Soft factors and interaction vertices from light-cone actions

Abstract: Universal factors associated with the emission of a soft boson in gauge theories and gravity, formulated in the light-cone gauge, are presented. The inverse-soft method, for constructing higher-point amplitudes from lower-point ones, using these factors is reviewed. These ideas are then examined in (light-cone) superspace and applied to both the N=4 super Yang-Mills and N=8 supergravity theories. One highlight is a compact result for the quartic interaction vertex in N=8 supergravity, a crucial ingredient for finiteness analyses.