Re: Speed limit at C a misconception?

don@xxxxxxxxxxxxxx (Don Klipstein) wrote in
news:slrndgdvsh.kvc.don@xxxxxxxxxxxxxx:

> 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)

I like your visualization, Don. Works well for the mathematically
challenged.

So far all the responses make sense to me and help alleviate
my lack of knowledge about phase and group velocities.

The thing that's still bugging me though is that phrase about
nothing going faster than light is a misconception. I suspect
this is an incorrect statement.

Even though the phase or group velocities might in and of themselves
be going faster than c, the light itself isn't going faster than c,
right?

If 'they' are arguing that something *is* going faster than c (the
group or phase velocity) and that therefore the statement that nothing
can go faster than c is false, I say 'they' are picking nits.

After all, are not concepts like phase or group velocity just
mathematical constructs? They aren't "real", right?

Brian
--
http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism

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Relevant Pages

  • Re: Speed limit at C a misconception?
    ... >>>I can accept phase velocity going faster than light, ... >> When the waves are electromagnetic waves, the group velocity does not ... "Group Velocity" refers to speed of a pulse of a cluster of waves! ...
    (sci.optics)
  • Re: Speed limit at C a misconception?
    ... >> exceed C even when the phase velocity does. ... phase and group velocity can be larger than c. ... signal at a rate exceeding C for distances more than a "wavelength" (or ... >a gaussian pulse. ...
    (sci.optics)
  • Re: Speed limit at C a misconception?
    ... > When the waves are electromagnetic waves, ... > exceed C even when the phase velocity does. ... phase and group velocity can be larger than c. ... If you send a gaussian pulse into a medium and the tail of that pulse is ...
    (sci.optics)
  • Re: Speed limit at C a misconception?
    ... >velocity, which they eventually do. ... >I can accept phase velocity going faster than light, ... The wake often has a bunch of wavelets. ... velocity" is that of the wavelets, and the "group velocity" is that of the ...
    (sci.optics)