Max Planck, April 23, 1858 – October 4, 1947, is thought to be the father of the Quantum Theory. He determined that atoms can emit quanta, which is the plural of quantum. Quantum is the smallest quantity of radiant energy.
His work was useful to Einstein when he was investigating photons.
A fun but tragic fact regarding Planck's personal life is that one of his two sons was executed because he was a part of an unsuccessful attempt to assassinate Adolf Hitler during his rule of the Nazi Regime. This happened in 1944.