Experiments performed in a uniformly accelerating reference frame
with acceleration a are indistinguishable from the same experiments
performed in a non-accelerating 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.
Source http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/grel.html#c1
Source http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/grel.html#c1