It is well known that Einstein never accepted the probabilistic formulation of quantum mechanics. In a vigorous discussion with Niels Bohr, he often used photons to disprove the quantum presentation. At the fifth Solvay Conference (Fig. 6.1), in 1927, Bohr and Einstein discussed the new quantum mechanics. In the morning, Einstein came to Bohr with some gedanken (thought) experiment that disproved the theory, and at dinner, Bohr solved the problem. In some cases, photons were the protagonists.