Albert EINSTEIN is the answer to the crossword clue 'HE POSTULATED E = MC^2'. He was a German-born theoretical physicist who developed the theory of relativity, one of the two pillars of modern physics. His famous equation, E = MC^2, states that energy and mass are equivalent and can be converted from one to the other. This equation has become one of the most famous equations in science and is widely used in many fields, including nuclear power and particle physics.