String theory is a theory of quantum gravity, and is a broad and varied subject that attempts to address a number of deep questions of fundamental physics. It is thus very complictaed. Thankfully, this cache has absolutely nothing to do with it :)
This cache has a lot more to do with the generally more simple string, the one we use to tie things with. Although after figuring the puzzle out, you may think you've been studying string theory all along, because this one could make you want to tear your hair out. As far as these types of puzzles go, this one is quite simple, but may be frustrating.
Double double toil and trouble, this is infact a string puzzle
This forms part of the Puzzle Containers Series, the others can be found here:
Puzzle Containers 1 - Bottlelock = GC67T4P
Puzzle Containers 2 - The Ultimate Renaissance Man = GC67WVH
Puzzle Containers 3 - Spin the Box = GC67KXR
Puzzle Containers 4 - Electricity Rebooted = GC6A98V
Puzzle Containers 6 - Nursery Rhymes = GC69XG7
Puzzle Containers 7 - Big In Japan? = GC6BPKQ
Puzzle Containers 8 - Balancing Act = GC6E22F
Puzzle Containers 9 - Deep Breath = GC6FR68
Puzzle Containers 10 - The Key is the Key = GC6JNEZ
Puzzle Containers 11 - Music Boxes = GC6MK81
Puzzle Containers 12 - Pipe = GC6W4C5
Puzzle Containers 13 - Dexterity = GC70N4E
Puzzle Containers 14 - Choo Choo = GC74NGV