Two of the most popular subjects in space art are things nobody can visit and, until recently, nobody had seen. Black holes and wormholes come out of the same theory, Einstein's general relativity, and they are often confused. One is an established fact of astronomy. The other is a possibility on paper. This post explains both, and what the pictures get right.
What a black hole is
A black hole is a region of space where gravity is so strong that nothing can get out, including light. It forms when a great deal of matter is squeezed into a very small volume.
The usual way this happens is the death of a large star. A star shines by burning fuel in its core, and the pressure from that burning holds it up against its own weight. When the fuel runs out, the core collapses. If the star was heavy enough, nothing can stop the collapse, and a black hole is left.
The parts of a black hole
- The event horizon. The boundary. Outside it, escape is possible. Inside it, every path leads inward. It is not a surface you could touch. It is the point of no return.
- The singularity. The centre, where the theory says the matter is crushed to a point. Physicists think this is a sign the theory is incomplete there.
- The accretion disk. Gas and dust falling towards a black hole don't drop straight in. They spiral round it in a flat disk, rubbing together and heating to millions of degrees. The disk glows fiercely. This is the bright part.
- Jets. Some black holes throw out narrow beams of matter from their poles at close to the speed of light.
So a black hole itself is dark, and its surroundings can be among the brightest things in the universe.
How big are they?
They come in at least two kinds. Stellar black holes, made from dead stars, are a few times to a few dozen times the mass of the Sun, and only miles across. Supermassive black holes sit at the centres of galaxies and are millions or billions of times the mass of the Sun. Our own galaxy has one, called Sagittarius A*, about four million times the Sun's mass.
Two myths
- They don't suck things in like a vacuum cleaner. From a distance, a black hole pulls no harder than a star of the same mass. If the Sun were replaced by a black hole of equal mass, the Earth would keep to its orbit. It would be very cold.
- They aren't holes. A black hole is an object, a concentration of mass. It looks like a hole because no light comes from it.
What a black hole looks like
For a long time this was guesswork. Then, in April 2019, a team called the Event Horizon Telescope released the first picture of one: the supermassive black hole in the galaxy M87, about 55 million light years away. They had linked radio telescopes across the world to act as one dish the size of the Earth. In 2022 they published an image of Sagittarius A*.
Both pictures show the same thing: a fuzzy ring of orange light round a dark centre. The dark centre is called the shadow. The ring is light from hot gas, bent round the black hole by its gravity. The orange was chosen by the scientists to show brightness. The telescopes recorded radio waves, which have no colour.
Gravity bends light, an effect called gravitational lensing. Near a black hole it is extreme. Light from the far side of the disk is bent up and over the top, so you see the back of the disk as an arch above the shadow and another below. This is the look of the black hole in the 2014 film Interstellar. The physicist Kip Thorne worked with the film's effects team to calculate it, and the result was close to what the real images later showed.
What a wormhole is
A wormhole is a hypothetical tunnel through space and time, joining two distant places by a short cut. Picture a sheet of paper with two dots far apart. Fold the paper so that the dots touch, and push a pencil through. The pencil hole is the wormhole.
The idea comes from the same equations as black holes. In 1935, Albert Einstein and Nathan Rosen found a solution that linked two regions of space by a bridge. It is still called an Einstein-Rosen bridge. The name wormhole was coined by the physicist John Wheeler in the 1950s.
The catch
- Nobody has ever observed one. There is no evidence that wormholes exist.
- They would collapse. The simple kind pinches shut too fast for anything to pass through, even light.
- Holding one open would need a kind of matter with negative energy, sometimes called exotic matter. Nothing like it is known in the amounts required.
So a wormhole is allowed by the mathematics and unsupported by observation. It is a serious idea, and physicists study it. It may also be nothing more than an idea.
What would one look like?
Probably not a swirling tunnel. Calculations suggest the mouth of a wormhole would look like a sphere, a ball in space. Looking into it, you would see a distorted view of the place at the other end, like a scene in a glass globe. The tunnel of light that films show is an invention. It is a very good one, because it shows motion and distance at a glance.
The difference in one line
A black hole is a place things fall into and don't come out of. A wormhole would be a passage things go through and come out of somewhere else. The first is observed. The second is not.
How artists draw them
Artists were drawing black holes long before anyone had a photograph. A few conventions grew up.
- The spiral. A whirlpool of light turning into a dark centre. It shows matter circling inward, which is true of the accretion disk, and it gives the picture movement.
- The ring. A bright circle round a black disc. This is closest to the real images.
- The tunnel. For wormholes: rings or streaks of light running towards a vanishing point.
- Colour. Real accretion disks are so hot that they would look white or blue-white. Artists use orange, purple, pink and blue, because colour shows heat, energy and strangeness.
Our pieces
- Black Hole. Silver and white on black. It leaves colour out, which is close to the truth: a real disk would be nearly white.
- Purple Pink Wormhole. A spiral of purple and pink turning into a black centre. It follows the tunnel convention, in synthwave colours.
Neither is a diagram. They are pictures made for a wall, drawing on the science and on fifty years of space illustration.
Why we like looking at them
A black hole is a limit: of light, of knowledge, of what can be seen. A wormhole is a door. One is the end of the road and the other is a way out. People have always been drawn to pictures of the edge of the known world, and these are the current ones. On a wall, they also happen to be strong shapes: a circle, a spiral, a point that pulls the eye in. See what a nebula actually is for more of the science, and cosmic and celestial canvas art for more of the art.
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