Black Hole

The idea

Mass curves spacetime, and light follows that curvature. Near a black hole the bending is extreme enough that the geometry itself becomes visible: rays from the far side of the accretion disk are bent up over the top of the hole and down under it, so you see the back of the disk twice — arched above and below the central shadow. That double arc is not an artistic flourish; it is what a thin bright disk around a Schwarzschild black hole actually looks like, and it is why the Interstellar rendering and the Event Horizon Telescope images of M87* and Sgr A* share a family resemblance.

Two more relativistic effects shape the picture. The gas orbits at a good fraction of the speed of light, so the side coming toward you is beamed brighter and bluer while the receding side dims and reddens — that is why the ring in the EHT images is a lopsided crescent, not an even donut. And light climbing out of the gravity well loses energy on the way, so the innermost disk, deepest in the well, is dimmed and reddened again on top of everything else.

What to look for

• The thin bright circle hugging the shadow is the photon ring — light that circled the hole one or more times before escaping. Photons can (unstably) orbit at 1.5 Schwarzschild radii.
• The upper and lower arcs are two images of the same far side of the disk, one from rays bent over the hole, one from rays bent under it.
• One side of the disk is distinctly brighter: that is relativistic Doppler beaming, and it tells you which way the gas is orbiting.
• Orbit the camera and watch the lensed background starfield smear and wrap around the shadow — background objects near the line of sight are stretched into arcs.

Getting it right

• A black hole does not vacuum up its surroundings. Replace the Sun with a solar-mass black hole and the planets keep their orbits; far from the horizon, gravity is ordinary. Falling in takes losing angular momentum, which is hard.
• The dark shadow is not the event horizon. It is the capture cross-section for light — about 2.6 times the horizon's radius — so even the famous images show something bigger than the hole itself.
• The disk does not glow because the hole "burns" anything. Orbiting gas rubs against itself, friction heats it to millions of degrees, and hot gas shines. The black hole only supplies the gravity.

Turn the knobs

• scenario picks the two canonical viewpoints: 'interstellar' sits near edge-on, where the over/under lensed arcs dominate; 'eht' looks nearly face-on, where the shadow and photon ring dominate.
• inclination sweeps continuously between those extremes — watch the arcs fold down into a ring as you approach face-on.
• beta sets the disk's orbital speed. Raise it and the bright/dim asymmetry sharpens; set it low and the crescent relaxes toward an even ring.
• disk_outer grows or trims the disk, which changes how much lensed material wraps the shadow.