In particular, one may ask how the zero-point energy depends on the shape s of the cavity. Each energy level E n depends on the shape, and so one should write E n (s) for the energy level, and ⟨ E (s)⟩ for the vacuum expectation value. At this point comes an important observation: The force at point p on the wall of the cavity is equal to the change in the vacuum energy if the shape s of the wall is perturbed a little bit, say by δs, at p. That is, one has