Research · Research notes
Doppler beaming on a black hole's disc
BlackHoles.fyi draws real black holes from the physics of light and gas near them. These are our working notes on one part of that picture: what we found while drawing it, and where the research is still unclear.
A black hole that is feeding, pulling in gas from around it, has a disc: a flat ring of hot gas that circles it at a good fraction of the speed of light. The side coming towards the camera looks brighter and bluer, the side going away dimmer and redder: Doppler beaming. The site follows a ray of light back from each pixel to the gas it came from, and works out from that ray's path how far its light is shifted in colour and brightness, so the effect comes from the physics rather than being painted on. How strongly it shows on screen is a separate question: the last step, turning the picture's light into screen brightness, can flatten it.
How much brighter the near side looks depends on how hot the gas is. Cooler gas glows mostly in infrared, so the shift moves a lot of its light into or out of the colours we can see; hotter gas already glows across them, so the shift changes it less. Temperatures here are in kelvin (K), the scale that starts at absolute zero; the Sun's surface is about 5 800 K. Here is how much brighter, in visible light, the side coming towards you looks than the side going away, for the gas nearest the hole at three temperatures:
- 4 800 K: about 1 000 times
- 30 000 K: about 5.5 times
- 127 000 K: about 3.2 times
On screen the difference is smaller. In the pictures we measured, the near side showed about 3.4 times as bright as the far side, and since early November 2026, when we changed how the picture's light is mapped to screen brightness, about 1.7 times. That is a choice about the look, not the physics, so we've decided to add a control to the picture for how strongly the effect shows.
How strong the effect should look is still open. One idea we haven't taken up comes from A Slower Speed of Light, a game made at MIT's Game Lab, which slows light down so the shift is impossible to miss.
Updated 8 November 2026