Black hole

How it works

A black hole curves spacetime so strongly that light near it does not travel in straight lines. The event horizon has the Schwarzschild radius rₛ = 2GM/c²; for a black hole of 10 solar masses that is about 29.5 km. At 1.5 rₛ lies the photon sphere, where light could circle on an unstable orbit. The fate of a ray depends on its impact parameter b, the distance by which it would miss the centre without gravity: below 3√3/2 rₛ ≈ 2.598 rₛ the ray crosses the horizon, above it the ray escapes.

rₛ = 2GM / c²photon sphere: r = 1.5 rₛcritical parameter: b = 3√3/2 · rₛ ≈ 2.598 rₛ

Try it yourself

1. Choose “Curved fly-by” and then “Capture”. Compare how the path bends when the ray aims closer to the centre.
2. Click “Two sides of the boundary”. Rays with b = 2.597 rₛ and 2.599 rₛ start almost identically, but the first ends beyond the horizon and the second escapes after a turn around the photon sphere.
3. Move “Black hole mass” from 10 to 100 M☉. The horizon radius grows tenfold, from 29.5 km to about 295 km, but the shape of the paths measured in units of rₛ stays the same.
4. Switch to the “03 · Black hole appearance” view. The NASA visualisation shows how gravity bends the light of the accretion disk, so its far side is visible above and below the black hole.

Model limitations

The demo computes light paths in a plane around a non-rotating (Schwarzschild) black hole. The geometric views are coordinate drawings, not the image a distant observer would see. Black hole spin, redshift and time delays are not shown, and the accretion disk is only a schematic layer.

Related experiments

• Gravitational lensing
• Special relativity
• Gravity and orbits