Speed of Light in Cesium Vapors

Ralph Spencer
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The speed of light in a gas is very close to, but slightly less than, the speed of light in a vacuum. Depending on how you define "speed" (group velocity, phase velocity,...), it is possible to get materials in which the speed of light is greater than in vacuum. However, there is no way that would happen in a gas made out of neutral atoms. None of this has any implications for relativity.
 
If I misinterpreted what http://www.nature.com/nature/journal/v406/n6793/full/406277a0.html" experiment meant, could you please explain it?

The speed of light in a gas is very close to, but slightly less than, the speed of light in a vacuum.

Relativity postulates that speed of light is same for all observers. How can speed of light change?
 
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Ralph Spencer said:
Relativity postulates that speed of light is same for all observers. How can speed of light change?
Relativity postulates that speed of light in vacuum is same for all observers.

Sometimes the words "in vacuum" are omitted because it's assumed (wrongly) that everyone knows that.
 
Ralph Spencer said:
If I misinterpreted what http://www.nature.com/nature/journal/v406/n6793/full/406277a0.html" experiment meant, could you please explain it?

Do you understand the definition of "group" and "phase" velocity of light? Look at the abstract:

"In this situation, the group velocity of a light pulse can exceed c and can even become negative16, 17."

What does this sentence tell you?
What does mainstream physics refer to when it talks about light speed?

In order to help you understand what the paper is talking about, I have done a short writeup on phase and group velocity. The bottom line is that group or phase (not both at the same time) velocity can exceed c without violating relativity.
 

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Ralph Spencer said:
If I misinterpreted what http://www.nature.com/nature/journal/v406/n6793/full/406277a0.html" experiment meant, could you please explain it?

I stand corrected. I hadn't realized that it could happen in an ordinary atomic gas, rather than a metamaterial.

But in any case, it has no implications for relativity.

As DrGreg points out, constancy of c in relativity is constancy of the speed of light in vacuum.

Actually, it would have consequences for relativity if a signal could propagate at >c in a medium. There would then be frames of reference in which the signal was received before it was transmitted, leading to problems with causality. But cases like this one don't actually allow signals to propagate at >c. This is why it's important to make distinctions like group velocity versus phase velocity.
 
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