Principle of EquivalenceExperiments performed in a uniformly accelerating reference frame with acceleration a are indistinguishable from the same experiments performed in a nonaccelerating reference frame which is situated in a gravitational field where the acceleration of gravity = g = a = intensity of gravity field. One way of stating this fundamental principle of general relativity is to say that gravitational mass is identical to inertial mass. One of the implications of the principle of equivalence is that since photons have momentum and therefore must be attributed an inertial mass, they must also have a gravitational mass. Thus photons should be deflected by gravity. They should also be impeded in their escape from a gravity field, leading to the gravitational red shift and the concept of a black hole. It also leads to gravitational lens effects. 
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Advance of Mercury's Perihelion

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Advance of Mercury's Perihelion

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Gravitational Deflection of Light

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