Albert Einstein published his special theory of relativity in 1905. The theory first appeared in his paper “On the Electrodynamics of Moving Bodies,” published in the journal Annalen der Physik.
The paper was one of four major scientific works Einstein produced during his famous annus mirabilis, or “miracle year.” Special relativity rested on two central ideas: the laws of physics are the same in all inertial frames, and the speed of light in a vacuum is constant for all inertial observers.
Those principles led to results that seemed surprising from a Newtonian perspective, including time dilation, length contraction, and the relativity of simultaneity. Later in 1905, Einstein also published a separate paper deriving the relationship between mass and energy associated with the famous equation E = mc².
The year 1915 is a common distractor because Einstein completed the field equations of general relativity then. General relativity is the theory that extends relativity to gravity; its final published formulation appeared in 1916. Thus, 1905 belongs to special relativity, while 1915–1916 belongs to general relativity.