``...If the group of scientists who asserted that light has slowed down were right, it would have been a blow to one of the cornerstones of modern science...''
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Light slows down. How, when, where, by what? The above doesn't seem to say.
Looking around I found this (http://focus.aps.org/v3/st37.html):
``...In EIT [electromagnetically induced transparency] a laser manipulates the quantum states in an opaque cloud of atoms and makes them transparent to a narrow range of wavelengths of light. According to electromagnetic theory, this narrow transmission band leads directly to an index of refraction that depends strongly on wavelength in this range, although in a low-pressure gas it may not stray far from the vacuum value of one. (The index of refraction gives the `phase velocity'--the speed at which pure sine-wave light beams travel.) Several years ago researchers demonstrated that the wavelength-dependent phase velocity causes the light's `group velocity'--the speed at which energy and all signals travel--to slow by more than one hundred times...'' (Slow Light..., 06/29/1999, Focus, on a Phys Rev Lett. 42, 5229 article)
Is the Guardian piece discussing the same phenomenon? Only difference I can see would be that the Guardian article is referring to some (more recent?) astronomical observations that demonstrate the effect the paragraph above is discussing.
It may not overthrow Einstein, since you can argue that the constant velocity only holds for a vacuum, and obviously a dense gas is not a vacuum. And further, the speed of light as an upper limit to velocity, doesn't discount slowing of light as such.
On the other hand it would have profound effects on the cosmology. There was always an alternate school of argument against a Doppler induced redshift, that held the red-shift due to recession was apparent, and that apparence was due to aging effects of photons (slowing down) as they collide with atoms in the intervening space.
An empirical support for such aging effects could nullify the expansion theories, and put the cosmic background radiation (black body) support for a big bang into question.
Well, guessing, anyway...
Chuck Grimes