[Etherlord87]

Light moves with `c` speed regardless of medium. Whenever we say light moves slower in a medium, we simply mean it is measured to be slower, it is macroscopically slower, it's as if having a hypothetical vehicle that, when it moves, it always moves with a constant speed, but you measure it by taking the time of departure in place A, time of arrival in place B, measure the distance |AB| on a map, and from that calculate the speed of the vehicle. Your measurement will be affected by exact path shape (which isn't a straight line), as well as the number of times the driver decided to take a break to sleep in a motel, eat something, go to a toilet on a gas station etc.

[jncfhnb]

This example seems… bad to me. Are you simply saying that light moves slower from A to B through a medium like water because it takes a path that is less direct to navigate the medium?

[felbane]

I agree, that answer is misleading. The way I've always understood it: light is an EM wave, and it interacts with medium that it travels through. When traveling through a vacuum, the "beam" source is the origin, but when traveling through a medium the "beam" is a propagation of emissions from the matter absorbing, oscillating, and re-emitting a photon. These interactions take (an extremely small, but nonzero) amount of time, but the light being absorbed and emitted always travels at c.

[withinboredom]

No. It actually goes slower. https://youtu.be/uo3ds0FVpXs?si=b7sDxNuQkTuwAkcP gives a pretty good overview.

Edit: wrong one https://youtu.be/yP1kKN3ghOU?si=hsBj0RpzOb3JZWdS the one above is the "why."

[dcow]

No. It propagates slower.

https://youtu.be/KTzGBJPuJwM?si=Pab2F0EFsB4oGhUo

[withinboredom]

I trust the quantum physicist more than a pop-sci YouTuber.

[dcow]

Both videos come to the same conclusion so your comment betrays the fact you didn't even watch the one I linked.

That or you misunderstood the physicist. You need to watch both videos to understand what's happening here.

The speed of light is not "slower in water". Light propagates more slowly through water. The subtle difference is not just pedantic semantics. It's the key to understanding how we don't have a paradox on our hands.

[Etherlord87]

The absorptions are the "breaks" in the analogy.