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Why doesn't gravity change the speed of light?

How come that the speed of light "c" doesn't change at all, even slightly, when the light passes closely to a star or some similar big object. We know that light bends in those situations, but what about "c"?

Yep, although light bends around a massive object like a black hole, the speed of that light in a vacuum is always the same. This is because the speed of light is directly dependent on the speed of the interaction between the electric and magnetic fields (light is an electro-magnetic wave, after all!). That speed of interaction is the same no matter where the light is or who is watching it. Therefore, the speed of light is the same for all observers at all points in space-time.

In other words, the light bends around the massive object, but the graduate students all along the light's path will always measure it moving at the same speed: the speed of light.

April 1999, Dave Kornreich (more by Dave Kornreich) (Like this Answer)

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