Complex sentence: The idea that relativistic theory could be usefully extended with the introduction of extra dimensions originated with Nordstöm's 1914 modification of his previous 1912 and 1913 theories of gravitation, which added an additional dimension resulting in a 5-dimensional vector theory.
Split into simpler sentences: The idea that relativistic theory could be usefully extended with the introduction of extra dimensions originated with Nordstöm's 1914 modification of his previous 1912 and 1913 theories of gravitation. In this modification, he added an additional dimension resulting in a 5-dimensional vector theory.