Time is an illusion – at least in a toy model of the universe made of two particles of light. The experiment shows that what we perceive as the passage of time might emerge from the strange property of quantum entanglement
. The finding could assist in solving the long-standing problem of how to unify modern physics.
Physicists have two ways of describing reality, quantum mechanics for the small world of particles and general relativity for the larger world of planets and black holes. But the two theories do not get along: attempts to combine their equations into a unified theory produce seemingly nonsensical answers. One early attempt in the 1960s was the Wheeler-DeWitt equation, which managed to quantise general relativity – by leaving out time altogether.
"It means that the universe should not evolve. But of course we see evolution," says Marco Genovese at the National Institute of Metrological Research in Torino, Italy.
In 1983 theorists Don Page and William Wootters suggested that quantum entanglement might provide a solution to the Wheeler-DeWitt "problem of time". When quantum objects are entangled, measuring the properties of one changes those of the other. Mathematically, they showed that a clock entangled with the rest of the universe would appear to tick when viewed by an observer within that universe. But if a hypothetical observer existed outside the universe, when they looked in, everything would appear stationary.