New experiment provides powerful testbed for ideas in quantum cosmology and gravity - theories relating to the early universe can be tested in the laboratory.
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 two laser beams of different frequency to create an observed (‘bright’) and an unobserved (‘dark’) region.
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. The experiment allows the sequence of events to be reconstructed from within the mini-universe itself, without any reference to an external laboratory clock.
The experiment demonstrated that time could emerge from changes happening inside a quantum system, rather than existing as something external that ticks along independently.
Using the ‘mini universe’ demonstrated that ‘time’ could be created from the disorder or spread (entropy) of atoms and how they behaved in a system. Atoms could move between ‘bright’ and ‘dark’ regions, but the system was otherwise isolated from the outside world.
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 called this process ‘entropic time’, after finding that this version of time:
Correctly orders events, even in a system expanding and contracting like a mini cosmos
Speeds up or slows down depending on how entropy moves around
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 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 '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 (Schrödinger) can still be written using entropic time – enabling predictions of how the ‘probability cloud’ of a quantum system will change over time.
The experiment addresses a long-standing question in physics - in some theories of the universe, there is no built‑in clock so how do you tell what comes ‘before’ and ‘after’ without external time?
Professor Barontini showed that the system follows the standard equations of quantum physics and demonstrates that deep questions about the nature of time - usually discussed only in theories about the universe as a whole - can be tested in controlled laboratory experiments.
The experiment provides a powerful testbed for ideas in quantum cosmology and gravity, meaning that ideas relating to the early universe can now be tested experimentally in the lab.
The approach could be extended to more complex systems, potentially allowing researchers to probe the physics of the Big Bang and the ‘Big Crunch’. It could also be used to simulate black holes in the lab or test competing theories about how time emerges in the universe.
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Professor of Physics
Staff profile for Professor Giovanni Barontini
