In a letter of 4 December to Lise Meitner et al., beginning, "Dear radioactive ladies and gentlemen", he proposed the existence of a hitherto unobserved neutral particle with a small mass, no greater than 1% the mass of a proton, in order to explain the continuous spectrum of beta decay.
U pismu datiranom 4. prosinca, koje je uputio Lisi Meitner, a šaljivo započeo "Dragi radioaktivni dame i gospodo", predložio je postojanje dotad neotkrivene neutralne čestice iznimno male mase, ne veće od 1 % mase protona, kako bi objasnio kontinuirani spektar β-raspada.