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Is Light Red or Blue Shifted When it Bends Around a Black Hole?

-- Dan | October 6, 2020

Question:

If I shine a white light out into space, and it bends around a black hole and comes straight back to me so I can see it in my telescope, is the light red or blue shifted, or neither?

For sake of the thought experiment, let’s leave out cosmic expansion that would stretch it on it’s journey, and let’s say my frame of reference doesn’t move so we can leave out any doppler shift. I am purely interested if there is any effect on the light from going in and out of gravity wells. Does it return to its original wavelength (if viewed from the reference frame of its origination)?

-- Dan

Answer:

I believe that the light you see returning from the black hole will be redshifted, due to the General Relativistic prediction of a gravitational redshift that the light experiences near the black hole. You can read a nice description of a measurement of the gravitational redshift from the light of a star that passed near the black hole at our galaxy’s center in the article “Why Starlight Turns Red Escaping from Black Hole at Heart of Milky Way”.

-- Jeff Mangum

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