How black holes work

We start sixty times further out than the edge of the hole, which for the black hole at the centre of our galaxy is still 760 million kilometres away. The black disc is where the hole has claimed space for itself. Inside its edge, the event horizon, gravity is strong enough that light can't climb back out, and nothing travels faster than light.

The glowing band is gas circling the hole. Look above it and you'll see the band again, arching over the top. When I first saw that I assumed it was a second ring. It's the back of the same disk, its light bent up and over the hole on the way to us.

I hadn't realised that anything becomes a black hole if you squeeze it small enough. Karl Schwarzschild worked out how small in 1915, weeks after Einstein published general relativity, while serving as a soldier on the Russian front.

Squeeze the Earth and watch the speed you'd need to escape from its surface. When it reaches the speed of light, you have a black hole.

I've switched the gas off and painted a grid on the distant sky. Near the hole the grid wraps into rings. That's gravitational lensing: the hole bends the light from everything behind it, so the sky near its edge is smeared into circles.

The thin bright edge around the shadow is the photon ring, light that went round the hole one or more times before escaping towards us. In principle it holds an image of the whole sky for every lap.

We find real black holes by the gas around them. As it spirals in it rubs against itself and heats to millions of degrees, hot enough to shine in X-rays. I've tuned the colours to visible light, but the shapes are what the equations give.

One side is much brighter. The gas there is moving towards us at around half the speed of light, which bunches its light up in our direction. The team behind Interstellar left this out because they thought a lopsided black hole would confuse audiences. Switch off Doppler beaming and you get their version.

This was the part I found hardest. Einstein's idea is that mass curves space and time, and everything simply follows the straightest path it can through that curved geometry. From the outside, that path looks like falling, or like an orbit.

The funnel is a picture of that curvature with one dimension taken away. Seen from above, the rings near the middle look close together, but measured along the surface they're much further apart. That extra distance is what curved space means. The white dot is you, twelve horizon radii out, and the gold one is a probe orbiting at six.

Far out, orbits behave as Newton expected. Closer in, an elliptical orbit stops closing on itself and swings round a little further each lap, drawing a rosette. Mercury does the same around the Sun, by 43 arcseconds a century, and explaining that tiny number was one of the first things Einstein's theory got right where Newton's couldn't.

At three horizon radii is the last stable orbit. Inside it, the slightest nudge sends you spiralling in, so once we pass it we'll need rockets to stay put. Launch a few probes and see which survive.

Light has no mass, so I'd assumed gravity couldn't touch it. But light follows the shape of space like everything else. Rays passing far away bend a little. Closer ones whip round the hole. At one and a half horizon radii, the photon sphere, light can in principle circle the hole forever.

Any ray aimed within 2.6 horizon radii of the centre is swallowed, which is why the shadow you saw at the start looked bigger than the horizon itself.

Gravity slows time. Hovering here, your clock ticks noticeably slower than one far away, and you don't need a black hole to measure it. GPS satellites sit 20,000 kilometres up where gravity is weaker, and their clocks gain about 38 microseconds a day on ours. Without correcting for that, your phone's position would drift by around 10 kilometres a day.

Drag yourself closer to the horizon and watch the far clock run away.

From here the black hole fills most of the sky. Every star in the universe, and the glowing gas, has been squeezed into the bright window beside it, the gas blueshifted into a blinding band.

Feet first, your feet are closer than your head and get pulled harder. That difference is a tidal force, the same thing the Moon uses to raise tides on Earth, and near a small black hole it stretches you into a strand. Astronomers really do call it spaghettification.

What I didn't expect is that bigger black holes are gentler. At Sagittarius A* you'd cross the horizon without feeling a thing, and there'd be no moment where you could tell you'd passed the point of no return. What happens after that, nobody knows.

In 2019 the Event Horizon Telescope, radio dishes across the planet working together as one telescope the size of the Earth, published the first image of a black hole: M87*, 55 million light-years away, with the mass of six and a half billion Suns. In 2022 it imaged Sagittarius A*, at the centre of the Milky Way.

Both look like this view from nearly above: a dark shadow inside a lopsided ring. The size of the shadow matches the equations Einstein wrote down in 1915, more than a century before anyone could take the picture.

If you want to keep going, Brian Cox has a short piece on how black holes could unlock the mysteries of the universe and a longer interview on the unsolved paradox of what happens to information that falls in. His book with Jeff Forshaw, Black Holes: The Key to Understanding the Universe, goes further still.

Everything is unlocked. If something looks strange, it's probably physics, though it could be a bug in my code.