I don’t know. Is there any physical reason that would resist a photon from speeding up. It has been demonstrated that photons can travel slower than the speed of light, doesn’t this possibly indicate that light can also speed up.
The results of Arthur Stanley Eddington's 1919 solar eclipse experiment showed that massive gravity changes the direction of photons. Doesn’t this mean that as a photon approaches a mass, it is pulled faster by the warped timespace?
Are the only things that restrain a photon from traveling faster than the speed of light some hundred year old flawed theories?
Remember, those theories were made my scientists that thought the galaxy was the Universe.
I hope someone who knows more about this than me can answer this question, without relying solely on theory. I'm sure someone has looked into this, but I sure can't find it.
An upstart astrologer ... This fool wishes to reverse the entire science of astronomy; but sacred Scripture tells us that Joshua commanded the sun to stand still, and not the earth. ~ Martin Luther on Copernicus
I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use. ~ Galileo Galilei
All great truths begin as blasphemies. ~ George Bernard Shaw