Then in §6.2, he expressed this in continuous variables,

I mentioned my results to Niels Bohr, during a walk. That is nice, he said, that is something new. I told him that I was puzzled by the extremely simple form of the expressions for the interaction at very large distances and he mumbled something about zero-point energy. That was all, but it put me on a new track. —H. B. G. Casimir [1] The Casimir Force was predicted in 1948 by Dutch physicist Hendrick Casimir. Casimir realized that when calculating the energy between two parallel uncharged conducting plates, only those virtual photons whose wavelengths fit an integral number of times into the gap should be counted. Each mode contributes to a pressure on the plates, and the infinite number of modes outside the plates is in some sense greater than the infinite number inside the plates, resulting in a small force drawing the plates together. This experimentally-confirmed force is one observable consequence of the existence of the vacuum electromagnetic field. Theoretical Background First, let us review the tools we have developed through Loudon to analyze the quantum vacuum. In §4.4, when he quantized the electromagnetic field, Loudon expressed the radiation Hamiltonian a