Re: Speed limit at C a misconception?

In article <11gdap6lor3bc47@xxxxxxxxxxxxxxxxxx>, Skywise wrote:
>http://www.livescience.com/technology/050819_fastlight.html
>
>"Scientists Mess with the Speed of Light"
>
>Ok, so I'm reading this article and they're talking about
>making light go faster than the speed of light in optical
>fibers. I'm thinking they've got to be talking about phase
>velocity, which they eventually do. But there's this one
>paragraph that has me puzzled,
>
> "Light in a vacuum travels at approximately 186,000 miles
> per second, but a popular misconception is that, according
> to Einstein?s special theory of relativity, _nothing_ in the
> universe can travel faster than this speed."
>
>A "popular misconception"? uhhhh....when did this change?
>
>I can accept phase velocity going faster than light, but phase
>velocity isn't a "thing", as in a tangible physical entity. The
>cosmic speed limit applies to physical particles. Phase velocity
>isn't physical, is it?
>
>Educate me please! :)

In some optical fibers and waveguides, you have "phase velocity" and
"group velocity".

Ever see a wake of a passing boat while on the shore of a calm lake or
calm slow river? The wake often has a bunch of wavelets. The "phase
velocity" is that of the wavelets, and the "group velocity" is that of the
cluster of them, as in the wake. Often you can see wavelets materializing
at the trailing edge of the group, moving to the leading edge of the
group, then dying out.
Another thing you can usually see in this case is the group getting
wider and more blurred as it travels.

When the waves are electromagnetic waves, the group velocity does not
exceed C even when the phase velocity does.

Another thing: When phase velocity and group velocity differ, it
appears to me that usually the phase velocity varies with frequency.

- Don Klipstein (don@xxxxxxxxx)
.

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