# String theory question

1. Sep 20, 2007

### ehrenfest

1. The problem statement, all variables and given/known data
For problem 6.5 I did a lot of manipulation and I get that the following holds at the endpoints of an open string:

$$c = |\frac{\partial{\vec{X}}}{\partial{t}}|$$

This probably seems like silly intuitive queston, but I cannot explain why this implies that the string endpoints move at the speed of light. Since X is a four-vector not a three-vector, why can we draw that conclusion?

2. Relevant equations

3. The attempt at a solution