Hideki Yukawa
What holds the nucleus together?
That time, that place
Protons sit crowded into a very small space inside the nucleus, and like charges repel — the more strongly the closer they are.
The nucleus should have flown apart long ago. Something far stronger must be holding it.
Why this question
The force has a strange property. Move even an atom's width away and it is not felt at all. It is powerful only at very short range and effectively absent beyond.
Gravity and electric force weaken with distance but reach out forever. How can a force simply stop at a certain range?
What was found
Yukawa took force to be carried by the exchange of particles — the idea that electromagnetism works by exchanging photons was already in play.
A photon has no mass and can travel indefinitely. If the exchanged particle has mass, the range becomes limited — the heavier it is, the shorter the reach.
So you can work backward: what mass gives a range about the size of a nucleus? The answer was around two hundred times the electron's — far lighter than a proton, far heavier than an electron, a mass no one had ever seen.
The weight of an unseen particle, computed on paper.
A force was simply pulling or pushing
A mass back-calculated from the condition that the force act only at nuclear range
Two years later a particle of similar mass turned up in cosmic rays, but with the wrong properties. The particle he predicted was confirmed only in 1947. In 1949 he became Japan's first Nobel laureate — four years after the war's end