According to our current understanding of physics, nothing can travel faster than light in a vacuum. The speed of light in a vacuum is considered to be the universal speed limit and is approximately 299,792,458 meters per second (or about 186,282 miles per second).
However, it is possible for light or other electromagnetic waves to travel slower than their maximum speed in a dense medium, such as water. This phenomenon is known as the refractive index of the medium. The refractive index is a measure of how much the speed of light is reduced when it passes through a particular substance.
When light travels through a medium like water, it interacts with the atoms or molecules of that medium. These interactions cause the light to slow down due to absorption and re-emission processes. As a result, the effective speed of light in the medium is lower than its speed in a vacuum.
The refractive index of water is approximately 1.33, meaning that light travels about 1.33 times slower in water compared to its speed in a vacuum. This reduction in speed leads to phenomena like refraction, where light changes direction as it passes from one medium to another.
It's important to note that even though light slows down in a dense medium, it still remains faster than any other particle or object in that medium. The speed of light in water is still extremely fast compared to other objects, but it is slower than its speed in a vacuum.
To summarize, while light can travel slower than its maximum speed in a dense medium like water due to interactions with the medium's particles, it cannot travel faster than the speed of light in a vacuum.