The theory for which Nordström was arguably most famous in his own lifetime, his theory of gravitation, was for a long time considered as a competitor to Einstein's theory of general relativity which was published in 1915, after Nordström's theory. In 1914 Nordström introduced an additional space dimension to his theory, which provided coupling to electromagnetism. This was the first of the extra dimensional theories, which later came to be known as Kaluza–Klein theory. Kaluza and Klein, whose names are commonly used today for the theory, did not publish their work until the 1920s. Some speculations as to why Nordström's contribution fell into oblivion are that his theory was partly published in Swedish and that Einstein in a later publication referenced to Kaluza alone. Today extra dimensions and theories thereof are widely researched, debated and even looked for experimentally.