Time from Disorder: How Atoms Revealed an Internal Clock ✛

Original: "Testing the problem of time with cold atoms"

arXiv:2509.07745 · 2025-09-09 · CC BY 4.0 · 1 min

General Relativity Quantum Gases Quantum Physics

Physicists have shown: time is measured not by ticking, but by the growth of disorder inside a system.

Scientists took a cloud of atoms, colder than space, and cut it in half with a laser. One half was in plain sight, the other hidden. Instead of clocks, they measured the growth of disorder — entropy. Through it, all events lined up in a clear chain, even as the visible part expanded and contracted. Disorder worked like clock hands: the greater the chaos, the further in time. In a room without windows, growing disorder is the only sign of time's passage. Based on the same principle, entropy made it possible to write an equation describing the hidden part — the shadow found its voice. The idea was predicted by John Archibald Wheeler and Bryce DeWitt back in the 1960s: time is not absolute, it arises from relationships. Now it's an experimental fact. This approach paves the way to understanding time inside black holes, at the moment of the Big Bang, and wherever spacetime is curved to its limits.

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Disorder in a closed system never decreases: that's what gives time its direction from past to future.

\tau(\lambda) = \frac{\sigma}{k_B} \int_{\lambda} \frac{dS}{d\phi} |d\phi|

Entropic time τ is proportional to the integral of the entropy change dS along the field trajectory φ, where σ is an arbitrary time unit and kB is the Boltzmann constant.

i\hbar \partial_\tau \psi(\tau, a) = \Phi(\tau)\psi(\tau, a) + \Lambda(\tau) H_{\text{geom}}\psi(\tau, a)

The evolution of the wave function ψ with respect to entropic time τ is determined by a global phase Φ and an entropy pump Λ multiplying the geometric Hamiltonian H_geom.

Scientists

Ludwig Boltzmann Rudolf Clausius Jacob Bekenstein Wilhelm Wien Charles Bennett John von Neumann

Concepts

time evolution schrodingers cat Schrödinger equation Bose-Einstein condensate arrow of time laboratory experiment entropy driven self assembly Boltzmann equation schrodinger cat state observation atomistic simulation retrospective forecast atom trap Wheeler-DeWitt equation Gross–Pitaevskii equation ergodic theorem second law of thermodynamics entropy quantum dynamics quantum clock

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Time from Disorder: How Atoms Revealed an Internal Clock ✛