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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

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:

For an overview of the one-way speed of light check out the wiki page:

The script was written in consultation with subject matter experts:
Prof. Geraint Lewis, University of Sydney
Prof. Emeritus Allen Janis, University of Pittsburgh
Prof. Clifford M. Will, University of Florida
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) (German)

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.

Response to Greaves et al. paper —
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.

A review article about simultaneity — Janis, Allen, "Conventionality of Simultaneity", The Stanford Encyclopedia of Philosophy (Fall 2018 Edition), Edward N. Zalta (ed.)

Will, C. M. (1992). Clock synchronization and isotropy of the one-way speed of light. Physical Review D, 45(2), 403.

Zhang, Y. Z. (1995). Test theories of special relativity. General Relativity and Gravitation, 27(5), 475-493.

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.

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:
Henry Reich, Minutephysics:
My Patreon supporters

Additional music from "Observations 2"

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