Einstein discovered the relativity of simultaneity and, in his special theory of relativity, unified:
space and time;
mass and energy;
electricity and magnetism.
Newton: space, time and particles; Faraday and Maxwell: space, time, fields and particles; Einstein, 1905: spacetime, fields and particles;
Einstein, 1915: fields and particles; Dirac (and see here): fields are particles;
quantum mechanics: spacetime and quantum fields;
quantum gravity: covariant quantum fields.
Einstein's general theory of relativity identifies space-time with the gravitational field.
Carlo Rovelli, Reality Is Not What it Seems (Allen Lane, 2016), pp. 51-107.