Catastrophe theory—or the study of gradually changing forces that lead to so-called catastrophes (or abrupt changes)—was popularized by French mathematician René Thom (1923–2002) in 1972. Unable to use differential calculus in certain situations, Thom used other mathematical treatments of continuous action to produce a discontinuous result. Although it is not as popular as it once was, it is often used in biological and optical applications. (Note that this term is often used in geology, too—but don’t confuse them. Some 18th-century scientists once believed that geologic features were created by sudden and violent events, such as earthquakes and volcanic eruptions. Today, most geologists think Earth’s features form slowly, punctuated by the occasional catastrophic event.)