Link to the paper: https://journals.aps.org/prresearch/abstract/10.1103/1h9j-df4k Follow me on LinkedIn: https://www.linkedin.com/in/leo-cui-ph-d-cfa-59531439/ I am a venture capital investor at FoundersX Ventures, an AI and deep-tech VC fund based in Menlo Park, Cambridge and NYC. I previously did my Ph.D. at Harvard, postdoc at MIT. 📊 Want the infographics from this video? Join the channel membership for downloadable infographics, ad-free videos, early access, and the ability to propose research topics. In a lab in England, a physicist took about 24,000 atoms, cooled them to a billionth of a degree above absolute zero, and used them to build a miniature model of a universe that expands and then collapses again — published here: https://journals.aps.org/prresearch/abstract/10.1103/1h9j-df4k The model mimics aspects of a big bang and a big crunch. But the point wasn’t to create another universe. It was to investigate how we describe change. Along the way, Giovanni Barontini built a clock out of what was happening inside a cloud of atoms. And at certain moments, that clock stops advancing — even while the atoms nearby keep moving. 🎯 You’ll learn: How time normally enters physics, using a ball dropped off a roof: you supply the interval, and the equation tells you where the ball is Why the stopwatch doesn’t make the ball fall — and why that distinction turns out to matter enormously The two theories that don’t fit together: quantum mechanics, where you supply time from outside, and general relativity, where time is part of the bending shape of spacetime The Wheeler–DeWitt equation, and why it sets a condition the universe has to satisfy rather than telling you how things change from moment to moment What it actually means that there’s no external time parameter to plug in — and why that does not mean time is unreal or the universe is frozen Relational time: describing how one part of a system changes relative to another, using quantities from inside the system itself Inside the experiment — rubidium atoms cooled until they share a quantum state and move together as a single wave, held in an optical trap, with a thin sheet of light dividing it into two regions atoms can cross between Why the width of the cloud ends up playing the part of “the size of the universe,” and why a sealed, fully measurable system makes such a useful test bed The first two candidates for an internal clock, and exactly why each one fails: the cloud’s position passes back through readings it already visited, and its size grows and shrinks through the same values Entropy explained with milk poured into coffee — how many microscopic arrangements fit the coarse picture you can see Why reading the entropy alone has the same problem, since one value can belong to two parts of the cycle The shop-door counter that clicks for everyone going in or out: how something can keep accumulating while the thing it tracks rises and falls How the clock is really built — combining measured entropy changes with the direction the cloud’s centre is moving — and the honest caveat that this is not a general rule that entropy pushes time forward The two tests: does ordering events by entropic time match the lab clock, and can an equation written in entropic time reproduce the measured width of the cloud? Why the result isn’t an independent forecast, and what it does establish The moments when entropic time simply doesn’t elapse at all, while the lab clock carries on — and what a higher barrier does, including the state the paper compares to a heat death ⏱️ CHAPTERS 0:00 A Mini Universe Made of 24,000 Atoms 0:41 A Falling Ball, and What Time Normally Does 1:39 The Problem of Time, and Wheeler–DeWitt 2:58 Relational Time: A Clock from Inside the System 3:40 Inside the Experiment 5:00 Why Position and Size Fail, and Entropy Instead 7:17 Does Entropic Time Actually Work? 8:17 When the Clock Stops, and What This Shows 🔬 The research: Giovanni Barontini, University of Birmingham, published in Physical Review Research. Everything in this video comes from that paper. One cloud of cold atoms is a long way from the cosmos, and none of this solves the problem of time for the real universe. Quantum gravity remains an area of active research rather than a tested theory. What the experiment offers is a controlled system where one specific proposal for building time from the inside can be checked against real measurements — and pushed until it breaks. ⚠️ This video is for educational purposes only. Some infographics are AI-generated and may contain errors; corrections will be pinned in the comments. 👍 If this was useful, subscribe and turn on the bell — I publish deep dives like this every week. #Physics #QuantumPhysics #Time #Entropy #BoseEinsteinCondensate #QuantumGravity #WheelerDeWitt #BigBang #BigCrunch #ColdAtoms #Science #Relativity #Thermodynamics #Birmingham