Often attributed to Albert Einstein, the quote by science-fiction writer Ray Cummings still serves as the most succinct explanation for one of the most fundamental features of our universe.
To Newton, time marched at a constant pace beyond the physical realm. Einstein’s relativity revealed time and space to be intimately wedded, shrinking and expanding under acceleration’s influence.
Just what causes things to “not all happen at once”, however, remains an open question. So University of Birmingham physicist Giovanni Barontini decided to go back to basics and build a whole new universe to watch time unfold from scratch.
His version was a little simpler than the one we’re used to looking at, consisting of 24,000 rubidium atoms. Chilled to a fraction of a degree above absolute zero, they were all forced to share the same quantum identity.
The resulting quantum condensate was then divided into two sections, one that could be measured and another that remained dark to outside observers. Allowing the isolated system to expand and waves to slip between the two realms provided Barontini with enough of an approximation of an expanding cosmos to test an intriguing hypothesis.
While it lacks stars and black holes, this version of the universe contains enough material to represent a model known as the Wheeler-DeWitt framework – a mathematical unification of general relativity and quantum mechanics that describes everything as part of a wave function.
And by everything, that includes time.
“This study provides the first controlled experimental evidence that ‘time’ can be defined by changes within a system rather than as the external ‘ticking clock’ we think of as time,” says Barontini.
“It offers new insight into the nature of time in quantum gravity that could be used to describe dynamics just as effectively as conventional time.”
The experiment confirms a suspicion that researchers have long held – in a Wheeler-DeWitt framework, “before” and “after” are features of disorder emerging within a system. In this case, disorder – also known as entropy – is a mathematical way to describe a loss of quantum information as the Universe expands.
By measuring features of his expanding and contracting cloud of cold rubidium atoms, Barontini constructed an order of events that fit what we might think of as time, which flows in a consistent direction and changes rate in relation to changes in entropy.
Current models of our universe struggle to reconcile gravity with quantum physics, leaving us in the dark on the mechanics of black holes or how the Big Bang unfolded.
Mini-universes such as this could provide researchers with much-needed insights into the temporal factors of our expanding Universe, finally giving us a good reason for why everything hasn’t already happened.
This research was published in Physical Review Research
Source: University of Birmingham
Fact-checked by Bronwyn Thompson
