Physicists create a miniature universe in which time emerges from entropy, without a clock

The experiment used two laser beams at different frequencies to create a thin barrier dividing the cold atom cloud into an observed, or bright, region and an unobserved, dark region. Although the overall system remained isolated from its surroundings, atoms could move between the two regions, and the bright region underwent repeated cycles of expansion and contraction resembling simplified versions of the Big Bang and the Big Crunch. The experiment did not recreate those cosmic events; it provided a quantum model in which a sequence of events could be reconstructed without reference to a clock outside the system.

The work bears on a puzzle in quantum gravity. The Wheeler-DeWitt equation appears in attempts to combine quantum mechanics with gravity; in this framework, the universe can be described as a single, unchanging quantum state with no external clock, raising the question of how anything can happen in a particular order. One possibility, which the experiment tested, is that time emerges from changing relationships between different parts of the universe. Many fundamental physical equations work equally well whether time is reversed or runs forward, yet everyday experience has a clear direction: people remember the past, not the future, and events unfold in a recognizable sequence.

Barontini tracked progression through changes in how atoms were distributed within the miniature universe, measuring the spread of particles rather than seconds or minutes. When the distribution increased or decreased in its spread, the system continued moving forward according to this internal measure; when the distribution remained unchanged, the progression of this form of time effectively stopped. The researcher called the concept entropic time. The study found that it moves consistently in one direction, establishing the arrow of time, places events in the correct sequence even as the miniature universe expands and contracts, and does not always advance at the same rate: its progression can speed up or slow down depending on how entropy changes within the system.

Barontini also demonstrated that a version of the Schrödinger equation, one of the foundational equations of quantum mechanics, can be expressed using entropic time, meaning the internally generated measure could still be used to make predictions about the quantum system's behaviour much as conventional time is.

Barontini said: "In some theories of the universe, especially quantum gravity, time doesn't appear as a built-in feature. Yet in everyday life, time flows from past to future -- why is this so, when most basic laws of physics work the same way forwards and backwards? 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 does not establish that time is an illusion or prove that the universe lacks fundamental time. It shows that a quantum system can possess a meaningful internal measure of time without depending on an external clock. Future experiments could extend the approach to more complicated quantum systems, allowing researchers to explore models of the Big Bang and the Big Crunch in greater detail, and similar techniques might eventually help simulate aspects of black holes or distinguish between competing explanations of how time emerges.