Although we often refer to “the speed of light” as a singular value (c = 299,792,458 m/s), the truth is that when we see light, it is actually moving slower than this speed. This is because the light we see is not moving in a vacuum, but through our atmosphere instead. Light moves at different speeds through different mediums (like air), and, interestingly, different types of light are affected differently by different mediums.
A material’s refractive index determines how much it slows down the light that passes through it, with an index of 1 being equal to a vacuum and a higher value meaning that the light is slowed. What is strange is that some materials can have refractive indices lower than 1, and some even have negative values. This essentially means that the material absorbs light extremely quickly. But perhaps the most surprising result of mediums’ effect on light is that some particles are less affected by some mediums than light is—meaning that sometimes, particles can go faster than (slowed-down) light. Sadly, this does not mean we are any closer to time travel.