Experiments have unequivocally proven that both subluminal motion of light in a vacuum and superluminal motion (not necessarily in a vacuum) are possible. However the scientific community cannot react properly because Einsteinians bombard it with contradictory and confusing explanations, often involving "group velocity" and "phase velocity". The experimentalists themselves have to include such explanations in their articles - otherwise there would be no publication. Yet even "group velocity" and "phase velocity" turn out to be insufficiently confusing and Einsteinians resort to the ultimate weapon - the speed of some mysterious entity called "information" (as if Einstein had based his 1905 second postulate on the speed of information, not on the speed of light). This speed always gloriously conforms to Divine Albert's Divine Theory:
Robert W. Boyd, Daniel J. Gauthier, Controlling the Velocity of Light Pulses: "So why do laboratory results of fast light not necessitate the superluminal transfer of information? It is believed that the explanation lies in the distinction between Vg [group velocity] and the information velocity. The group velocity can take on any value. However, the information velocity can never exceed c and, according to many models, is always equal to c."
If it were not for the schizophrenic and confusing atmosphere created by Einsteinians, the inconstancy of the speed of light would be an obvious experimental fact:
"Spatially structured photons that travel in free space slower than the speed of light" Science 20 Feb 2015: Vol. 347, Issue 6224, pp. 857-860
"Physicists manage to slow down light inside vacuum [...] ...even now the light is no longer in the mask, it's just the propagating in free space - the speed is still slow. [...] "This finding shows unambiguously that the propagation of light can be slowed below the commonly accepted figure of 299,792,458 metres per second, even when travelling in air or vacuum," co-author Romero explains in the University of Glasgow press release."
"The speed of light is a limit, not a constant - that's what researchers in Glasgow, Scotland, say. A group of them just proved that light can be slowed down, permanently."
"Although the maximum speed of light is a cosmological constant - made famous by Einstein's Special Theory of Relativity and E=mc^2 - it can, in fact, be slowed down: that's what optics do."
"For generations, physicists believed there is nothing faster than light moving through a vacuum -- a speed of 186,000 miles per second. But in an experiment in Princeton, N.J., physicists sent a pulse of laser light through cesium vapor so quickly that it left the chamber before it had even finished entering. The pulse traveled 310 times the distance it would have covered if the chamber had contained a vacuum. Researchers say it is the most convincing demonstration yet that the speed of light -- supposedly an ironclad rule of nature -- can be pushed beyond known boundaries, at least under certain laboratory circumstances. [...] The results of the work by Wang, Alexander Kuzmich and Arthur Dogariu were published in Thursday's issue of the journal Nature."
Nature 406, 277-279 (20 July 2000): "...a light pulse propagating through the atomic vapour cell appears at the exit side so much earlier than if it had propagated the same distance in a vacuum that the peak of the pulse appears to leave the cell before entering it."
"Researchers at the University of Ottawa observed that twisted light in a vacuum travels slower than the universal physical constant established as the speed of light by Einstein's theory of relativity. [...] In The Optical Society's journal for high impact research, Optica, the researchers report that twisted light pulses in a vacuum travel up to 0.1 percent slower than the speed of light, which is 299,792,458 meters per second. [...] If it's possible to slow the speed of light by altering its structure, it may also be possible to speed up light. The researchers are now planning to use FROG to measure other types of structured light that their calculations have predicted may travel around 1 femtosecond faster than the speed of light in a vacuum."
"Speed of light broken with basic lab kit. Electric signals can be transmitted at least four times faster than the speed of light using only basic equipment that would be found in virtually any college science department. Scientists have sent light signals at faster-than-light speeds over the distances of a few metres for the last two decades - but only with the aid of complicated, expensive equipment. Now physicists at Middle Tennessee State University have broken that speed limit over distances of nearly 120 metres, using off-the-shelf equipment costing just $500. [...] While the peak moves faster than light speed, the total energy of the pulse does not. This means Einstein's relativity is preserved, so do not expect super-fast starships or time machines anytime soon."
"Light that travels... faster than light! [...] This is exactly what the EPFL team has demonstrated. Using their Stimulated Brillouin Scattering (SBS) method, the group was able to slow a light signal down by a factor of 3.6, creating a sort of temporary "optical memory." 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."
"Light Pulses That Travel Faster Than Light Created [...] The technique developed at NIST is called four-wave mixing, and it works by altering some parts of each individual light pulse. This makes the light move forward faster than it normally would when traveling through a vacuum. [...] The physicists explain that the new research does not violate Albert Einstein's theory on general relativity - which states that the speed of light in a vacuum is the fastest achievable in the Universe. They say that a sort of loophole exists in this theory. By careful tuning of the light source and advanced calculations, it is possible to nudge portions of the light pulses so that they arrive at their destination ahead or behind the main pulse. [...] With four-wave mixing, the NIST investigators produced laser pulses that arrived at their destination a full 50 nanoseconds faster than photons traveling through a vacuum."
"Light hits near infinite speed in silver-coated glass. A nano-sized bar of glass encased in silver allows visible light to pass through at near infinite speed. The technique may spur advances in optical computing. [...] In a vacuum the refractive index is 1, and the speed of light cannot break Einstein's universal limit of 300,000 kilometres per second. Normal materials have positive indexes, and they transmit at the speed of light in a vacuum divided by their refractive index. Ordinary glass, for instance, has an index of about 1.5, so light moves through it at about 200,000 kilometres per second. The new material contains a nano-scale structure that guides light waves through the metal-coated glass. It is the first with a refractive index below 0.1, which means that light passes through it at almost infinite speed, says Albert Polman at the FOM Institute AMOLF in Amsterdam, the Netherlands. But the speed of light has not, technically, been broken. The wave is moving quickly, but its "group velocity" the speed at which information is travelling is near zero."