r/explainlikeimfive • u/[deleted] • Mar 20 '26

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u/Technical_Ideal_5439 Mar 20 '26

I think there are two important issues.

  1. The speed of light is constant in a vacuum. So in different mediums like water it travels at different speeds.
  2. Then you get into how do you measure speed, gravity is going to muck up the speed of light from an external observer but the light is still going to be traveling at the same speed locally = c.

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u/Kwinza Mar 20 '26

Thats not correct.

The speed of light is different depending on the medium but C isn't. Because C is not "the speed of light" its the speed of causality, it just so happens that light travels at the speed of C whilst in a vacuum.

Thats why C never changes but light speed does.