In 1948, the Dutch physicist Hendrik Casimir theorized that an attraction would arise when two metal plates are brought close together in a vacuum. In essence, charge fluctuations on one plate induce fluctuations on the other, and the interaction of the two produces an attractive force. Soon after, he realized that as the distance between the plates grew larger, the increasing time needed for electromagnetic influences to flit back and forth between the plates would make the attraction weaker than he had first supposed. Then in 1956, the Russian physicist Evgeny Lifshitz worked out a more general treatment of the problem for materials other than metals, separated by some fluid rather than a vacuum.