TIME really is an illusion – at least in a model universe made of two particles of light. The experiment could assist in uniting our two best theories for describing reality: quantum mechanics and general relativity.
So far, attempts to combine the equations in these disparate theories have produced seemingly nonsensical answers. In the 1960s the Wheeler-DeWitt equation quantised general relativity – by leaving out time altogether, something that runs counter to our practical evidence.
Then in 1983 theorists Don Page and William Wootters suggested that time might be an illusion that emerges from the phenomenon of quantum entanglement. They showed mathematically that an observer within the universe would sense time, while an outside observer would see it as static, with no time passing.
Now Marco Genovese at Italy’s National Institute of Metrological Research in Turin and his colleagues have shown how time can emerge from quantum entanglement in a physical “universe”, albeit one that contains only two photons. The team start with a pair of entangled photons that each have a particular polarisation, which rotates as both photons pass though a quartz plate and on to a set of detectors.
The entangled photons exist in a superposition of horizontal and vertical states simultaneously, until they are observed. 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 photon is treated like a clock with a “tick” that is in fact an alternation between horizontal and vertical polarisation. Because of entanglement, reading this clock will affect the polarisation value of the second photon. An observer who reads the clock influences the photons’ universe and becomes part of it. The observer is then able to gauge the polarisation of the other photon based on quantum probabilities.
Since photons passing through a thicker quartz plate experience a different tick rate, repeating the experiment with multiple plates confirms that the second photon’s value varies with time (arxiv.org/abs/1310.4691).
In another mode, the experimenter is a “super-observer” who exists outside of the photons’ universe. From that vantage point, the state of both photons taken together is always the same, giving the appearance of a static universe.
Not everyone thinks tossing out time is the correct route to unification of the quantum and classical worlds (see “The now delusion“). Genovese acknowledges that this result does not clinch the issue. “It’s a visualisation of the phenomenon, it’s not a proof,” he says. “You should look to the universe itself for that.”
This article appeared in print under the headline “Step outside the universe to kill time”