time dilation) does show that an object going faster than light goes backward in time, the same concept can be described using world lines.