Modern physics is an effort to understand the underlying processes of the interactions of matter using the tools of science and engineering. In a literal sense, the term modern physics means up-to-date physics. In this sense, a significant portion of so-called classical physics is modern. However, since roughly 1890, new discoveries have caused significant paradigm shifts : especially the advent of quantum mechanics (QM) and relativity (ER). Physics that incorporates elements of either QM or ER (or both) is said to be modern physics. It is in this latter sense that the term is generally used.