Explore the cutting-edge intersection of high-energy string theory, celestial holography, and tensionless strings in this comprehensive 79-minute lecture delivered at the Galileo Galilei Institute. Delve into advanced theoretical physics concepts as Stephan Stieberger examines the fundamental connections between string theory at high energies and the emerging framework of celestial holography, while investigating the unique properties and implications of tensionless string configurations. Gain insights into how these interconnected areas of theoretical physics contribute to our understanding of quantum gravity, spacetime structure, and the holographic principle, with particular emphasis on celestial amplitudes and their role in modern string theory research.