string endpoints move at light speed--? In Zweibach's introduction to string theory I encounter the assertion that the endpoints of an open string move at the speed of light. A few other sources on the web agree with this. But nobody bothers to tell me what the heck it means (not even in https://www.physicsforums.com/archive/index.php/t-49119.html , where it is stated that only the endpoints move at light speed, not the rest of it). Where are the endpoints going so fast? Are they just vibrating in a tiny space? Are the endpoints massless, so as not to violate relativity? What am I missing?