In 1900, Lord Kelvin summarized the problems in classical physics that had arisen in the nineteenth century. He concluded that there are two clouds posing threats to its solid foundation. One is the existence of ether, and the other is related to the equipartition theorem. The Michelson-Morley experiment confirmed that the speed of light is constant. The existence of ether is contradictory to the constant speed. The solution to the first cloud resulted in the discovery of the theory of special relativity. The second cloud originated from the inconsistency between the Black body observation and the theoretical prediction. The solution to this inconsistency is the theory of quantum mechanics. Readers interested in a systematic, detailed introduction are referred to the book by Strange (Relativistic Quantum Mechanics, 1st ed., 1998) or the book by Martin for computational applications (Electronic Structure, 1st ed., 2004).