Physicists created a tiny universe and watched time emerge without a clock

Key points

• A University of Birmingham physicist built a miniature universe from about 24,000 ultracold atoms.
• Time in the experiment emerged from entropy changes inside the system, not an external clock.
• The internal measure, called entropic time, set events in order and moved in one direction.

A physicist at the University of Birmingham has created a miniature universe in the laboratory using thousands of atoms cooled to temperatures barely above absolute zero, the university reports. Professor Giovanni Barontini showed that the passage of time can be measured without an external clock, with a form of time emerging from the behavior of particles inside the system. The findings were published in Physical Review Research.

The experiment used about 24,000 atoms confined in an isolated quantum system. Two laser beams at different frequencies formed a thin barrier splitting the system into an observed, or bright, region and an unobserved, or dark, region, and atoms could move between them.

How time was measured

The bright region went through repeated cycles of expansion and contraction, resembling simplified versions of the Big Bang and a Big Crunch. Barontini tracked time through changes in how atoms were distributed, using entropy rather than seconds or minutes. He called the concept entropic time.

The experiment showed this form of time moves consistently in one direction, places events in the correct sequence and can speed up or slow down depending on how entropy changes. When the particle distribution stayed unchanged, the internal measure of time stopped advancing, though time did not literally stop in the laboratory.

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.

— Professor Giovanni Barontini

What the researcher said

Barontini said the study offers the first controlled experimental evidence that time can be defined by changes within a system rather than an external ticking clock. He said it gives new insight into the nature of time in quantum gravity.

The work also expressed a version of the Schrödinger equation using entropic time. The experiment does not prove that time is an illusion or that the universe lacks fundamental time, but shows a quantum system can have a meaningful internal measure of time without an external clock.