Photograph of Albert Einstein (1879-1955) and Wolfgang Pauli (1900-1958). Between 1900 and 1930 physics changed dramatically because of the development of two theories; quantum mechanics, which described the behaviour of atoms, and relativity which described the properties of space and time. Albert Einstein published his theories of relativity between 1900 and 1916. Pauli worked on quantum mechanics, particularly the Pauli Exclusion Principle (1924), which required that no two electrons in an atom could be in the same quantum state. This explained a lot about atomic structure.
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