This chapter focuses on the very important developments in quantum theory that occurred during the interregnum. Paul Dirac’s development of a formulation for coupling quantum theory and relativity was remarkable, and a singular achievement. E. H. Kennard’s quantum force was similar to the quantum potential due to Madelung, which provided a correction to induce quantum effects, such as interference. The connection between the quantum probability and that of a classical statistical system may seem a bit too much to hope for. A. Einstein and Neils Bohr would face off once again at the 1930 Solvay conference. This time the topic was magnetism, which implied that there would be discussion about the role of quantum mechanics and electron spin, all of which contributed to magnetic phenomena. Dirac would publish his take on quantum mechanics in 1930. The quantum probability would transition into a classical certainty. This was an essential part of the philosophy of Bohr and the acolytes.