  What is the microscopic interaction responsible for high temperature
superconductivity (HTSC)? Here data on temporal relaxation of Tc and the room
temperature conductivity in YBa2Cu3O(6+x) after abrupt alteration by light
pulses or pressure changes are analyzed. The analysis proves, independently of
microscopic details, that only electron-phonon interactions can cause HTSC in
the cuprates; all other interactions are excluded by experiment.
