help

Speed of Light in a Gravitational Field

-- John | April 4, 2023

Artist's conception of a gamma ray burst

Question:

GRAVITY AND SPEED OF LIGHT

According to the General Theory of Relativity, time passes slower in areas of higher gravitational fields. I have been told that atomic clocks, one set on a floor and one set on a table, have confirmed this aspect of the GT.

The physics dept of a university at the equator measures the speed of light at noon when the Sun and Moon are overhead during an equinox. They get a value C1.
They repeat the measurement 12 hours later at midnight when the Sun and Moon are on the opposite side of the Earth getting C2. (Could also do this by measurements on Earth and the Moon.)

At noon, the gravity of the Sun and Moon would reduce the net gravitational field of the Earth at the university and, by the GT, time would pass faster. At midnight, they all combine to increase the gravity resulting in time slowing down.

Since the distance would be the same in both, would C1 be slower than C2. If C1 = C2 then the speed of light has changed.

And, isn’t the speed of light at the Event Horizon for a Black Hole zero?

-- John

Answer:

I believe that this question has been answered, in a rather general sense, in my answer to “Is the Speed of Light Changed by Gravity?“. The speed of light is always the same. It is the paths that light travels when moving through space which contains different objects (gravity fields), and the time that it takes to traverse those paths, that change.

-- Jeff Mangum

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.