Why doesn't gravity change the speed of light? (Beginner)

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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.

This page was last updated June 27, 2015.

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.