Concepts explained
Where Light Cannot Leave
Make something massive enough and packed tightly enough, and leaving it would require going faster than light. Nothing is, so nothing does.
How hard depends on how massive the body is and how tightly it is packed. Same mass, smaller ball, and leaving the surface is harder.
So imagine something so massive and so tightly packed that leaving it would require going faster than light.
There is a boundary. Cross inward and you do not come back. It is called the event horizon, after the way a horizon hides what lies beyond. It is not a wall — it is empty space, and crossing it feels like nothing at all.
How do you photograph the invisible?
No light leaves it, so you cannot photograph it. You photograph around it.
Matter spiralling in whirls at enormous speed, heats up and shines fiercely. What gets captured is a black hole punched through the middle of that bright background.
In the picture released in 2019 — dark centre, bright doughnut rim — what we saw was not the black hole. It was the hole it leaves in the light.
Does truly nothing escape? In the 1970s someone calculated that a black hole emits a very faint something and shrinks, exceedingly slowly. It has not yet been observed.
The question that remainsIf light is the only messenger, is the inside of a place light cannot leave 'unknowable' to us, or simply 'not there'?