In other words, time is an entanglement phenomenon, which places all equal clock readings (of correctly prepared clocks – or any objects usable as clocks) into the same history. This was first understood by physicists Don Page and William Wootters in 1983. They proposed to address the problem of time in systems like general relativity called conditional probabilities interpretation. It consists in promoting all variables to quantum operators, one of them as a clock, and asking conditional probability questions with respect to other variables. They arrived at a solution based on the quantum phenomenon of entanglement. Page and Wootters showed how quantum entanglement can be used to measure time.