 It neglects many-body interactions and retardation.A more rigorous approach accounting for these effects, called the "macroscopic theory" was developed by Lifshitz in 1956.Langbein derived a much more cumbersome "exact" expression in 1970 for spherical bodies within the framework of the Lifshitz theory while a simpler macroscopic model approximation had been made by Derjaguin as early as 1934.