World History

...practical implications, in my view, is the paper on the photoelectric effect, inasmuch as it established the physical reality of quantum physics as opposed to a mathematical approach, developed by Planck to explain away the "ultraviolet catastrophe" associated with the Rayleigh-Jeans law.

Although later in his career Einstein would develop some reservations about interpretation of quantum theory, in this sense he was a founder of quantum mechanics.

Special and general relativity would need to wait a few years to have practical import.