Wolfgang Pauli once said (regarding the neutrino):

> I have done a terrible thing. I have postulated a particle that cannot be detected.

Why did he figure it couldn't be detected? Was this because he thought it was massless? According to Wikipedia, "neutrinos were long believed to be massless".

If so, why did they think it was massless? I thought the particle was hypothesised in order to maintain the conservation of momentum in a [beta decay][1]. If it was massless, this wouldn't have any effect, right?

  [1]: https://en.wikipedia.org/wiki/Beta_decay