The problem had already been solved by Pontecorvo in 1968. In 1959, a powerful accelerator (that was never built) was being designed, and he began considering experiments that could be performed with it. He contemplated a project investigating muons. Julian Schwinger had hypothesised that particles experience the weak interaction through exchanging W bosons. The W boson would not be discovered until 1983, but a problem immediately surfaced. Gerald Feinberg pointed out that this implied that some interactions that had never been observed should occur, but conceded that this was only true if the neutrinos associated with electrons were the same as those associated with muons.