The Event Horizon
The event horizon is the edge of a black hole. It is the boundary where gravity becomes so strong that nothing inside can ever escape, not even light. There is no fence or wall to mark it. If you crossed it, you would not notice anything change at the moment you went over. But you would have just left the entire universe behind, with no way back.
- What is it?The point of no returnOnce you cross, you cannot come back
- ShapeA bubbleA sphere around the black hole
- How big?Depends on mass3 km per solar mass
- A 10-Sun BHapprox. 30 km acrossAbout the size of a small city
- Sgr A*approx. 24 million km acrossAbout 17 times the Sun's diameter
- First photographed2019Galaxy M87, by the Event Horizon Telescope
How big are event horizons?
The bigger the black hole, the bigger the event horizon. (km across)
A stellar black hole event horizon would fit inside a small city. M87*'s event horizon is bigger than our entire Solar System out to Neptune.
What is an event horizon?
Imagine you are in a swimming pool with a strong drain at the bottom. Far from the drain you can easily swim against the pull. As you get closer the pull gets stronger. At some point even your strongest swim is not enough, and you start to be sucked towards the drain whatever you do.
A black hole works the same way, except instead of water it is gravity, and instead of a swimmer it is anything trying to escape, including light. The event horizon is the place where escape becomes impossible no matter how fast you can travel. Since nothing in the universe goes faster than light, and light cannot get out, neither can anything else.
How big is it?
The size of an event horizon depends only on the black hole's mass. For every solar mass of black hole, the event horizon is approx. 3 km across. So:
- A small stellar black hole (10 Suns) has a 30 km event horizon, about the size of a small city.
- Sagittarius A* (4 million Suns) has one approx. 24 million km across, around 17 times the diameter of the Sun.
- M87* (6.5 billion Suns) has one of approx. 38 billion km, bigger than the orbit of Pluto.
Does time slow down near a black hole?
Yes. The closer you get to a black hole, the slower your clock runs compared to clocks far away. This is called time dilation. It is a real effect predicted by Einstein's theory of relativity and tested with ordinary atomic clocks on aeroplanes and satellites.
If you were falling into a black hole and someone was watching you from a safe distance, they would see your clock tick more and more slowly. As you got near the event horizon, you would appear to almost stop, and the light coming from you would shift to redder and redder colours until it faded out. From their point of view, you would seem to freeze on the edge of the black hole forever, never quite crossing it.
From your own point of view, nothing strange would happen. You would just keep falling, with your clock ticking at the normal rate, and would cross the event horizon in a flash.
Photographing the unseeable
In April 2019 the world saw the first picture of an event horizon. The dark central spot is the shadow of the supermassive black hole in galaxy M87, surrounded by a bright orange ring of glowing gas. The photo was made by linking eight radio telescopes around the world into one giant virtual telescope the size of the Earth, called the Event Horizon Telescope. The shadow you see is slightly bigger than the event horizon itself because of the way the black hole bends the light around it.
A second image was released in 2022, this time of Sagittarius A* at the centre of our own galaxy. Both images closely matched what Einstein's equations had predicted more than a hundred years earlier.
Deeper dive: the Schwarzschild radius, the ergosphere and Hawking radiation
The size of the event horizon for a non-rotating black hole is given by the Schwarzschild radius, R = 2GM/c² where G is Newton's gravitational constant, M is the black hole's mass, and c is the speed of light. Plug in the numbers and you get approx. 2.95 km per solar mass, which is why a 10-solar-mass black hole has an event horizon roughly 30 km across.
Real black holes rotate, and a rotating (Kerr) black hole has a more complicated structure. Instead of a single spherical event horizon, it has two: an outer event horizon and an inner Cauchy horizon. It also has a region called the ergosphere just outside the outer event horizon, where space itself is dragged around with the spin. Anything that enters the ergosphere is forced to rotate with the black hole, no matter how powerful its engines. The Penrose process is a hypothetical way to extract rotational energy from a Kerr black hole by sending objects into the ergosphere.
Stephen Hawking discovered in 1974 that quantum effects near the event horizon make a black hole glow very faintly, an effect now called Hawking radiation. Pairs of virtual particles constantly appear and annihilate near the horizon, but occasionally one falls in while the other escapes, carrying energy away. The black hole slowly loses mass as a result. For a stellar-mass black hole the temperature of the Hawking glow is approx. 60 nanokelvins, far colder than the cosmic microwave background, so they actually grow rather than shrink. Only very small primordial black holes would have measurable Hawking radiation today, and only the smallest of those would have evaporated by now.
The relationship between the event horizon, entropy and information is one of the biggest open questions in physics. The Bekenstein-Hawking entropy of a black hole is proportional to the area of its event horizon, not its volume, which has led to ideas like the holographic principle: that all the information in a three-dimensional region of space can be encoded on its two-dimensional boundary. Whether the information that falls into a black hole is lost forever, scrambled, or eventually returned via Hawking radiation, is the famous "information paradox", still actively debated by physicists.
For more on the giant black holes at the centres of galaxies, read about supermassive black holes. To learn how small black holes form, see stellar black holes.