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C decay.
In order to observe this effect, an extremely long travel path for neutrinos and photons is needed. The supernova SN1987A provided such a path—it is 186,000 light years from Earth. (So the supernova actually occurred 186,000 years ago.) There are theoretical reasons to believe that the flash of light would have been delayed about 3 hours from the neutrino burst, since the photons had to pass through the exploding star, interacting with it, whereas the neutrinos had no such interaction. But the observation of the light flash was 7.7 hours later than the neutrino burst, indicating that it took 4.7 hours longer to travel to Earth. This is consistent with the predicted slowing of photons due to vacuum polarization.
In order to observe this effect, an extremely long travel path for neutrinos and photons is needed. The supernova SN1987A provided such a path—it is 186,000 light years from Earth. (So the supernova actually occurred 186,000 years ago.) There are theoretical reasons to believe that the flash of light would have been delayed about 3 hours from the neutrino burst, since the photons had to pass through the exploding star, interacting with it, whereas the neutrinos had no such interaction. But the observation of the light flash was 7.7 hours later than the neutrino burst, indicating that it took 4.7 hours longer to travel to Earth. This is consistent with the predicted slowing of photons due to vacuum polarization.
==Has the speed of light changed through the history of the universe?==
Briefly, no. There is no credible evidence for this. There is a simple logical argument. There are photons that have been traveling since near the beginning of the universe. They obey the formula
:<math> E = pc </math>
where E and p are the energy and momentum, respectively. If the speed of light had changed while a photon was in flight, either its energy or its momentum, or both, would have had to change. Exact conservation of energy and momentum are cornerstones of physics. If they weren't valid, the way the universe works would be radically altered.
Now there have been a few careful investigations of whether the [[fine structure constant]], α, could have changed slightly. These involve examination of spectra from very distant stars, and examination of the isotopic mix of the fission products from the Oklo event. But these results are speculative and far from convincing.
Some Fundamentalists and Creationists posit a serious decline in the speed of light over the eons, but these are typically in the context of a view of the universe that is radically different from the accepted view. For example, some suggest that the Earth was in some kind of "time dilation field", sometimes gravitationally caused, for some period of time. These theories are rarely found outside of Fundamentalist web sites<ref>http://creation.com/speed-of-light-slowing-down-after-all</ref>.
Theories of serious decline in the speed of light are often called [[C decay]]. One of the proponents of this, [[Barry Setterfield]], suggests the use of different time scales, an "atomic" scale and a "gravitational" one, to get the effect. He published graphs showing the consequent C decay, with the curves conveniently converging to the modern value, always just in time to stay within the ever-increasing precision of measurements, and then extrapolated backward over many orders of magnitude. Interestingly, his redefinition of the time scale meant abandoning the "Young Earth" theory and accepting an age of the universe close to the scientifically accepted value of about 13 billion years.
==Notes==
==Notes==