Inside a glass cell at the University of Birmingham, Professor Giovanni Barontini has trapped 24,000 ultracold rubidium atoms to build something extraordinary: a laboratory version of the universe itself, where time doesn't tick from an external clock but emerges from the atoms' own behavior. The question he's testing is one of physics' most profound: What actually is time?
Most of us take time for granted—a steady backdrop against which everything else happens. But deep theories of physics, including the Wheeler–DeWitt equation, suggest something stranger: at the universe's deepest level, there might be no built-in time at all. The cosmos might exist as a single, unchanging quantum state, and our familiar sense of past flowing toward future could emerge somehow from the internal workings of reality itself. For decades, this idea remained abstract speculation. Barontini has now made it testable in the lab.
The experiment works by dividing the cloud of atoms into two regions using two laser beams of different frequencies: a "bright" sector that researchers observe directly, and a "dark" sector they don't. The bright sector undergoes repeated cycles of expansion and collapse—something like a miniature Big Bang followed by a Big Crunch, the hypothetical scenario where cosmic expansion reverses. Atoms can move between the two regions, but the entire system is otherwise sealed off from the outside world, cut off from any external reference.
Here's where it becomes elegant: Barontini found that time could be measured from within the system itself, based on how entropy—the disorder or spread of the atoms—changed. When the distribution of atoms in the bright sector increased or decreased as particles moved between regions, the system was "moving forward in time." When the distribution remained static, time effectively stopped. This he calls "entropic time," and it exhibits three remarkable properties. It flows in one consistent direction, giving the universe a clear "arrow of time"—explaining why we remember the past but not the future. It correctly orders events, even in a system expanding and contracting like a mini cosmos. And it speeds up or slows down depending on how entropy moves around.
The findings, published in Physical Review Research, offer what Barontini describes as "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." More intriguingly, he showed that Schrödinger's equation—the fundamental equation of quantum mechanics—can still be written using entropic time, meaning physicists could predict how quantum systems evolve without invoking an external clock at all.
What makes this work transformative is its bridge between the abstract and the empirical. Questions about the nature of time have belonged almost exclusively to theoretical physics and cosmology—the realm of thought experiments and mathematical equations. By creating a controlled laboratory system that tests these ideas experimentally, Barontini has opened a powerful test bed for quantum cosmology itself. Ideas about the early universe, previously confined to blackboards and papers, can now be probed in the lab.
The implications ripple outward. If time truly emerges from internal system changes rather than existing as a fundamental feature of reality, it reshapes how we think about quantum gravity and the cosmos's deepest nature. For Barontini and his colleagues, this Birmingham laboratory is just the beginning—the apparatus and method can be extended to test even more complex scenarios about how reality, at its most fundamental level, might actually work.