supermassive black hole. These typically have a mass a hundred thousand to a billion times larger than our sun. They reside in the center of the galaxy, surrounded by a dense cluster of stars. Just as a dust-mote is knocked about by the tiny atoms surrounding it, the black hole is knocked about by the (relatively) tiny stars surrounding it. Obviously we can’t observe this motion in real time, but its effect is clearly measurable.
There is an important difference between dust-motes and black holes. For traditional Brownian motion, the atoms move very much like billiard balls. An atom moves freely through space until it collides with the dust-mote, the collision happens very quickly, and then the atom moves freely again. But stars surrounding a black hole do not interact like billiard balls. For stars and black holes, the interaction varies depending on how close a star is to the black hole. This means that while the billiard-ball type model for Brownian motion can be used to model stars and black holes, you also have to take into account how the black hole’s gravity affects the distribution of stars in the first place.
Typically, the Brownian motion of a black hole has been modeled by starting with a galaxy of stars in an equilibrium state, then adding the black hole to the model to see what happens. But fellow RIT faculty David Merritt and his team modeled a galaxy of stars in equilibrium with the central black hole from the beginning. What they were able to show was that this new approach makes a significant difference in your predicted outcomes. Essentially, the presence of the black hole means that closer stars have more kinetic energy on average than more distant stars, and these closer stars in turn create most of the Brownian motion of the black hole.
The reason this matters is that Brownian motion can be used to determine the mass of the black hole in the center of our Milky Way galaxy. Measure the distribution of stellar speeds near the center of our galaxy and you can determine the mass of the central black hole. Merritt and co. determined the mass of our galactic black hole to be about 1.2 million solar masses. Pretty big, but smaller than older measurements which gave a value of about 3 million solar masses.
All this from treating a huge black hole as a cosmic speck of dust.