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Black Holes and Gravitational Redshifting of Emission

-- | October 18, 2015

Question: What do black holes look like on radio telescope images? Specifically how does a visible light image of a black hole compare to a radio telescope image of the same black hole?

I’m curious whether gravitational time dilation and redshifting causes visible light to change into radio frequency EM energy, and therefore not be visible. What happens to visible light when it is redshifted too much? Can it turn back into visible light when it leaves the gravity field? Is there a frequency at which there is a required energy to quantize the photons back into visible light? — Ray

Answer: Let me first answer your first question about what black holes look like when measured with a radio telescope. In fact, there are no direct measurements that we can make that tell us what a black hole looks like. Black holes are always measured through indirect means, such as measuring objects in their gravity field that tell us about the black hole’s existence. For example, at radio wavelengths we can measure emission from the water molecule that traces the gas in the disk around a black hole. By measuring the velocity of this water emission we can infer the mass of the black hole.

As for your second question about gravitational time dilation and redshifting, the expansion of the universe does shift emission at shorter wavelengths down to longer wavelengths, such as radio waves. The redshifting of EM waves does not wrap around back to shorter wavelengths, though. The shifting to longer wavelengths monotonically approaches infinite wavelength.

Jeff Mangum

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