Einstein was a full professor at the German University in Prague in 1911 when he was asked to write, for inclusion in a book, a treatise, “On the special theory of relativity.” Because of the First World War, the book’s publication was delayed and his contribution was ultimately withdrawn. The handwritten manuscript, which began in 1912, still exists. Section 14, “The inertia of energy,” contains another thought experiment. It imagined a “rectangular parallelepiped-shaped plate” floating freely in space that simultaneously radiates two identical electromagnetic wave trains in opposite directions.