0 = non-spinning (Schwarzschild). Higher spin drags the ISCO — the disk's inner edge — inward (6M → 1.24M) and reddens the deep inner gas. Verified Kerr (Bardeen/Page-Thorne).
Off = fast Schwarzschild light-bending: spin reshapes only the disk, the shadow stays a circle. On = real per-pixel Kerr ray-marching: the shadow itself goes asymmetric (D-shape + frame-drag) and disk + lensed background stay consistent with it. Heavy (per-pixel ray-marching) — if FPS drops, lower render scale (System tab) for speed; physics is identical at any scale. Desktop only.
Inner edge sits at the ISCO — physics, not a setting. Its radius tracks the black-hole spin a★ (6M when non-spinning, down to ~1.24M near extremal).
Collimated plasma beams along the spin axis, relativistically beamed — the jet pointing toward you is far brighter (M87's one-sided jet). Spin-powered (Blandford–Znajek): raise a★ to fuel them; gone at a★=0.
A lensed, Doppler-beamed, gravitationally-redshifted companion star spirals in and is swallowed by the hole. As it nears the horizon its light bends around the shadow, beams brighter on the approaching side and reddens (gravitational redshift). “Drop it in” plunges it now.
Choose Black and switch off the disk and the hole vanishes — a black hole is visible only through lensed light and its disk.
Standard/High download a large image — you'll be asked first, with the size.
Below 1× trades sharpness for speed (handy for the heavy Kerr mode); 1× = native; 2–3× = supersampled, much sharper edges. Watch FPS in the HUD.
Integration accuracy of the lensed arcs near the hole (the shadow size is exact at every setting). Higher = smoother, lower FPS.
Volumetric accretion-disk quality — the biggest FPS lever. Lower it if the frame rate drops; raise for richer gas detail.
Drag = orbit around it · Shift+drag = look · ⌥/Alt+drag = move the black hole · scroll = zoom toward it. Background: NASA Milky Way by default (switchable in Modules), gravitationally lensed.
A black hole emits no light. Everything here is other light, bent by its gravity.
The shadow — the black disk. Not the event horizon itself: it's the silhouette of all the light the hole swallows, about 2.6× wider than the horizon. Any ray aimed within the critical impact parameter $b = 3\sqrt{3}\,M \approx 5.2\,M$ falls in and never returns.
The photon ring — the razor-thin bright rim hugging the shadow. Light that whipped around the photon sphere ($r = 3M$) one or more times before escaping, piling up right at the edge. It sits just outside the shadow — nothing escapes from within.
Gravitational lensing — the smeared, warped stars. Mass bends light, so the background sky is distorted and you can even see around the far side of the hole.
The accretion disk — the glowing gas (switch it on in Modules). It spirals inward and stops at the ISCO ($6M$), the innermost stable orbit — that's the disk's sharp inner edge.
The arcs over and under the hole — the disk's far side: its top face is lensed up over the hole and its underside beneath it. Tilt the disk to 90° (edge-on) for the iconic line-plus-two-crescents look.
One side brighter (Doppler beaming) — the part of the disk rotating toward you is relativistically boosted, so it blazes while the receding side dims.
Reddening (redshift) — light climbing out of the gravity well loses energy and shifts red; deeper in, time itself runs slow (watch $d\tau/dt$ in the readout).
The bent-light image is scale-invariant — a stellar-mass hole and a billion-sun one are identical in these units. Mass only sets the real size and clock: pick a preset (Sgr A*, M87*) to read the horizon in km and the orbital period.
Black-hole spin a★ — a rotating (Kerr) hole drags spacetime around with it. Turn on High-fidelity Kerr geodesics (Physics tab) and the shadow itself becomes asymmetric: a flattened "D" pushed to one side by frame-dragging. Spin also pulls the disk's inner edge inward (6M → 1.24M near extremal).
Relativistic jets — spin-powered plasma beams along the axis (Modules). Moving near light-speed, they're relativistically beamed: the one aimed at you blazes while its twin nearly vanishes — exactly why M87's jet looks one-sided.
Quantum & thermodynamics — pick a real mass to read the hole's Hawking temperature, Bekenstein–Hawking entropy (set by horizon area — the holographic principle), and evaporation time. Real holes are colder than the cosmic microwave background, so they grow rather than evaporate; a primordial micro-hole runs hot and is evaporating about now.
Drag orbits around the hole · Shift+drag looks around · ⌥/Alt+drag moves the hole · scroll zooms. The three tabs tune the physics, the disk, and rendering quality.
The physics follows the primary literature:
The real-time Kerr engine (Cartesian Kerr-Schild geodesics), the Page–Thorne disk, the jets, and the thermodynamics readouts are adapted, with thanks, from steeltroops-ai/blackhole-simulation (MIT). Background sky: NASA/ESA imagery.
A black hole emits no light of its own — you see it only by how it bends the light around it. Here that happens in real time: the real Milky Way, warped by gravity.
Five short labs explain what you're seeing — then you end up back here, understanding every detail.
Or read the quick guide →