Likewise in 1905, Einstein developed a theory of Brownian motion in terms of fluctuations in the number of molecular collisions with an object,[4] providing further evidence that matter was composed of atoms. A few weeks earlier, he had derived the Einstein relation for diffusion, which was the first example of the general fluctuation-dissipation theorem and allowed a good estimate of the Avogadro constant.[5] Additionally In 1905, Einstein developed the theory of special relativity, which reconciled the relativity of motion with the observed constancy of the speed of light (a paradox of 19th-century physics).[6] Special relativity is now a core principle of physics. Its counterintuitive predictions that moving clocks run more slowly, that moving objects are shortened in their direction of motion, and that the order of events is not absolute have been confirmed experimentally.