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One of the most fundamental physics facts is that the speed of light in a vacuum is constant for all observers. But can we really be sure that the speed of light wasn’t different in the past, or perhaps in other parts of the universe? In fact, variable speed of light theories have long been used to try to explain everything from dark energy to gravity itself. Let’s explore how constant this fundamental constant really is.

All Previous Episodes Referenced
   • The Speed of Light is NOT About Light  

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Hosted by Matt O'Dowd
Written by Fernando Franco Félix & Matt O'Dowd
Post Production by Leonardo Scholzer, Yago Ballarini, Adriano Leal & Stephanie Faria
Directed by Andrew Kornhaber
Associate Producer: Bahar Gholipour
Executive Producers: Eric Brown & Andrew Kornhaber
Executive in Charge for PBS: Maribel Lopez
Director of Programming for PBS: Gabrielle Ewing
Assistant Director of Programming for PBS: John Campbell

Spacetime is produced by Kornhaber Brown for PBS Digital Studios.
This program is produced by Kornhaber Brown, which is solely responsible for its content.
© 2023 PBS. All rights reserved.

End Credits Music by J.R.S. Schattenberg:    / multidroideka  

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00:00 Introduction
00:31 Light & Relativity
01:49 Is the Speed of Light Invariant?
05:13 First VSL Theory
06:43 VSL & The Horizon Problem
09:09 Moffat's VSL Proposal
09:46 Albercht & Mageuijo's VSL
10:37 Are VSL Theories Testable?
12:58 VSL & Refractive Index of the Universe
13:56 Is there VSL Evidence?
14:51 Comments