Squarespace Website Builder

Head to https://squarespace.com/physicsexplained to save 10% off your first purchase of a website or domain using code PHYSICSEXPLAINED Why does time only go one way? Why do broken things never unbreak, and why does the universe remember the past but not the future? The laws of physics work just as well forwards as backwards in time—so where does the arrow of time really come from? In this video, we follow entropy from simple coin tosses and mixing gases all the way to black holes and the Big Bang and uncover a staggering result first estimated by Roger Penrose: the initial state of our universe was unbelievably special, with odds of about 1 in 10^(10^123). References and Additional Material Baez, J. C. (2024). What Is Entropy? arXiv:2409.09232. https://arxiv.org/abs/2409.09232 Carroll, S. M. (2014). In What Sense Is the Early Universe Fine-Tuned? arXiv:1406.3057. https://arxiv.org/abs/1406.3057 Egan, C. A., & Lineweaver, C. H. (2010). A Larger Estimate of the Entropy of the Universe. The Astrophysical Journal, 710(2), 1825–1834. Journal: https://iopscience.iop.org/article/10.1088/0004-637X/710/2/1825 arXiv: https://arxiv.org/abs/0909.3983 Fantl, J. (2025). What Is Entropy? https://jasonfantl.com/posts/What-is-Entropy/ Lineweaver, C. H. (2014). The Entropy of the Universe and the Maximum Entropy Production Principle. In A. Kleidon & R. D. Lorenz (Eds.), Beyond the Second Law: Entropy Production and Non-Equilibrium Systems. Springer. Chapter PDF: https://www.mso.anu.edu.au/~charley/papers/Chapter22Lineweaver.pdf Penrose, R. (1989). Difficulties with Inflationary Cosmology. Annals of the New York Academy of Sciences, 571, 249–264. https://onlinelibrary.wiley.com/doi/10.1111/j.1749-6632.1989.tb50513.x Penrose, R. (1989). The Emperor’s New Mind: Concerning Computers, Minds, and the Laws of Physics. Oxford University Press. Publisher page: https://global.oup.com/academic/product/the-emperors-new-mind-9780198784920 Penrose, R. (2004). The Road to Reality: A Complete Guide to the Laws of the Universe. Jonathan Cape. Publisher page (Penguin UK): https://www.penguin.co.uk/books/357292/the-road-to-reality-by-penrose-roger/9780099440680 Penrose, R. (2006). Before the Big Bang: An Outrageous New Perspective and Its Implications for Particle Physics. In Proceedings of EPAC 2006, Edinburgh (Paper THESPA01). https://proceedings.jacow.org/e06/PAPERS/THESPA01.pdf Penrose, R. (2010). Cycles of Time: An Extraordinary New View of the Universe. The Bodley Head. Publisher page (Penguin UK): https://www.penguin.co.uk/books/392219/cycles-of-time-by-roger-penrose/9780099505945 Price, H. (1996). Time’s Arrow and Archimedes’ Point: New Directions for the Physics of Time. Oxford University Press. Book page: https://global.oup.com/academic/product/times-arrow-and-archimedes-point-9780195117981 Related preprint (arrow of time): https://prce.hu/w/preprints/origins.pdf Siegel, E. (2024, February 22). Did the Universe Have Zero Entropy When It First Began? Big Think (Starts With A Bang). https://bigthink.com/starts-with-a-bang/universe-zero-entropy/ Wald, R. M. (2005). The Arrow of Time and the Initial Conditions of the Universe. arXiv:gr-qc/0507094. https://arxiv.org/abs/gr-qc/0507094 WS5. (n.d.). Roger Penrose: Spacetime, Relativity, Quantum Physics, and Quantum Gravity. https://www.ws5.com/Penrose/ 🤝 Support the channel Patreon → https://www.patreon.com/physicsexplained 🔗 Follow me Instagram → https://www.instagram.com/physics_explained_ig X (Twitter) → https://twitter.com/PhysicsExplain1