Quantum string (or just string) is a one-dimensional object operating at levels much, much smaller than subatomic particles. Strings can be open or closed, forming lines or loops respectively, and trace two-dimensional worldlines as they move.

For strings to work in the way that humans suspect, the universe would need to have eleven dimensions (according to M-theory), most of which would be tightly rolled up on themselves. It also might imply brane cosmology, where the universe was created by M-theory's predicted interactions.