help

Is the Speed of Light Changed by Gravity?

-- Dayne | April 17, 2020

Question:

Obviously I have a question; hoping you might have an answer.

My knowledge of astronomy is very slim. With that said, I was attempting to wrap my head around the world of gravity. While doing so, I came across a question that I haven’t been able to find any relevant information. I understand we can determine the distance of a light-year based on the speed of light and how far light would travel through a standard earth year. My question stems from more than one year though. When you’re speaking of light traveling over great distances, wouldn’t the light travel at different speeds depending on how strong, weak or non-existent the gravitational field is that the light is traveling through.

For instance, when measuring the distance between us (earth) and other celestial bodies, aren’t we viewing objects so far away that by the time the light from said objects reaches earth it would have traveled through many different “bubbles” of space and time. So if a black hole, for example, were to be near the path between the viewing objects and earth, wouldn’t the light travel significantly slower through the gravity surrounding the black hole. And thus lengthening the amount of time the light has been traveling from the perspective of the viewer outside of the gravity field. The effect would cause the object opposite of the black hole to appear much farther away than it physically is. Since we can’t see a black hole, then we would just assume the distance is actually that far away.

-- Dayne

Answer:

The short answer is no, the speed of light that you measure locally is unchanged by gravity. In Einstein’s General Theory of Relativity, space and time can be visualized as a four-dimensional construct that gets warped under the influence of gravity. If for example light travels from a distant star to Earth and passes by a black hole, the path of the light will get bent as it passes by the black hole, which will lengthen its travel time. The actual speed of light at your location, though, is unchanged.

-- Jeff Mangum

Facebook

Twitter

The National Radio Astronomy Observatory is a facility of the U.S. National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
	Founded in 1956, the NRAO provides state-of-the-art radio telescope facilities for use by the international scientific community. NRAO telescopes are open to all astronomers regardless of institutional or national affiliation. Observing time on NRAO telescopes is available on a competitive basis to qualified scientists after evaluation of research proposals on the basis of scientific merit, the capability of the instruments to do the work, and the availability of the telescope during the requested time. NRAO also provides both formal and informal programs in education and public outreach for teachers, students, the general public, and the media.
	The U.S. National Science Foundation (NSF) is an independent federal agency created by Congress in 1950 "to promote the progress of science; to advance the national health, prosperity, and welfare; to secure the national defense..."
	AUI collaborates with the scientific community and research sponsors to plan, build, and operate cutting-edge facilities. We cultivate excellence, deliver value, enhance education, and engage the public.