Searching for a response: the intriguing mystery of Feynman’s theoretical reference amplifierVincenzo d’Alessandro, Santolo Daliento, Marco Di Mauro, Salvatore Esposito and Adele NaddeoEur. Phys. J. H, 44 4-5 (2019) 331-347DOI: https://doi.org/10.1140/epjh/e2019-90071-6