How can so many clocks be wrong? (Image: Peter Dazely/Getty)

Editorial: “Solving the eternal mystery that is time“

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.

Photon clock

For the first time, Genovese and colleagues have demonstrated this effect in a physical system, albeit in a “universe” that contains only two photons. The team started by sending a pair of entangled photons along two separate paths. The photons start out polarised, or orientated, either horizontally or vertically, and the polarisation rotates as both photons pass though a quartz plate and on to a series of detectors.

The entangled photons exist in a superposition of both horizontal and vertical states simultaneously until they are observed. But the thicker the plate, the longer it takes the photons to pass through and the more their polarisation evolves, affecting the probability that either one will take a particular value.

In one mode of the experiment, one of the photons is treated like a clock with a tick that can alternate between horizontal and vertical polarisation. Because of entanglement, reading this clock will affect the polarisation value of the second photon. That means an observer that reads the clock influences the photons’ universe and becomes part of it. The observer is then able to gauge the polarisation value of the other photon based on quantum probabilities.

Since photons passing through a thicker quartz plate experience a different degree of change, repeating the experiment with plates of different thicknesses confirms that the second photon’s polarisation varies with time.

In another mode, the experimenter is a “super-observer” that exists outside of the universe, and so measures the quantum state of the system as a whole. From that vantage point, the state of both photons taken together is always the same, giving the appearance of a static universe.

Quantum cosmos?

“It’s very nice these people have done an experiment to illustrate this effect and show how in practice it can occur,” says Page, who is now at the University of Alberta in Edmonton, Canada.

But not everyone thinks the Wheeler-DeWitt equation is the correct route to unification of the quantum and classical worlds, says Lee Smolin at the Perimeter Institute in Waterloo, Ontario, Canada. “They have verified in the context of a laboratory system that quantum mechanics is working correctly,” he says. But Smolin argues that any correct description of the universe must include time.

Genovese acknowledges that the result does not cinch the issue. Instead, he sees the work as a hint that quantum equations can in some ways mesh with general relativity, offering hope for a unified theory. The next step will be moving beyond the toy universe and seeing whether a similar effect scales up to explain what we see on a cosmic level.

“It’s a visualisation of the phenomenon, it’s not a proof,” Genovese says of the experiment. “You should look to the universe itself for that.”

Journal reference: arxiv.org/abs/1310.4691