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What a black hole really looks like

A black hole bends the light around it, so what you see near one is distorted in some surprising ways.

A black hole doesn't give off light of its own. What you see is everything around it, seen through the strongest gravity in the universe. Light passing close by follows a curved path, so the black hole acts like a giant, badly made lens. Gravity Lab draws its black holes by following that bending exactly, one ray of light per pixel. Every feature below follows from that one rule.

The shadow

The event horizon is the point of no return. Inside it, nothing can escape, not even light. But the dark disc you see is larger than the horizon. At 1.5 times the horizon's radius is the photon sphere, where light can orbit the black hole in a circle. Any light that comes closer than that spirals in. Bending makes the resulting shadow look about 2.6 times as wide as the horizon itself.

This shadow is what the Event Horizon Telescope photographed: M87* in 2019 and Sagittarius A*, at the centre of our own galaxy, in 2022. Both appear as a dark centre inside a bright, lopsided ring, which is what the physics predicts.

The disk that wraps over the top

Many black holes are surrounded by an accretion disk: gas spiralling inward, heated to millions of degrees by friction. Look at a black hole from nearly edge-on and you see something strange. The disk seems to arch up over the top of the shadow and curl under the bottom.

You're actually seeing the back of the disk. Light from the far side, which the black hole should block, is bent up and over it toward you, and some is bent underneath. So you see the front of the disk crossing the shadow, plus the back of it rising behind. The physicist Jean-Pierre Luminet produced the first image like this, by hand calculation, in 1979. The film Interstellar (2014) made it famous with help from the physicist Kip Thorne.

The photon ring

Some light loops part of the way, or even all the way, around the black hole before escaping toward you. It piles up into a thin, bright photon ring right at the edge of the shadow. In Gravity Lab, the photon ring sits behind the near side of the disk, just as it does in reality.

Why one side is brighter

The gas in the disk orbits fast, at a sizeable fraction of the speed of light near the inner edge. Gas coming toward you looks brighter and bluer. Gas moving away looks dimmer and redder. This is relativistic Doppler beaming, and it's why real black hole images are lopsided. On top of that, light climbing out of the black hole's gravity loses energy, which makes it redder. This is gravitational redshift. The inner disk also orbits faster than the outer disk, so the bright streaks in it shear into spirals as they turn.

Lensed stars

The stars behind a black hole are lensed too. They're smeared into arcs, and a star almost exactly behind it becomes an Einstein ring around the shadow. As you move around a black hole in Gravity Lab, watch the background stars stretch and slide around it.

How Gravity Lab draws it

For every pixel, Gravity Lab traces the ray of light backward from your eye, step by step, bending it with the equation for light near a non-spinning (Schwarzschild) black hole. A ray that falls inside the photon sphere is drawn black. A ray that crosses the disk picks up the disk's light at that point, with the right Doppler shift and redshift. A ray that escapes shows the stars it reaches, after bending. It all runs on your graphics card many times per second. The Ultra quality setting uses more steps per ray for a sharper result.

Simplification: real black holes usually spin, which pulls the shadow slightly off-centre and changes the disk. The rendering uses the non-spinning case. The Theory Lab's Time near a black hole page uses the full spinning (Kerr) solution to work out how time slows down near one.

Real black holes to visit

Search for any of them with /. In Sandbox Mode you can make your own, from a micro black hole smaller than an atom to a supermassive one, and drop it into the Solar System.