30th July 2007 - 02:31 PM
I do not know if the speed of light was considered according to this context on the Forum.
I found an interestin link :
It is at this point in the discussion that a consideration of light-speed becomes important. It has already been mentioned that an increase in vacuum energy density will result in an increase in the electrical permittivity and the magnetic permeability of space, since they are energy related. Since light-speed is inversely linked to both these properties, if the energy density of the vacuum increases, light-speed will decrease uniformly throughout the cosmos. Indeed, in 1990 Scharnhorst  and Barton  demonstrated that a lessening of the energy density of a vacuum would produce a higher velocity for light. This is explicable in terms of the QED approach. The virtual particles that make up the "seething vacuum" can absorb a photon of light and then re-emit it when they annihilate. This process, while fast, takes a finite time. The lower the energy density of the vacuum, the fewer virtual particles will be in the path of light photons in transit. As a consequence, the fewer absorptions and re-emissions which take place over a given distance, the faster light travels over that distance [49, 50].
However, the converse is also true. The higher the energy density of the vacuum, the more virtual particles will interact with the light photons in a given distance, and so the slower light will travel. Similarly, when light enters a transparent medium such as glass, similar absorptions and re-emissions occur, but this time it is the atoms in the glass that absorb and re-emit the light photons. This is why light slows as it travels through a denser medium. Indeed, the more closely packed the atoms, the slower light will travel as a greater number of interactions occur in a given distance. In a recent illustration of this light-speed was reduced to 17 metres/second as it passed through extremely closely packed sodium atoms near absolute zero . All this is now known from experimental physics. This agrees with Barnett's comments in Nature  that "The vacuum is certainly a most mysterious and elusive object...The suggestion that the value of the speed of light is determined by its structure is worthy of serious investigation by theoretical physicists."http://www.ldolphin.org/setterfield/vacuum.html