Most of the strong force glues quarks into protons and neutrons. But a fraction of this force leaks out, binding protons and neutrons into atomic nuclei (see illustration). Without this force, nuclei would fly apart.

The force that binds is delicately balanced by a force that repels. A nucleus — less than one trillionth the size of an atom — is a tightly packed space. When protons and neutrons become even more tightly packed, the force reverses and they repel each other. If this didn't happen, nuclei would collapse.

That nuclei exist at all is one of nature's most remarkable miracles. The Jefferson Lab upgrade can help us unlock the secrets of this balancing act.