# Massive objects traveling near the speed of light

## Main Question or Discussion Point

Hey everyone, I have a question that might be pretty obvious to everyone else, but I just want to be clear. Relativity says that nothing that has mass can travel at the speed of light because it will take an infinite amount of energy for it to accelerate to that. I think I get that...but I do have a question. If an object with mass is accelerating in a medium with an index of refraction greater than 1, is the speed of light in that medium the upper limit for how fast a massive object can travel?

For example, in a vacuum, the speed of light is about 3e8 m/s (right..?), so a massive object has to travel less than that. But in a medium that has an index of refraction of 2, making the speed of light 1.5e8 m/s, is the massive object now limited to traveling 1.5e8 m/s?

So, I guess what I'm trying to ask, is a massive object's velocity limited by c, or by the speed of light in the medium it's traveling through?

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