Veritasium - The Speed of Light

00:00 - Derek: The speed of light is the universe's speed limit. Nothing with mass can reach it. But here is what most people get wrong about it. 01:45 - Derek: Light does not actually travel at the speed of light through most materials. In water, it slows to about 75% of its vacuum speed. In glass, even slower. In a Bose-Einstein condensate, light has been slowed to just 17 meters per second. 04:00 - Derek: The really mind-bending part is that the speed of light in a vacuum, c, is not really about light at all. It is the speed of causality — the maximum rate at which any cause can have an effect elsewhere in the universe. 07:30 - Derek: Einstein showed that as you approach c, time slows down relative to a stationary observer. At exactly c, time stops. This is not a metaphor — it is measured with atomic clocks on planes and satellites every day. GPS satellites have to correct for this. 10:15 - Derek: One of the strangest consequences is that from a photon's perspective, no time passes at all. A photon emitted from a star 100 light-years away arrives at your eye having experienced zero elapsed time. 13:00 - Derek: So why can not we go faster? Because as you add energy to accelerate an object, its mass increases. To reach c, you would need infinite energy. The math literally prevents it.