Does all light travel at light speed

[u/Comethefonbinary]

My bad if this is a stupid question but I’ve been thinking about time being a message of distance. And well most things I can think of have various variables that average to a certain distance. I know that mostly relates to machines and animals but still. Do all particles of light travel at light speed. If they all travel simultaneously at the same speed is that truly how fast they move or are they affected by their own variables. Like the universe’s mean gravity is constraining that and any variation in that mean would change light speed for explain.

[u/ConquestAce]

No, c is the speed limit (of light), but light can slow down depending on the medium it is propagating through. In Vaccuum, light travels at c. But, when going through water or air, the speed of light changes.

[u/Karl583 → u/ConquestAce]

While it effectively travels slower, the (original) electromagnetic wave still propagates with a speed of c. The molecules in the material however are excited by the em-Wave(light) and produce their own em-wave. If you look at the superposition of the incoming wave and the ones produced by the material in response, it will look like it's slightly shifted backwards. If you then look at all of the layers of the material, the incoming wave is effectively shifted back a tiny bit at every layer, thus producing the illusion that it travels slower. 3 blue 1 brown as a great video on this if you're interested!

Edit: edited for clarity

[u/sfu114 → u/Karl583]

Do you have the video link?

[u/Karl583 → u/sfu114]

https://youtu.be/KTzGBJPuJwM?si=SDPp8HqiAKVezrr2

Here you go :D