How is Einstein's concept of Individual time different from a second dimension of time? I am thinking of the example of the twins; one twin goes into space at a velocity approaching the speed of light and the other remains on Earth. When the traveler returns he is actually younger than his twin depending on his speed and distance. If I were to graph the timeline from point A (departure) to point B (return) for both twins, the traveller's timeline would be shorter. If I were graphing these as two separate line segments it suggests that there would be points on one line that would not exist on the traveller's line. Why doesn't this suggest a plane, or two dimensional view of time? Please forgive me if this seems too elementary a question, but I am not a scientist or even a science student - I am just a curious layperson.