In 1993, he argued that if a black hole starts in a pure quantum state and evaporates completely by a unitary process, the von Neumann entropy of the Hawking radiation initially increases and then decreases back to zero when the black hole has disappeared. This is known as the Page curve, and the turnover point of the curve the Page time. For many researchers, deriving the Page curve is synonymous with solving the famous black hole information paradox.