““Positional, or potential energy, is the energy we add to a rock when we lift it from the floor and put it on a table. Except for its mass energy, the total energy of any object is the sum of its motion energy and its positional energy. For a quantum mechanical system that attractive forces keep from moving all over space, this total energy cannot vanish; but there is a minimum permissible energy. This is what we call zero-point energy, the energy of the quantum mechanical ground state. This fixed ground state energy means that the smaller the space to which we confine a particle, the higher the velocities it attains and the larger its motion energy. Its positional energy is small, because it cannot move far away from the point with positional energy zero. What we call zero-point energy is the smallest energy to which quantum mechanics permits motion energy and positional energy to add up.””
