Where light goes to bend
Why the first picture of a black hole is a glowing ring, not a black dot.
What you see above isn’t an illustration. It’s being calculated in your browser right now: for every pixel, a ray of light is traced backwards from your eye, bent by gravity step by step, until it either hits glowing gas, escapes to the stars, or never comes back.
Here’s how a region of space that gives off no light at all ends up as one of the brightest-looking things in the universe.
Gravity bends light
Einstein’s general relativity says mass curves the paths light follows. During the 1919 solar eclipse, astronomers measured starlight grazing the Sun and found it bent by 1.75 arcseconds, just as predicted. Around a black hole the effect is extreme. These paths are computed, not drawn.
Closer means a sharper turn
Light passing farther out is nudged slightly. Light passing closer is swung hard around. The black disc in the middle is the event horizon, and the rest of the picture is scaled in multiples of its radius.
The event horizon
Inside the horizon, nothing, not even light, can climb back out. Its radius depends only on mass: r = 2GM/c². Squeeze the Sun into a black hole and its horizon radius would be about 3 km. For Earth, about 9 mm.
The photon sphere
At 1.5 times the horizon radius, light can travel in a circle around the black hole. The orbit is knife-edge unstable: a ray aimed exactly right loops around before it either escapes or falls in.
The shadow
Any ray aimed inside a certain distance, about 2.6 horizon radii, is swallowed. So from far away you see a dark patch about 2.6 times wider than the horizon itself. That patch is the black hole’s shadow.
Add a disk of hot gas
Real black holes are often surrounded by gas spiralling inwards, heated until it glows. Here’s a thin disk reaching in to 3 horizon radii, drawn first as if light travelled in straight lines: just a flat ring around a dark ball.
Now let gravity bend the light
The far side of the disk, hidden behind the hole, appears lifted over the top. Light from its underside is bent under the bottom. A thin bright ring hugs the shadow: light that skimmed the photon sphere.
One side outshines the other
The gas near the inner edge moves at around half the speed of light. Gas rushing towards us is beamed and brightened. Gas moving away is dimmed. This is relativistic Doppler beaming.
Look from above
Now tilt the view until we look down almost along the black hole’s axis. We are thought to see M87’s black hole from close to its axis: jet measurements suggest within roughly 13–27°. The disk becomes a ring of light around a dark centre.
M87*, 2019
Blur it to the sharpness of an Earth-sized telescope and you get something like the first image of a black hole, published by the Event Horizon Telescope in 2019: a lopsided ring 42 microarcseconds across, surrounding a shadow, around a mass of 6.5 billion suns.
A shadow, not a hole
The EHT never photographed the event horizon itself. It photographed light bent around it. That’s why the picture is a ring. The ring’s size matched the shadow general relativity predicts for a black hole of that mass, another test Einstein’s theory passed.
Go deeperThe numbers behind the pictures
r_s = 2GM/c² · photon sphere = 1.5 r_s · shadow radius = √27·GM/c² ≈ 2.6 r_s
For a non-spinning black hole the innermost stable circular orbit is 3 r_s (6GM/c²), which is where our disk begins. The gas there circles at about half the speed of light. Light grazing the Sun is deflected by 4GM/(c²b) ≈ 1.75″.
EHT (2017 data): ring diameter 42 ± 3 µas. Mass (6.5 ± 0.7) × 10⁹ solar masses, assuming a distance of 16.8 Mpc (about 55 million light-years). The 2018 data gave a 43.3 µas ring, with the bright region shifted by about 30°.
Go deeperWhat this simulation leaves out
Our ray tracer models a non-spinning (Schwarzschild) black hole with an infinitely thin disk. M87* almost certainly spins, and its glowing gas is a thick, turbulent flow rather than a neat disk. The EHT team compared their data with large libraries of full simulations.
The sense of rotation (clockwise as seen from Earth, with the bright part at the bottom) is inferred by comparing the image with models, given the jet’s known orientation. Our final frame is styled to match, not fitted to the data.