Professor Giovanni Barontini at the University of Birmingham created a miniature quantum system using approximately 24,000 atoms cooled to a few billionths of a degree above absolute zero. The experiment demonstrated that time can be measured without an external clock, emerging instead from changes within the system itself.
Two laser beams divided the quantum system into an observed bright region and an unobserved dark region. The bright region underwent cycles of expansion and contraction resembling simplified versions of a Big Bang and Big Crunch. As atoms moved between regions, changes in particle distribution provided an internal measure of time, which Barontini termed "entropic time."
The findings, published in Physical Review Research, show that entropic time moves consistently in one direction, establishing the arrow of time distinguishing past from future. Crucially, entropic time does not always advance at the same rate; its progression speeds up or slows down depending on how entropy changes within the system.
Barontini demonstrated that the Schrödinger equation, a foundational equation of quantum mechanics describing how quantum systems evolve, can be expressed using entropic time. This means the internally generated measure of time could still make predictions about the system's behavior, much as conventional time does in standard quantum mechanics.
Barontini stated: "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. 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 provides a testing ground for theories of quantum gravity and cosmology where time is not assumed to exist independently of the system being studied. The approach could eventually help explore models of black holes or test competing explanations of how time emerges from quantum systems.