Physics students learn the speed of light, c, is the same for all inertial observers but no one has ever actually measured it in one direction. Thanks to Kiwico for sponsoring this video. For 50% off your first month of any crate, go to https://kiwico.com/veritasium50Huge thanks to Destin from Smarter Every Day for always being open and willing to engage in new ideas. If you haven't subscribed already, what are you waiting for: https://ve42.co/SEDFor an overview of the one-way speed of light check out the wiki page: https://ve42.co/wiki1wayThe script was written in consultation with subject matter experts:Prof. Geraint Lewis, University of Sydney https://ve42.co/gflProf. Emeritus Allen Janis, University of PittsburghProf. Clifford M. Will, University of Florida https://ve42.co/cmwThe stuff that's correct is theirs. Any errors are mine.References:Einstein, A. (1905). On the electrodynamics of moving bodies. Annalen der physik, 17(10), 891-921.(English) https://ve42.co/E1905 (German) https://ve42.co/G1905Greaves, E. D., Rodríguez, A. M., & Ruiz-Camacho, J. (2009). A one-way speed of light experiment. American Journal of Physics, 77(10), 894-896. https://ve42.co/Greaves09Response to Greaves et al. paper — https://arxiv.org/abs/0911.3616Finkelstein, J. (2009). One-way speed of light?. arXiv, arXiv-0911.The Philosophy of Space and Time - Reichenbach, H. (2012). Courier Corporation.Anderson, R., Vetharaniam, I., & Stedman, G. E. (1998). Conventionality of synchronisation, gauge dependence and test theories of relativity. Physics reports, 295(3-4), 93-180. https://ve42.co/Anderson98A review article about simultaneity — Janis, Allen, "Conventionality of Simultaneity", The Stanford Encyclopedia of Philosophy (Fall 2018 Edition), Edward N. Zalta (ed.) https://ve42.co/janisWill, C. M. (1992). Clock synchronization and isotropy of the one-way speed of light. Physical Review D, 45(2), 403. https://ve42.co/Will92Zhang, Y. Z. (1995). Test theories of special relativity. General Relativity and Gravitation, 27(5), 475-493. https://ve42.co/Zhang95Mansouri, R., & Sexl, R. U. (1977). A test theory of special relativity: I. Simultaneity and clock synchronization. General relativity and Gravitation, 8(7), 497-513. https://ve42.co/SexlResearch and writing by Derek Muller and Petr LebedevAnimations by Ivàn TelloVFX, music, and space animations by Jonny HymanFilmed by Raquel NunoSpecial thanks for reviewing earlier drafts of this video to:Dominic Walliman, Domain of Science: https://ve42.co/DoSHenry Reich, Minutephysics: https://ve42.co/MPMy Patreon supportersAdditional music from https://epidemicsound.com "Observations 2"