Re: Semi-on-topic: Light that travels faster than the speed of light?

[David Hagel <david.hagel () gmail com>]

I asked about this article to someone who works on optical properties
of materials. Here's what he says (I don't pretend to understand
everything though):

" This is called superluminal propagation, and many groups have shown
it in different media; this one is in fiber.  However, this does not
violate anything apparently because it is only the leading edge of a
pulse, and information still cannot go faster than c.

 I have been trying to understand what Einstein actually said.
Apparently he said that "information" cannot be transmitted faster
than c.  Now light has a phase velocity which exceeds c all the time. 
The textbooks then say that it is the group velocity that cannot
exceed c.  But I found out while writing my book that even that is
possible near resonances.  Then I saw somewhere that "energy velocity"
cannot exceed c.  Well, I tried deriving that in a general medium and
cannot see why it is fundamentally impossible.  I asked around, and
one of my colleagues says that it is far more subtle than even
Einstein may have realized..it is the leading edge of a pulse (or
something like that) that can exceed c, but the whole pulse itself
cannot.  I really don't understand that part, and haven't found any
text describing it.  (Need to find one)."


On 8/22/05, up () 3 am <up () 3 am> wrote:
> No, they were actually over the speed of light for a "portion of the
> signal":
>
> "They were also able to create extreme conditions in which the light
> signal travelled faster than 300 million meters a second. And even though
> this seems to violate all sorts of cherished physical assumptions,
> Einstein needn't move over  relativity isn't called into question, because
> only a portion of the signal is affected."
>
> On Sun, 21 Aug 2005, Steve Brown wrote:
>
> > Okay, guess I should have read the article first, given the title is "Light
> > that travels faster than the speed of light"
> >
> > Steve
> >
> > > Perhaps they are referring to being able to vary the speed while it is
> > > below the speed of light. That is, slowing it down to 1/10th the speed of
> > > light, and then speeding it up to 1/5th the speed of light.
> > >
> > > Steve Brown
>
> James Smallacombe                     PlantageNet, Inc. CEO and Janitor
> up () 3 am                                                     http://3.am
> =========================================================================