using classical supergravity. But what makes the conjecture so tantalizing also makes it so enigmatic: no proof yet exists, and the different regimes of tractable applicability mean one may never be found. 

## 2.3.1 History of String Theory 

There’s a certain irony that string theory is again purported to describe the strong nuclear force. Created in the 1960’s as a phenomenological model to explain the mass spectrum of hadrons, string theory originally had some suc- cesses. A number of consequences unseen in Nature falsified string theory as a theory of hadrons. Undiscouraged, the practitioners audaciously proposed string theory as the theory of everything. Having possibly lost its usefulness as a theory in the landscape, the theory is back as a tool to analytically cal- culate, appropriately, properties of strongly coupled QCD. Surprisingly, string theory has rather humble roots (for a very nice historical overview of string theory developments until 1985 from one of its founders, see Schwarz [313] and references therein). In the early 1960’s there was a successful quantum field theory of electrodynamics, but Geoffrey Chew argued that a weak-coupling field theory approach was inappropriate for the strong force. Even worse, no hadron appeared more fundamental than another: there was a paralyzing nu- clear democracy (while Gell-Mann had already published his Eightfold Way quarks were considered merely a mathematical construct; the pivotal Ruther- fordian Slac measurements had not yet been made). 

The order of the day was to focus on physical quantities, especially the S 

