I've started to study superstring now and I have a little doubt. If you consider the open string with free ends, Polchinski's book says that the ends move with the speed of light. But if the ends move this fast, as the length of the string is fixed the rest of it must have the same velocity. If it depends only on the boundary conditions, massive states of the string will have the same velocity... It should not be this way, what am I doing wrong?