For a time in the 1960s, quantum field theory was in doubt and the study of this object was dominant, as led by Chew and Gell-Mann.
It is thought that in quantum gravity in asymptotically flat spacetime, it is the only gauge-invariant observable.  For a time in the 1960s, quantum field theory was in doubt and the study of this object was dominant, as led by Chew and Gell-Mann. Chew's "bootstrap" was intended to provide a foundation for the study of hadron physics in terms of analyticity and unitarity properties of it.  It provides the probabilities for transitions between different well-defined asymptotic particle states, and as such is particularly useful for obtaining predictions for collider experiments.  FTP, what is this operator whose name reflects its derivation from "scattering"?
It is thought that in quantum gravity in asymptotically flat spacetime, it is the only gauge-invariant observable.  
S-matrix