Can the Speed of Light Change or Increase?

[meteor]

Just for the record, I am not convinced about vgr being able to be vgr > c.

Just to add more to your confussion (not that I want to make you suffer, but...):
take a look at this experiment of Lijun Wang, where he reports a negative group velocity of -310c
http://physicsweb.org/articles/news/4/7/8/1

 

[Gonzolo]

Well I was just about to call 911 until I read :

"There is no widespread agreement among physicists about the speed at which information is carried by pulses in such experiments."

and :

"The team intend to analyse this further..."

Whenever I get the chance, I'll look into this further. Cool stuff.

 

[meteor]

I've been thinking today about this oddity called negative group velocity, then i',ve searched if there was such a thing called negative phase velocity and I've found this:
http://www.innovations-report.de/html/berichte/physik_astronomie/bericht-32117.html
"Whether viewed dimly through the haze and lights of a city or in all their glory in a pristine wilderness, the stars that surround the Earth are magnificent, and one day Earthlings will travel to some of the new planets that astronomers are locating. However, the stars we see are not necessarily where we think they are, according to an international research team.

"We know that the light from distant stars takes a very long time to reach the Earth," says Dr. Akhlesh Lakhtakia, distinguished professor of engineering science and mechanics, Penn State. "But, taking into account the distance a star will have moved while that light travels, we still may not be able to accurately locate the star.

Negative phase velocity media or materials with negative refractive index may be responsible for this locational uncertainty. Recently, materials researchers at the University of California San Diego, working with micro and nano materials, developed a metamaterial that had a negative refractive index for microwaves, proving that negative phase materials could exist at least in the microwave part of the electromagnetic spectrum. Their requirements for this material were that both the relative permittivity, a measure of the charge separation in a material, and the relative permeability, a measure of how electrons loop in materials, of a substance must be less than zero."
I'm glad that at least there's no such a thing called negative front velocity or negative signal velocity. at least according to google