If you have a guitar string and you pluck it nothing happens. Not unless it is tensioned by pulling both ends tightly. In string and brane theory do the strings have something that keeps them tensioned? Or don't they need tension to maintain vibrating; unlike normal strings? Are they free floating vibrations or tensioned vibrations? Sorry, I think I put this in the wrong area (I'm not a science researcher); can it please be moved to general physics?