First look
The idea, in plain words
A black hole has an edge, called the event horizon. It is not a solid surface. There is nothing there to touch. It is simply the line beyond which there is no way back.
Think of a river running towards a waterfall. Far upstream the water is slow and a swimmer can go wherever they like. Nearer the falls the water runs faster. At some point it runs faster than anyone could ever swim. Nothing marks that point, but once past it, every swimmer goes over.
At the event horizon, space itself is the river, and to get back you would need to move faster than light.
In 2019 astronomers published the first picture of this. It shows a glowing ring of hot gas round a dark middle. The dark middle is the shadow of a black hole 6.5 billion times as heavy as the Sun, in a galaxy about 55 million light years away.
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
- Event horizon
- The edge of a black hole: the boundary from inside which nothing can get out.
- Black hole
- A place where so much mass is packed so tightly that nothing which goes in, not even light, can come out.
- Singularity
- The point at the centre of a black hole where today's physics says density becomes endless.
- Redshift
- A stretching of light to longer, redder wavelengths.
- Tide
- The stretching of one body by another's gravity, which pulls harder on the near side than on the far side.
- Accretion disc
- A flat, spinning disc of gas spiralling in towards a star or a black hole.
- 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
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.