A mini-universe of ultracold rubidium atoms in a glass trap. There is no clock inside this box. Yet a “time” reading appears below — computed only from how the atoms' disorder changes. Heat the gas, exchange entropy, and watch time flow. Cool and isolate it, and time freezes.
Physicists working with ultracold rubidium-87 atoms — cooled with lasers and evaporative cooling to nanokelvin temperatures, where thousands of atoms can collapse into a single quantum state called a Bose–Einstein condensate — built what amounts to a tiny, well-isolated “mini universe.” Inside it they asked a radical question: can the system tell time without any external clock?
The answer reported: yes. A usable time parameter can be defined purely from the system's internal changes — and it advances when entropy is exchanged between the system and its surroundings. No entropy flow, no internal change, no time. As the researcher put it: “this time increases when entropy is exchanged.”
This simulator does the honest version of the same trick. The chamber is divided into a 4 × 4 × 4 grid of cells. Each frame we count the fraction of atoms pᵢ in every cell and compute the Shannon entropy of that spatial distribution:
The internal clock τ is then defined so it advances only when S increases: dτ = k · max(0, dS). Condensed atoms huddled in a few cells give low S; heating spreads them out and S climbs — and only then does τ tick. Cool and compress the gas and τ simply stops, even though your wall clock keeps running. That gap between the two clocks is the whole point.
Microscopic physics is essentially time-symmetric: Newton's and Schrödinger's equations run equally well forwards and backwards. What breaks the symmetry is statistics. Boltzmann's insight was that there are astronomically more disordered arrangements than ordered ones, so isolated systems overwhelmingly drift toward higher entropy — the Second Law, ΔS ≥ 0.
That one-way drift is the only fundamental arrow we know: milk mixes into coffee and never unmixes; you remember the past, not the future. In this view, “time passing” isn't a river flowing past us — it is the experience of entropy increasing. The cold-atom result makes that concrete: build a system where you control entropy flow, and you control its time.
The idea has deep roots. In quantum gravity, the Wheeler–DeWitt equation infamously contains no time variable at all — the “problem of time.” The Page–Wootters mechanism (1983) proposed a way out: for a frozen global state, time emerges relationally — one part of the system acts as a clock for the rest, and “evolution” is just correlation between the clock part and everything else.
Emergent-time experiments are laboratory-scale echoes of that idea: instead of assuming a cosmic clock, you define time operationally from internal change — entropy exchanged, correlations built, states distinguishable from their predecessors. A perfectly static, perfectly isolated system has no usable time. Play with the simulator: freeze it, and ask yourself when “now” was.
Honest caveat: this page's entropy is a classical spatial proxy, not the full quantum (von Neumann) entropy of a real BEC, and real experiments track entropy exchange far more subtly. The logic — τ defined by dS — is the same.