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Zero-point energy is generally known to be unphysical. Casimir effect,however, is often presented as a counterexample, giving rise to a conceptualconfusion. To resolve the confusion we study foundational aspects of Casimireffect at a qualitative level, but also at a quantitative level within a simpletoy model with only 3 degrees of freedom. In particular, we point out thatCasimir vacuum is not a state without photons, and not a ground state for aHamiltonian that can describe Casimir force. Instead, Casimir vacuum can berelated to the photon vacuum by a non-trivial Bogoliubov transformation, and itis a ground state only for an effective Hamiltonian describing Casimir platesat a fixed distance. At the fundamental microscopic level, Casimir force isbest viewed as a manifestation of van der Waals forces.

Is zero-point energy physical? A toy model for Casimir-like effect