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Can gravity affect the speed of light?
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The speed of light is something measured with a local apparatus
in an inertial reference frame, using the same meter stick and clock.
A gravitational field has zillions of such 'locally inertial reference
frames' which are described by freely-falling observers for short
intervals of time and small regions of space. In all of these tiny
domains, an observer would measure the same velocity for light as
guaranteed by special relativity. To ask what the speed of light
is over a domain where gravitational forces make a reference
frame 'non-inertial' and not moving at a constant speed, is an
ill-defined question in special relativity. As soon as you try
to measure the speed of such an impulse, you would be using a
clock and a meter stick which would not be the 'proper time and
space' intervals for the entire region where the gravitational field exists.

Gravity can affect the speed of light. If you measure the speed
over a large enough region that special relativity and its requirement of a
flat spacetime is not satisfied. In the presence of curved spacetime, conventional
local measurement techniques do not work and so you cannot define the speed of light
in exactly the same way that you do under laboratory conditions in 'flat' spacetime. In fact,
in curved spacetime even the concept of conservation of energy is not
easily efined because the curvature of space itself changes the definition. Conservation of energy only works in flat spacetime.

**Return to Dr. Odenwald's Gravity
page at the ***Astronomy Cafe Blog*.**
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