At the heart of galaxy NGC 4051 lies a black hole with a mass of 800,000 Suns. Around it swirls a blazing hot gas that shines brightly. This light doesn't reach us all at once: blue rays arrive first, then green, then red. This happens when light is born in different parts of the hot disk, like hearing the conductor's voice first, and then, with a delay, the sound of violins from the far end of the orchestra.
For two years, astronomers measured the brightness of NGC 4051 every day in seven colors. The delays between blue and red light turned out to be longer than expected. The infrared rays behaved especially strangely: they were delayed more than simple heating of the disk can explain. It seems there is something else between us and the black hole.
Scientists have proposed two explanations. First: the light passes through a cloud of cosmic dust, like a lantern through fog — the dust absorbs blue rays, so more red light reaches us. Second: the disk's light is joined by emission from distant gas clouds surrounding the black hole. This is like the conductor's voice being mixed with echoes from the hall's walls.
It's not yet clear which explanation is correct. But if dust is indeed to blame, astronomers will have to recalculate the brightness of many other black holes — perhaps they appear redder to us than they actually are. And that means we might have been estimating their distances with errors.
The delay of light from the near and far parts of the disk around a black hole can be several days, even though the disk itself is smaller than the Solar System.
In the movie 'Interstellar,' light from the accretion disk of the black hole Gargantua is also distorted, but there due to gravity, and here due to dust and gas.