Efforts to unify quantum mechanics and general relativity ultimately have to confront the challenge of the "problem of time." In the 1960s, physicists John Wheeler and Bryce DeWitt proposed the Wheeler-DeWitt equation, which successfully combined quantum mechanics and general relativity. However, solving one problem introduced a new one: their equation had no concept of time, and the universe it described was static. In 1983, physicists Don Page and William Wooters proposed a solution based on quantum entanglement, suggesting that the evolution of an entangled pair of particles could serve as a clock for measuring change. Their results depended on how observations were made.
To an observer outside the universe, akin to a god-like being, there would be no distinction between these entangled particles; however, when an observer inside the universe measures one particle in the entangled pair, they immediately observe a difference between the two. While an external observer might consider the universe to be static and unchanging, the entangled entities actually undergo extreme fluctuations. This theory posits that time emerges due to the nature of entanglement. Now, physicists from Italy have conducted the first quantum experiment to test the Page-Wooters theory, confirming that for internal observers, time is an emergent property of entanglement, while for external observers, time does not exist. The paper has been published on a preprint server.