On June 30, 1905 Albert Einstein published his paper on Special Relativity with the paper "On the Electrodynamics of Moving Bodies" in the journal Annalen der Physik (original German version in pdf here).
This was Einstein's third of what have become known as the Annus Mirabilis papers (Latin for "extraordinary year") and revolutionized the field of physics by reconciling Maxwell's equations for electricity and magnetism with the laws of mechanics. He was 26 years old. In this paper Einstein also dispelled with the concept of "luminiferous ether" (proposed by Isaac Newton in 1704), a hypothetical medium that light waves were thought to travel through in the same way that sound waves travel through air or water. His first paper, on the photoelectric effect, earned him the Nobel Prize in physics.
From the introduction:
We will raise this conjecture (the purport of which will hereafter be called the "Principle of Relativity'') to the status of a postulate, and also introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body. These two postulates suffice for the attainment of a simple and consistent theory of the electrodynamics of moving bodies based on Maxwell's theory for stationary bodies. The introduction of a "luminiferous ether'' will prove to be superfluous inasmuch as the view here to be developed will not require an "absolutely stationary space'' provided with special properties, nor assign a velocity-vector to a point of the empty space in which electromagnetic processes take place.