Is the Speed of Light Really Constant?

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If photons can follow a curved trajectory, may they be considered to accelerate in a (generally) relative manner?
 
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In a vacuum photons travel along geodesics, to be specific null-geodsics, which can be considered 'straight lines'. If the presence of an intermediary mass causes a light ray to be deflected, the path of the photon is still 'straight', it just happens to be along a surface of space-time that itself is curved by the presence of the gravitating mass.

So in this case the photon is not said to 'accelerate'.

Garth
 
It's a theory, I don't think anyone can agree on it.

Check out this link http://van.hep.uiuc.edu/van/qa/section/Light_and_Sound/Properties_of_Light/20040223072254.htm
 
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I believe that in GR the definition of a null Geodesic is the path traveled by light. So in that sense it is impossible for light to NOT follow a null geodesic. If you were able to travel with a photon you would never observe it to vary in its velocity. However if we consider our frame stationary and observe the path of a photon it can appear to vary its velocity. This is observed and called a gravitational lens. Also it was the supposed observation of the variation of the position of star near the limb of the sun during a solar eclipse which was the first verification of GR.
 
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