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Re: Semi-on-topic: Light that travels faster than the speed of light?
- From: David Hagel
- Date: Mon Aug 22 11:06:31 2005
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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
" 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, email@example.com <firstname.lastname@example.org> wrote:
> No, they were actually over the speed of light for a "portion of the
> "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
> email@example.com http://3.am