There are three basic elements to a black hole explained by general relativity. There’s the singularity itself, at the heart of a black hole and made of stellar matter compressed to an infinite density. Outside the singularity is the black hole’s interior space, a region that’s hard for even the most learned astronomers to imagine, where the rules of physics as we know them get bent and broken, and where space and time are stretched and compressed like putty.
If you’ve been zooming about interstellar space in your impossibly fast spacecraft, stumbled upon a black hole and entered its interior space, then it’s already too late for you because you’re past the third component of a black hole: its event horizon.
Also known as the Schwarzschild radius (after German physicist Karl Schwarzschild), it’s the part that lets us know where black holes are by outlining them in black. It marks the point of no return for anything falling beyond it, as to re-cross it would require travelling faster than the speed of light, which – as far as we know – is impossible.
Event horizons aren’t solely attributed to black holes – they’re just a noteworthy phenomenon that possesses them. According to some theories governing the expansion of the cosmos, there are areas that won’t ever be observable because light will never reach us from them. So the boundary limit at which we can observe the universe is also referred to as an event horizon.