Yukawa predicted a meson as the particle which holds the nucleus together. There is obviously a strong force in the nucleus, as there is a strong electric force, and an easy way to visualize the electric force is to say that a photon is emitted by one charged particle and absorbed by another, and so Yukawa said some other particle must be emitted to make the much stronger nuclear force. 'And I know', said Yukawa, 'that the nuclear force is short range, and I can easily do that by giving mass to that particle'. So he postulated a particle of intermediate mass which nowadays is called a meson, to transmit the nuclear force. The trouble was, and that trouble only became gradually evident, that the meson you saw in cosmic ray didn't have the properties that Yukawa had predicted - but I think that's anticipating. But I was terribly interested.

[Q] Then you actually work on trying to unravel or decipher all the meaning of a...

I worked on cosmic ray showers and I worked on neutrons and cosmic rays with Placzek.

[Q] And you worked on Yukawa Theory.

And I worked on Yukawa theory trying to apply it to the deuteron, and it worked very nicely for the deuteron and it was able to predict that the forces between neutron and proton are not purely central forces, and you needed that in order to explain that the deuteron has an electric quadrupole moment. That quadrupole moment had been observed at Columbia by Rabi's collaborators, and so the Yukawa theory gave possibility of explaining why there should be non-central forces between nucleons which would explain the quadrupole in the deuteron.