In string theory, if all elementary particles have their properties due to the specific vibrations of identical strings, what "force" keeps this vibration from changing and therefore changing the elementary particle itself? I know it's only meant as a metaphor but the usual example given is that of an instrument. When I visualize this I think a string is only vibrating based on an exterior force (the pluck let's say), and it eventually slows vibrating or changes pitch. Wouldn't this be changing the particles themselves?