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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/veritasium50 Huge 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/SED For an overview of the one-way speed of light check out the wiki page: https://ve42.co/wiki1way The script was written in consultation with subject matter experts: Prof. Geraint Lewis, University of Sydney https://ve42.co/gfl Prof. Emeritus Allen Janis, University of Pittsburgh Prof. Clifford M. Will, University of Florida https://ve42.co/cmw The 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/G1905 Greaves, 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/Greaves09 Response to Greaves et al. paper — https://arxiv.org/abs/0911.3616 Finkelstein, 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/Anderson98 A review article about simultaneity — Janis, Allen, "Conventionality of Simultaneity", The Stanford Encyclopedia of Philosophy (Fall 2018 Edition), Edward N. Zalta (ed.) https://ve42.co/janis Will, C. M. (1992). Clock synchronization and isotropy of the one-way speed of light. Physical Review D, 45(2), 403. https://ve42.co/Will92 Zhang, Y. Z. (1995). Test theories of special relativity. General Relativity and Gravitation, 27(5), 475-493. https://ve42.co/Zhang95 Mansouri, 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/Sexl Research and writing by Derek Muller and Petr Lebedev Animations by Ivy Tello VFX, music, and space animations by Jonny Hyman Filmed by Raquel Nuno Special thanks for reviewing earlier drafts of this video to: Dominic Walliman, Domain of Science: https://ve42.co/DoS Henry Reich, Minutephysics: https://ve42.co/MP My Patreon supporters Additional music from https://epidemicsound.com "Observations 2"