String Theory Extends Quantum Field Theory

Quantum field theory is right within its domain of validity (like saying 'this is a chair'—deeper truths may emerge but won't refute it), while string theory attempts to go beyond it by positing that an electron is a tiny loop of string rather than a point; remarkably, making this consistent quantum mechanically predicts the existence of gravity, so string theory is the leading theory of quantum gravity, though we have no idea yet whether it actually describes nature better than other extensions.