Then in 1969 during his time at University of Wisconsin-Madison, Virasoro successfully generalized Veneziano's theory and discovered a formula (the Virasoro-Shapiro amplitude) which described the scattering of closed strings.
During his time at the University of Wisconsin-Madison in 1969, Virasoro built upon Veneziano's theory and discovered a formula (Virasoro-Shapiro amplitude) that described how closed strings interact with each other.