49 Chapter 2: Motivation intercepts for the trajectories of the π and ρ mesons gave them unphysically large masses. Although there were attempts to resolve these issues and advances were made, with the advent of QCD and the Standard Model—and the convinc- ing experimental evidence for them—string theory rapidly went out of favor. Undaunted the remaining string practitioners abandoned it as a theory of hadrons and shot for the moon in 1974 by proposing it as a theory of every- thing. By doing so the serious flaws of string theory as a theory of hadrons became assets: the massless spin-2 particle is the graviton and an inevitable consequence of strings; the theory is free of the usual UV divergences associ- ated with point-particle quantum gravity; the massles spin-1 particles of open strings are Yang-Mills gauge bosons; and the extra dimension dynamics are set by the geometry of gravity. It is interesting to note that this promotion of string theory changed the string tension by 38 orders of magnitude, from α ′ ∼ 1 GeV − 2 to α ′ ∼ 1 /M 2 $_{p}$, where the Planck mass in 4 D is M$_{p}$ ≃ 1 . 22 × 10 18 GeV. The string theory story until the 1990’s is of less interest to the nuclear physicist: susy and sugra emerged in the late 1970’s; superstrings, anomaly cancellation in the early 1980’s; and the discovery of heterotic strings and the compactification on Calabi-Yau manifolds of the mid-80’s, the latter of which produced the first set of multiplets reminiscent of Standard Model particle classification. This second superstring revolution caused a frenzy of activity that continues today. While appreciably closer to unifying the known forces,