Question: How can you measure the mass of a black hole with radio waves? What is the mass of a typical black hole? — Ray
Answer: One can measure the mass of a black hole by tracing the motion of the gas and dust near the black hole that is being pulled-on by the mass of the black hole. This can be done at many wavelengths, such as by measuring the positions of stars which orbit near a black hole at optical wavelengths. At radio wavelengths, we can measure the emission from a specific type of emission from the water (H2O) molecule, which occurs at radio wavelengths, to trace the motions of the gas circling the black hole. Recently, the mass of a black hole in a nearby elliptical galaxy was measured by tracing the velocities of gas traced by the carbon monoxide (CO) molecule using the Atacama Large Millimeter Array (ALMA) telescope in Chile.