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# What If The Speed of Light is Not Constant?
- URL: https://www.cascadepbs.org/show/pbs-space-time/episode/what-if-the-speed-of-light-is-not-constant-wqbiph/
- Published: 2024-04-02T23:12:07.000Z
- Updated: 2025-08-21T13:28:45.000Z
- Description: Let’s explore how constant this speed of light fundamental constant really is.
- Author: Cascade PBS Staff
- Tags: PBS Space Time, Season 9, Episode 19, #ZZZ_Episode_172eb3a2-063c-41aa-ae85-ce3cf34c7a28, #episode, #ZZZ_Show_ae70f836-13b6-4c9d-b61f-99f7b8bc29eb, #v1.16

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? 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.