What theory of relativity did Albert Einstein publish in 1905?

The theory of relativity Albert Einstein published in 1905 was special relativity. Einstein presented it in his paper “On the Electrodynamics of Moving Bodies,” published in the journal Annalen der Physik in September 1905.

Special relativity applies to objects moving at constant velocity and rests on two principles: the laws of physics are the same in all inertial reference frames, and the speed of light in a vacuum is constant for all observers. These ideas overturned the older assumption that time and space were absolute.

The theory leads to effects such as time dilation, length contraction, and the relativity of simultaneity. It also established the relationship between mass and energy expressed by E = mc², although Einstein discussed that result in a separate 1905 paper later the same year.

A common mix-up is general relativity, which Einstein published in completed form in 1915. General relativity extends relativity to accelerating frames and explains gravity as the curvature of spacetime. String theory and quantum theory are different frameworks, not the answer to this question.