Poets are the unacknowledged legislators of the world. — Shelley
Definite optimism as human capital - Dan Wang
Innovation Starvation - Neal Stephenson
Luck and Definite Optimism - Peter Thiel
Arthur C. Clarke: Project Spaceguard (Rendezvous with Rama), space elevator (Fountains of Paradise), general space infrastructure and normalisation of progress in spaceflight; counterpoint that HAL in 2001 is the original "bad AI"
Not just the 50s-60s:
Advertising and mass-media that incorporated specific positive visions of technical progress; World's Fair showcases as examples
Britain during Industrial Revolution had a vibe of definite optimism: "Recollect farther, that every natural and every artificial advantage is susceptible of gradual progression, and trace the yearly elevation to higher perfection. New societies for improvement …, new machines to advance our arts and facilitate labour; waste lands enclosed, roads improved, bridges erected, canals cut, tunnels excavated, marshes drained and cultivated, docks formed, ports enlarged: these and a thousand kindred operations which present themselves spontaneously to the mind’s eye, prove that we have not yet attained our zenith, and open an exquisite prospect of future stability and greatness."