# Speed of Light in Cesium Vapors

1. Aug 27, 2010

### Ralph Spencer

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2. Aug 27, 2010

### bcrowell

Staff Emeritus
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.

3. Aug 27, 2010

### Ralph Spencer

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

Relativity postulates that speed of light is same for all observers. How can speed of light change?

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4. Aug 28, 2010

### DrGreg

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.

5. Aug 28, 2010

### starthaus

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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6. Aug 28, 2010

### bcrowell

Staff Emeritus
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.

Last edited by a moderator: Apr 25, 2017