Massive objects traveling near the speed of light

  • Thread starter pete5383
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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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pervect
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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?
The short answer is that the limit is 'c', and has nothing to do with the speed of light in the medium.

The longer answer would point out that there is actually no need to invoke dynamics (forces) or "relativistic mass" to explain why obects cannot exceed 'c'. It is a consequence of how velocities add, a purely kinematical issue that doesn't need any dynamical concepts (i.e. mass).
 

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