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
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