The original Lifshitz 1955 paper proposed this method relying on quantum field theory principles, and is, in essence, a generalization of the Casimir effect, from two parallel, flat, ideally conducting surfaces, to two surfaces of any material. Later papers by Langbein, Ninham, Parsegian and Van Kampen showed that the essential equations could be derived using much simpler theoretical techniques, an example of which is presented here.