A Cloud of Cold Atoms Just Told Time Without a Clock

Strip away every clock in the universe. No ticking, no pendulums, no caesium atoms counting out the second. Now ask yourself a question that has bothered physicists for the better part of a century: how would you know that anything was happening at all? In some of our deepest theories of reality, this is not a thought experiment. It is the actual situation.

That uncomfortable gap between the equations and lived experience has a name. Physicists call it the problem of time, and for decades it has lived almost entirely in chalk dust and abstraction.

At the University of Birmingham, Giovanni Barontini has dragged it onto a laboratory bench. He cooled roughly 24,000 rubidium atoms to a few billionths of a degree above absolute zero, until they merged into a single shimmering quantum blob, and then he split that blob in two with a wall of laser light. One side he could watch. The other he deliberately could not. The trick, it turns out, is in what you choose not to look at.

Why bother with all this? Because some theories of physics, the Wheeler-DeWitt equation chief among them, describe the universe as a single frozen quantum state with no built-in time at all.

In that picture the cosmos simply is, complete and unchanging, like a film reel laid flat on a table rather than running through a projector. And yet here we are, ageing, remembering, watching coffee go cold. The challenge is to recover the river of time we all feel from equations that, frankly, never mention it.

Barontini’s answer is to let one part of his miniature cosmos keep time for the other. “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?” he says.

A Big Bang in a Bottle

Here is where it gets strange, and rather lovely. The observed half of the atom cloud, the “bright” sector, does not just sit there. It swells outward until it reaches a maximum, then contracts and collapses back, a whole cosmic life cycle playing out in about a tenth of a second: a tiny Big Bang followed by an equally tiny Big Crunch, over and over. Atoms leak across the laser barrier into the hidden “dark” sector and back again, and it is precisely this traffic, this spreading-out and bunching-up of particles, that Barontini uses as his clock. He calls the resulting quantity entropic time. When the spread of atoms changes, time moves. When nothing spreads, time simply stops. No external second-hand required.

And the thing actually behaves like time should. It runs in one direction, giving a clean arrow from past to future. It orders the events of each expansion and collapse in the right sequence.

It even speeds up and slows down depending on how briskly entropy sloshes between the two sectors, which is a property no ordinary clock has, and a slightly disorienting one to think about. Crank the laser barrier up high enough and the exchange of entropy dwindles toward nothing; the little universe drifts toward what Barontini, borrowing the old cosmological phrase, calls a “heat death,” a stationary state in which entropic time grinds to a complete halt. Time doesn’t end with a bang there. It just runs out of things to count.

From Chalkboard to Lab Bench

The reduced description of the bright sector, it turns out, is structurally a dead ringer for the so-called minisuperspace models that quantum cosmologists have scribbled for years, stripped-down toy universes with only a handful of moving parts. Barontini went further and wrote down a version of the Schrödinger equation, quantum mechanics’ central engine, run not on laboratory time but on his entropic time, then showed by simulation that it reproduces what the atoms actually did. Ordinary, strictly reversible quantum mechanics, it emerges, is just the special case you get when no entropy is flowing at all.

None of this means we have solved what time is, and Barontini does not claim as much. It is one isolated cloud of atoms, an analogue, a stand-in, not the genuine fabric of spacetime.

Still. “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,” he says, adding that the approach could describe the dynamics just as effectively as conventional time does. That is a bold thing to be able to demonstrate on a bench rather than argue on a blackboard.

What makes it more than a curiosity is where it might lead. Questions once reserved for cosmologists, whether the Big Bang hid a true singularity or merely a quantum bounce, how a black hole scrambles the order of events, whether different internal clocks in the same universe might disagree, are suddenly things you could, in principle, dial up and test with lasers and cold gas. Barontini lists analogue black holes and Big Crunch physics among the possibilities. The early universe, reduced to something you can fit on a table and run again tomorrow morning.

Time, in the end, may not be a stage the universe performs on. It might be something the universe does, a tally of its own restlessness. And now, for the first time, there is a small glass chamber in Birmingham where you can watch that tally being kept.

DOI / Source: 10.1103/1h9j-df4k, Physical Review Research

Frequently Asked Questions

How can anything keep time without a clock?

Instead of counting ticks from an outside timepiece, the experiment tracks how spread out its atoms are, a measure of entropy. Every time that spread changes, the system has effectively “moved forward,” and when it stops changing, time stops too. It is a way of reading time off the internal state of a system rather than imposing it from outside, and it behaves remarkably like the time we actually experience.

Is this really a universe, or just a clever metaphor?

It is an analogue, not a literal cosmos. A few billion atoms standing in for the whole of reality is a model, deliberately simplified. But the point is that the same mathematics used to describe toy universes in quantum cosmology also describes this atom cloud, which means abstract cosmic questions can be poked and prodded in a real laboratory for once.

Why do physicists say the universe has no built-in time?

In certain quantum gravity theories, notably the Wheeler-DeWitt equation, the cosmos is described as one unchanging quantum state with no external parameter ticking along. That clashes head-on with our everyday sense of past flowing into future. Reconciling the two has been a stubborn open problem, and experiments like this offer a fresh way to chip at it.

Could this approach actually tell us anything about the real Big Bang?

Not directly, but it opens a door. Because the atom cloud cycles through its own miniature Big Bang and Big Crunch, researchers can in principle test competing ideas, such as whether the cosmos began in a true singularity or bounced, in a controlled setting. The same platform might also be tuned to mimic black holes, turning thought experiments into measurements.