Einstein argued that the speed of light was a constant in all inertial reference frames and that electromagnetic laws should remain valid independent of reference frame – assertions which rendered the ether "superfluous" to physical theory, and that held that observations of time and length varied relative to how the observer was moving with respect to the object being measured (what came to be called the " special theory of relativity "). It also followed that mass and energy were interchangeable quantities according to the equation E = mc. In another paper published the same year, Einstein asserted that electromagnetic radiation was transmitted in discrete quantities (" quanta "), according to a constant that the theoretical physicist Max Planck had posited in 1900 to arrive at an accurate theory for the distribution of blackbody radiation – an assumption that explained the strange properties of the photoelectric effect.