Physicist Creates 'Mini-Universe' to Unravel Time's Mysteries

A groundbreaking experiment by physicist Giovanni Barontini at the University of Birmingham in the U.K. has provided the first direct, quantitative test of theories suggesting time emerges from relationships within a system. Barontini successfully created a "mini-universe" using a cloud of ultracold atoms, demonstrating how time can arise and behave even without an external reference point. The findings, published on June 11 in Physical Review Research, offer experimental evidence for long-standing ideas in quantum cosmology and thermodynamics.

For decades, physicists have grappled with the implications of the Wheeler-DeWitt equation, a cornerstone of quantum gravity. This equation describes the universe as a self-contained system, devoid of an external cosmic clock. This raises a fundamental question: If there is nothing outside the universe, where does our perception of time originate? One prominent hypothesis, known as relational time, proposes that time is not a fundamental property but rather an emergent phenomenon, arising from the interactions and relationships between different parts of a system, where one part acts as a clock for another.

Barontini's inspiration stemmed from observing his son play with building blocks, leading him to conceptualize that complex systems, even those built with "very expensive toys" in a laboratory, could mimic the fundamental processes of the universe. In his experiment, Barontini utilized a Bose-Einstein condensate, a state of matter achieved near absolute zero where thousands of atoms behave as a single quantum object. This highly isolated quantum system served as his "mini-universe." To simulate the absence of external influences, the condensate was carefully divided into two halves using a laser light barrier. One half was designated the "bright sector," which Barontini closely observed, while the other was the "dark sector," deliberately ignored.

The Emergence of Entropic Time

The experiment's pivotal moment involved monitoring the exchange of entropy, a measure of disorder, between the bright and dark sectors. Instead of relying on standard laboratory time to sequence events, Barontini constructed a clock based entirely on this entropy flow. He termed this "entropic time." The flow of atoms between the sectors, analogous to cosmic events like the Big Bang and Big Crunch, became markers for this internal clock. When entropy flowed freely between the two halves, the entropic time advanced rapidly. As the exchange slowed, so did the internal clock. Remarkably, when the two sectors reached equilibrium and no further entropy was exchanged, the internal clock effectively stopped.

"Time was speeding up or slowing down, or even stopping, depending on what the system was doing," Barontini stated, highlighting the surprising precision with which the experiment validated theoretical predictions. He further demonstrated that this internally generated entropic time could be used to accurately derive and reproduce a version of the Schrödinger equation, a fundamental equation in quantum mechanics, validating the emergent nature of time in his system. "This was quite surprising, how well everything came together," he added, noting the unusual neatness of the experimental results.

This research suggests that both the existence of time and its unidirectional flow—the "arrow of time"—may originate from a fundamental principle: observation and the relinquishing of information. By choosing to observe only the bright sector and ignoring the dark sector, Barontini effectively gave up knowledge about that part of the system. This act of "ignorance," encoded in the system's entropy, appears to be the very mechanism that generates time in the observed sector. "Both time and the arrow of time — maybe they just are born from ignorance," Barontini mused. "To have time and to observe, you have to give up some degrees of freedom." Barontini envisions this cold-atom apparatus as a versatile tool, capable of simulating a wide range of complex phenomena, including the conditions of the early universe, black hole analogues, and potential future cosmological events, all within a controlled laboratory setting.