The nuclear struggle between the sticky strong force and the repellent electrical force ultimately depends on the size of the nucleus. For small nuclei, the strong force wins out easily, and adding more protons and neutrons generally just makes it stronger. But the strong force can only act over very short distances, comparable to the size of a proton itself—anything much larger than a trillionth of a millimeter (a distance known as one fermi, after Enrico) is too much for it. After a certain point, the nucleus gets too big, the electric force starts to win the tug-of-war, and nuclei become
...more
