Black hole raytracer

What's this?

What you can do

• Point and drag to change the viewpoint around the black hole. Pinch or scroll to zoom.
• Play around with turning on the other checkboxes to render various toy objects around the black hole, including stars, planets, and reflective/transparent spheres. These objects are totally unphysical, but they help demonstrate that this is a real raytracer.
• Rotate the viewpoint until an object is directly behind the black hole to see an Einstein ring.
• Experiment with the Curvature slider to see how the strength of the curvature affects the bending of light around the black hole. This is not changing the mass of the black hole, but rather changes the geodesic equation to have less curvature, so that, at the other extreme, you can see what happens if the black hole didn't curve spacetime at all.
• Unfortunately the simulation does not allow you to travel past the event horizon. The raytracing would produce mostly nonsensical results, and not really worth attempting here.

What you're seeing, physically

• This simulation shows the gravitational lensing effects caused by a black hole, done using real-time ray tracing of light rays along the geodesic paths that light would follow around a Schwarzschild black hole (the simplest possible black hole: non-rotating, non-charged, spherically symmetric), as predicted by general relativity.
• Strictly speaking, what you see is an instantaneous snapshot of how light rays are bent by the presence of the black hole. Even though you can move around and change the viewpoint, it should not be interpreted as what you would actually experience at this distance from a real black hole, with time flowing normally. It's the view of an observer hovering motionless next to the black hole, frozen in time: nothing in the scene orbits or falls in. A real observer hovering there would need to fire their rockets constantly, which near a small black hole means an enormous acceleration, and would experience effects like time dilation, blueshifting, and tidal forces. This is currently outside the scope of this visualization.
• The accretion disk is also extremely simplified: a real accretion disk would be moving at relativistic speeds around the black hole, so the portion of the disk moving towards the observer would be significantly blueshifted and brighter, while the portion moving away would be redshifted and dimmer. This is why the real images from the Event Horizon Telescope appear asymmetric.
• The dark region is technically not the event horizon, but rather the "shadow" of the black hole. Seen from far away, it appears about 2.6 times the horizon's radius, because light passing nearby is bent around it. Its edge is determined by the photon sphere, at 1.5 horizon radii, where light can orbit the black hole in unstable circles: any light that crosses it heading inward can never get back out. Turn on "Checkered horizon" to see that the whole shadow is really a magnified, wrapped-around image of the horizon.
• Nevertheless, the simulation provides an accurate depiction of how light is bent by the black hole's gravity, and can be useful for building intuition for how gravity affects light at the extremes.