Searching for a response: the intriguing mystery of Feynman’s theoretical reference amplifier Vincenzo d’Alessandro, Santolo Daliento, Marco Di Mauro, Salvatore Esposito and Adele Naddeo Eur. Phys. J. H, 44 4-5 (2019) 331-347 Published online: 06 August 2019 DOI: 10.1140/epjh/e2019-90071-6