Max Karl Ernst Ludwig Planck (April 23, 1858 – October 4, 1947) was a German physicist. He is considered to be the founder of quantum theory, and therefore one of the most important physicists of the twentieth century. In 1894 Planck turned his attention to the problem of black-body radiation. By interpolating between Wien's law[disambiguation needed] and the Rayleigh–Jeans law, Planck found the famous Planck black-body radiation law, which described the experimentally observed black-body spectrum very well. The discovery of Planck's constant enabled him to define a new universal set of physical units (such as the Planck length and the Planck mass), all based on fundamental physical constants.