Time, from inside.

Physics has a puzzle about time at its heart. When physicists apply quantum theory to gravity and to the universe as a whole, one famous equation says the whole thing does not change at all. Yet everything inside it plainly does. They call it the problem of time.

In 1983 the physicists Don Page and William Wootters suggested a way through. Split the universe in two and let one part be a clock for the rest. Timed by that clock, from inside, the rest of the universe changes. Seen as a whole, from outside, nothing does.

In 2014 Ekaterina Moreva and her colleagues, at Italy’s national metrology institute in Turin, acted the idea out with two photons whose states were linked, so that measuring one tells you about the other. One photon served as the clock. Timed by it, the other changed. Then they measured the pair as a whole, as if from outside, and showed that it did not change at all.

It is an elegant idea and a debated one, and two photons are not a universe. The experiment does not say time is unreal. It shows how change can be real inside, read against a clock, while the whole stands still.