First look
The idea, in plain words
This is the black hole at the centre of our own galaxy, in a real picture made from radio waves. It weighs as much as 4.3 million Suns. Nobody can go there, and nothing that fell in could send word back. But Einstein’s equations of gravity say what a fall would be like, and in 2024 scientists at NASA ran them on a supercomputer for a black hole of this size. What follows is what the sums say.
From far off you see a disc of hot gas, glowing, circling a round patch of black. The gas is the only thing that shines. The black hole bends its light so strongly that you see the far side of the disc as well as the near side, lifted up over the top. Close round the dark patch is a thin bright ring. It is made by light that went right round the black hole before coming out again.
In NASA’s fall the camera starts 640 million kilometres out and takes about three hours to reach the edge, swinging nearly twice round on the way. The edge is called the event horizon, and it is 25 million kilometres across. Nothing marks it. There is no wall, no jolt and no sign. The camera goes through and, from that moment, every way it could turn leads inward.
A friend watching from a safe distance sees something quite different. As you near the edge, your clock seems to them to run slower and slower. Your last wave takes for ever. To their eyes you never cross at all: your picture slows and seems to freeze just outside. This is why black holes were once called frozen stars. Both stories are true. Time itself runs at different rates for the two of you.
Inside, the end is quick. Gravity pulls harder on the part of you that is nearer the centre, so a falling body is stretched lengthways and squeezed sideways until it is pulled into a thread. Scientists really do call this spaghettification. In NASA’s fall it comes 12.8 seconds after crossing the edge. A small black hole is far worse. Its pull changes so sharply from place to place that it would tear you apart before you ever reached the edge.
There is a way to feel all this and come home. NASA worked out a second journey, in which an astronaut swings close round the same black hole without crossing and then climbs away. It is a six-hour round trip. When she is back on her ship she is 36 minutes younger than the friends who stayed behind. Nothing went wrong with her watch. Less time passed for her.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Black hole
- A place where so much mass is packed so tightly that nothing which goes in, not even light, can come out.
- Event horizon
- The edge of a black hole: the boundary from inside which nothing can get out.
- Singularity
- The point at the centre of a black hole where today's physics says density becomes endless.
- Accretion disc
- A flat, spinning disc of gas spiralling in towards a star or a black hole.
- Tide
- The stretching of one body by another's gravity, which pulls harder on the near side than on the far side.
- Supermassive black hole
- A black hole of hundreds of thousands to billions of times the mass of the Sun, found at the centre of a galaxy.
- Speed of light
- 299,792,458 metres per second in empty space, the fastest that anything can carry news from one place to another.
Where this comes from
- NASA, New NASA black hole visualization takes viewers beyond the brink (6 May 2024)
- NASA Scientific Visualization Studio, Black hole accretion disk visualization (2019)
- NASA, Anatomy of a black hole
- NASA, Black holes
- ESO, Astronomers reveal first image of the black hole at the heart of our galaxy (12 May 2022)
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.