The "second great unification" was James Clerk Maxwell 's 19th century unification of electromagnetism. It brought together the understandings of the observable phenomena of magnetism, electricity and light (and more broadly, the spectrum of electromagnetic radiation). This was followed in the 20th century by Albert Einstein 's unification of space and time, and of mass and energy through his theory of special relativity. Later, Paul Dirac developed quantum field theory, unifying quantum mechanics and special relativity.