In 1973, Professor Gross, working with his first graduate student, Frank Wilczek, at Princeton University, discovered asymptotic freedom—the primary feature of non-Abelian gauge theories—led Gross and Wilczek to the formulation of quantum chromodynamics (QCD), the theory of the strong nuclear force. Asymptotic freedom is a phenomenon where the nuclear force weakens at short distances, which explains why experiments at very high energy can be understood as if nuclear particles are made of non-interacting quarks. The flip side of asymptotic freedom is that the force between quarks grows stronger as one tries to separate them. Therefore, the closer quarks are to each other, the less the strong interaction (or color charge) is between them; when quarks are in extreme proximity, the nuclear force between them is so weak that they behave almost as free particles. This is the reason why the nucleus of an atom can never be broken into its quark constituents.