Black hole · Frequently asked questions
Short, honest answers to the questions everyone asks. For the full derivations with equations, each answer links to the equations page.
Is it a real simulation or a graphical effect?
It is a real simulation. For every pixel we numerically solve the metric's exact null geodesic (Schwarzschild, and Kerr with spin): gravitational lensing, the photon sphere, the shadow and the photon ring all emerge from the general-relativity calculation, not from tricks. We trace rays from the camera backwards, but light paths in GR are reversible, so the image is exactly the one real light would produce.
Read more →Does the black hole's mass affect the spacetime grid?
No, and it correctly shouldn't. The geometry, measured in Schwarzschild radii rₛ, is identical for every mass: black holes are scale-invariant. Mass only changes the absolute scale (rₛ being 30 km or an astronomical unit) and the disk colour. What does deform the grid is spin: frame-dragging twists it.
Read more →Is the spin the true Kerr metric or an approximation?
It is the exact Kerr metric. With the Spin slider we integrate the exact Kerr null geodesics in real time (in Cartesian Kerr–Schild form, with no coordinate singularity). The asymmetric shadow, the displaced and flattened photon ring and the dragging of inertial frames all emerge — the same metric as Interstellar's Gargantua.
Read more →Does the accretion disk change with spin?
Yes. The inner edge is the prograde Kerr ISCO (Bardeen's formula): it shrinks toward the hole as spin increases. The Doppler and redshift follow the Kerr metric, and now the radial flux profile is the exact Kerr Page–Thorne flux computed per spin (the hot region tightens toward the smaller ISCO as spin grows). Only the disk's absolute luminosity is held fixed across spin: the real efficiency increase, up to ~100× the peak flux at extreme spin, would simply clip to white.
Read more →Are the star and black-hole sizes to scale?
No: in the playground they are compressed so both are visible. The real ratio depends entirely on mass: around a stellar-mass hole (~10 M☉, rₛ≈30 km) a star is thousands of times larger than the horizon; around a supermassive one (Gargantua) the horizon dwarfs any star.
Read more →Is there real plasma physics (magnetic fields, synchrotron)?
No. The disk emits as a blackbody at its temperature, with procedural turbulence for the gas structure; magnetized plasma, synchrotron emission and polarization are entirely absent — they would need a GRMHD solution, feasible only offline. The thermal side and the relativistic radiative transfer (redshift, beaming) are correct.
Read more →Is it identical to Interstellar's black hole?
It is the same Kerr metric. The difference is the computation: Interstellar's Gargantua was ray-traced offline (hours per frame); this runs in real time in the browser. Interstellar's disk is an artistic model too (no plasma), like ours.
Read more →Can I trust the equations?
Yes for the lensing and the orbits: integrating the geodesics (null and timelike) is exact and reproduces the photon sphere, Einstein ring, shadow, ISCO and periastron precession. Everything approximated — disk profile, procedural turbulence, stylized jets, un-lensed playground bodies — is stated openly.
Read more →What is the difference between the «Orbits» demo and the «Playground»?
The «Orbits» demo integrates the exact Schwarzschild timelike geodesic for a single body (exact precession and ISCO, with a conservation diagnostic). The Playground uses the Paczyński–Wiita pseudo-Newtonian potential, which reproduces strong-field effects but allows mutual N-body gravity and tidal disruption — an exactness/interactivity trade-off.
Read more →Why is space black instead of full of bright stars?
It is Olbers' paradox: the deep sky is nearly black and only discrete stars glow. We keep a near-zero floor for exactly this reason — and the background light you see around the hole is genuinely lensed by the curvature.
Read more →Why does it sometimes slow down on mobile?
Ray-tracing Kerr geodesics is heavy: each pixel integrates the photon's path, and the fragment shader runs on the phone's GPU. If it stutters, lower the Quality preset (Medium or Low): it reduces the integration steps and the resolution.
How did you choose the disk colours?
They are not invented. Each ring of the disk has a temperature (from the Page–Thorne flux via Stefan–Boltzmann) and we show the true blackbody colour of that temperature (Planckian locus → sRGB), then shifted by gravitational redshift and relativistic Doppler.
Read more →This lab is free, ad-free and built on real physics. If it is useful to you, you can buy me a coffee.
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