Corekeeper

Corekeeper holds on hard, but only up close. Step back a couple of femtometres and he lets go.

Inside a nucleus, protons repel one another electrically. In 1935 Hideki Yukawa proposed that nucleons hold each other by trading a particle, and saw that the mass of that particle would set the reach of the force. A briefly borrowed particle of mass m cannot travel far before it must be paid back, about ħ/mc. Make the reach the size of a nucleus and the carrier should weigh a couple of hundred electron masses. That is the first formula on this page: an attraction that dies away exponentially beyond the range λ.

The particle turned up in 1947 in photographic emulsions exposed to cosmic rays, and was named the pion. Cecil Powell received the 1950 Nobel Prize for the photographic method and its meson discoveries; Yukawa had received the 1949 prize for the prediction. Put the measured charged-pion mass, 139.57 MeV, into Yukawa's range and you get 1.41 femtometres, about the spacing of nucleons in a nucleus. Today this pull is understood as a residue of the strong force between quarks, much as the forces between neutral molecules are residues of the electric forces inside them.

The second formula measures how tightly he holds. Weigh a helium-4 nucleus and it comes up short. Two protons and two neutrons, weighed separately, total 28.3 MeV more than the nucleus they form: about 0.75 percent of the mass, missing. That missing mass is the binding energy, released when the nucleus was made, and the energy needed to pull it apart. Building helium from hydrogen releases energy on this scale, which is why it powers the Sun.

Corekeeper does not chase anyone. His grip is fierce at one femtometre and nearly gone at five.