How an argument between Einstein and Bohr changed quantum mechanics forever. Sponsored by NordVPN - 🌏 Get exclusive NordVPN deal here: https://NordVPN.com/veritasium It’s risk free with Nord’s 30 day money-back guarantee!✌ In 2025, we launched the Elements of Truth game on Kickstarter and were thrilled by the response. We’re now in the final stages of production, with Kickstarter rewards shipping soon. If you missed the campaign, you can now pre-order the retail edition through our new website here -https://ve42.co/BellsDesc If you’re looking for a molecular modelling kit, try Snatoms, a kit I invented where the atoms snap together magnetically - https://ve42.co/SnatomsV Sign up for the Veritasium newsletter for weekly science updates - https://ve42.co/Newsletter ▀▀▀ Thank you to Dr Adam Becker for his help with this video - check out his book here https://ve42.co/WhatReal And special thanks to Thierry Avignon, Benjamin Vest, and Lionel Jacubowiez of the Institut d'Optique Graduate School - Laboratoire d'Enseignement Expérimental (LEnsE) Correction: 30:53 Electrons and Positrons not Protons ▀▀▀ 0:00 The Speed of Gravity 3:07 Spooky Action at a Distance 5:21 The Copenhagen Interpretation 9:12 The EPR Paradox and Hidden Variables 17:25 Einstein vs Bohr 20:51 John Bell and Entanglement 22:14 Bell’s Theorem 29:30 The Bell Inequality Test 33:41 The Most Misunderstood Experiment in Physics 35:06 The Locality Problem 40:37 The Many-Worlds Interpretation ▀▀▀ References: Adam Becker (2018). What is Real?. Basic Civitas Books - ve42.co/WhatReal Einstein-Podolsky-Rosen paradox via Wikipedia - ve42.co/EPRWiki Einstein, A. et al. (1935). Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?. American Physical Society (APS) - ve42.co/EPR Solvay Conference via Wikipedia - ve42.co/SolvayConf Owen Willans Richardson Photo Galley via nobelprize.org - ve42.co/SolvayPhoto Bohr-Einstein debates via Wikipedia - ve42.co/BohrEinDeb Bacciagaluppi, G and Valentini, A (2009). Quantum Theory at the Crossroads. Cambridge University Press - ve42.co/Crossroads Bohr, N. (1935). Can Quantum-Mechanical Description of Physical Reality be Considered Complete?. American Physical Society (APS) - ve42.co/BohrResp The Solvay Conference via rarehistoricalphotos.com - ve42.co/SolvayImg Kragh, H. (2022). Chemists Without Knowing It?. Firenze University Press - ve42.co/BhorsConferenceImg Einstein Attacks Quantum Theory. The New York Times via nytimes.com - ve42.co/EinsteinAttackQT Oral History Interviews via repository.aip.org - ve42.co/BohrInterview Images & Video: Image and video references can be found here - https://ve42.co/BellsImRefs ▀▀▀ Special thanks to our Patreon supporters: Adam Foreman, Albert Wenger, Alex Porter, Alexander Tamas, Anton Ragin, armedtoe, Balkrishna Heroor, Bertrand Serlet, Blake Byers, Bruce, Damien, Dave Kircher, David Johnston, David Tseng, EJ Alexandra, Evgeny Skvortsov, Garrett Mueller, Gnare, gpoly, Hayden Christensen, Ibby Hadeed, Jeromy Johnson, Jon Jamison, Juan Benet, KeyWestr, Kyi, Lee Redden, Marinus Kuivenhoven, Matthias Wrobel, Meekay, meg noah, Michael Bush, Michael Krugman, Orlando Bassotto, Paul Peijzel, Richard Sundvall, Robert Oliveira, Sam Lutfi, Tj Steyn, Ubiquity Ventures, wolfee ▀▀▀ Writers: Mithuna Yoganathan, Casper Mebius & Derek Muller Producer & Director: Mithuna Yoganathan Editor: Spencer Wright Camera Operators: Mithuna Yoganathan, Derek Muller and Maxime Beauquesne Animators: Alex Drakoulis, Fabio Albertelli, Emma Wright, Andrew Neet and Peter Nelson Illustrators: Jakub Misiek, Katheryn Chan, Grace Nemanic, Isaac McCree and Natalky Zhuk Assistant Editor: James Stuart Researchers: Gabe Strong and Sophia Long Thumbnail Designers: Abdallah Rabah, Ren Hurley & Ben Powell Production Team: Katy Southwood, Josh Pitt, Anna Milkovic and Matthew Cavanagh Executive Producers: Derek Muller & Casper Mebius Additional video/photos supplied by Getty Images, Storyblocks Music from Epidemic Sound Notes *When we show the Stern Gerlach experiment we’ve chosen to flip the polarity of the machine when measuring a positron. This is because both its spin and charge should oppose the electron’s. This would mean both would bend in the same direction! We’re interested in seeing the difference in the spins so we flip the machine to make this clearer. **We show curved spacetime using Flamm’s Parabaloid, which we discovered is controversial, more notes on that here - https://ve42.co/HBDFlamm