Illustration of a supermassive black hole (bright yellow, centre) and a torus (ring shape, black) of dust surrounding it. Supermassive black holes are surrounded by an accretion disk (orange) made of material. Over time the disks get brighter and occasionally release flashes of light that travel away from the black hole toward the torus. The dust in the torus absorbs the light and releases infrared light. The torus of dust is said to be 'echoing' what happens to the disk. Astronomers propose a technique that measures distance using the time difference between the observations of the visible light (seen first) and the infrared (seen second). The luminosity of the disk can be used to calculate the distance to the galaxy by comparing it to the brightness of the galaxy as seen from Earth. This technique is called echo mapping or reverberation mapping.