The unique force/distance profile of the strong force has no classical equivalent and is difficult to model outside the realm of quantum mechanics. Its range is limited, on the order of the diameter of a proton, so its effects are undetectable at the human scale. Nevertheless, while the electromagnetic force and gravity decrease with the inverse square of the distance, the strong force actually increases with distance in a near-linear fashion, as shown in the initial 0.2 fm of Figure (a). Quarks cannot escape the nucleus because the farther the displacement the more tightly the strong force pulls them back. This phenomenon is called quark confinement, and Musulmanbekov's conception of the force/distance relationship is shown in Figure . Like the Coulomb barrier, confinement is a quantum mechanical phenomenon. Nothing like it exists in the classical domain. And like the Coulomb barrier, confinement forces may be modeled with an appropriately configured sequence of alternating and unequal charges, as shown in Figure .