It appeared that the law of conservation of energy did not hold for beta decay, something Meitner regarded as unacceptable. In 1930, Wolfgang Pauli wrote an open letter to Meitner and Hans Geiger in which he proposed that the continuous spectrum was caused by the emission of a second particle during beta decay, one that had no electric charge and little or no rest mass. The idea was taken up by Enrico Fermi in his 1934 theory of beta decay, and he gave the name " neutrino " to the hypothetical neutral particle. At the time there was scant hope of detecting neutrinos, but in 1956 Clyde Cowan and Frederick Reines did just that.