Combining cold-atom experiments with ideas from stochastic thermodynamics and quantum gravity offers a new lab perspective on one of physics' oldest conundrums.
Giovanni Barontini, Professor of Physics, at the University of Birmingham, with the apparatus to trap and cool rubidium atoms in the background.
A University of Birmingham scientist has built a 'mini universe' that takes a step towards answering one of science’s biggest questions: ‘what is time?’
Publishing his findings in Physical Review Research, Professor Giovanni Barontini shows how it is possible to measure the flow of time without using a clock at all. The new findings provide a scientific model where a version of time emerges from the experiment itself.
Some theories of physics, such as the Wheeler–DeWitt equation suggest that, at its deepest level, the universe has no built‑in time, but exists as a single, unchanging quantum state where particles exhibit both wave-like and particle-like properties. It treats the universe as a whole with no external clock, and any sense of time must emerge from internal relationships between parts.
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.
Professor Barontini used a cloud of 24,000 ultracold atoms - just a few billionths of a degree above absolute zero - to create a hermetically sealed quantum system that mimics a simple ‘universe’. The particles were trapped and divided with a thin barrier formed with a laser beam to create an observed (‘bright’) and an unobserved (‘dark’) region. Atoms could move between ‘bright’ and ‘dark’ regions, but the system was otherwise isolated from the outside world.
The ‘bright’ sector repeatedly expands and collapses, experiencing something like a Big Bang and a Big Crunch - a hypothetical scenario where the expansion of the cosmos eventually reverses.
Using the ‘mini universe’ Barontini demonstrated that an ‘entropic time’ could be created from the disorder or spread (entropy) of atoms and how they behaved in a system. In stochastic thermodynamics, entropy can briefly decrease due to randomness, but still increases on average, so ‘entropic time’ still has a clear direction.
When the spread of particles in the bright sector increased or decreased as atoms moved in or out, the system was ‘moving forward in time’. When this distribution of atoms did not change, time effectively stopped. Professor Barontini tested the theory of ‘entropic time’ in the ‘mini universe’ and found that this version of time:
Professor 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 clear 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 'cloud' inside the glass cell is a magneto-optical trap of rubidium atoms at a temperature of ~0.0001 degrees above absolute zero. It is only the first step to "build" the mini-universe.
The study also demonstrates that a version of the main equation in quantum mechanics (the Schrödinger equation) can be generalised using the entropic time emerging from within the system – enabling correct description of the dynamics of a quantum system and how it is likely to change over time.
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Professor of Physics
Staff profile for Professor Giovanni Barontini
