| What is the ‘event horizon’ as described by Stephen Hawking and what does it mean for time, space and light?

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*Edit*

Thank you to everyone for the enlightening responses, I’ve learnt many many things and many have them have been in such clear terms that a giant idiot like myself has been able to understand.

The internet is excellent at times.

In: Physics
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Its the place where information can only be pulled farther into a black hole. Time, space, and light, pretty much immediately lose any relevance of what they were before crossing this boundary. In our observations of what we think is a black hole, we are really only seeing the smear left over by the stars and gas and matter heating up as they approach the horizon.

Event horizons are definitionally a boundary beyond which events cannot affect the observer. Specifically, this is a concept that applies to black holes – in an ELI5 sense, imagine a marble in a deep pot with a curved bottom. By swirling the pot around, you can get the marble to climb the walls, but imagine that this pot had sides so steep and tall that no amount of swirling could get that marble out. If one were to imagine that marble is a photon, the highest point you could get the marble swirling would be the event horizon – if anything, marble, spaceship, rubber duck, were to get into the pot beyond that point, it doesn’t matter how fast it’s going, it’s stuck in there. Since light itself can’t escape the event horizon, you can’t actually see or even detect what goes on beyond that point – and due to relativity (if someone figured out an ELI5 on relativity, that would be entertaining to read) the closer an object got to the event horizon, the slower it would appear to go, so someone watching something go over the edge would just see it slow down to a complete stop as it entered. Black holes are wild.

EDIT: Kurzgesagt came out with a really neat video on this topic a few weeks ago – [check it out](https://youtu.be/QqsLTNkzvaY)!

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The closer you get to the centre of a black hole, the stronger it pulls, and the faster you need to move to escape that pull.

At some point the speed you need to escape its pull becomes greater than the speed of light. Because speed of light is the fastest speed anything can move with, nothing can escape it. This point is what’s called event horizon.

Event horizons are the “edge” of a black hole’s…blackness. It’s the point where the black hole’s gravity becomes so strong that light (and by extension everything else) can’t overcome it and gets pulled in, leaving only black nothingness.

This leads to another important aspect about black holes: The actual black hole object (the singularity) is an infinitely-small chunk of mass at the very center, and the hole part that we see is just an illusion, created because all the light that would otherwise travel to our eyes is being sucked into the hole.

Another important thing: If you were to fall into the black hole (assuming you weren’t ripped to shreds first), time would pass normally for you, and you would watch the universe shrink away behind you until you were surrounded by darkness and/or ceased to exist. Someone *watching* you fall in would see a different thing though. They would watch your descent slow down the closer you got to the event horizon, because the light “image” of you falling would become more and more distorted the closer you got. This would continue until you froze in-place completely, at which point you would just kinda fade away since the “image” of you falling in, also falls in with you.

And one last important thing: Most black holes have event horizons a few dozen miles wide at most. [But some, at the core of very bright galaxies, are orders of magnitude larger than the largest stars, and could swallow solar systems whole.](https://i.redd.it/lqtsqz7ahpq11.jpg)

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Think of this way: You throw a ball vertically up on Earth.. It goes a fair distance then falls back down on the ground. You need to throw it at 11.8 km/s or more for the ball to “escape” Earth and go into Space (I’m ignoring air resistance here).

Similarly, on Moon, you’d need to throw the ball at 2.38km/s, since Moon’s gravity is less than Earth.

This is known as the Escape Velocity.

For our Sun, the Escape Velocity is 615km/s, ie, anything coming out of the Sun slower than that will “fall” back into the Sun.

Escape Velocity also depends upon your distance from the gravitation source(center of the cosmological body, usually). So if you throw the ball from an airplane 35,000 feet in the air, the escape velocity required will be slightly low.

Now think of this way: Event Horizon is the boundary _around_ the black hole where the escape velocity is 3×10^5 km/s. Does this number look familiar to you? It’s the velocity of light. It’s also the Universal cosmological limit, meaning nothing can travel faster than that.

You can work out the next conclusion on your own but I’ll write it here: Since the escape velocity of a black hole at event horizon is “c”, nothing escapes it, not even light.

(Simplified explanation of course)

Think of it like getting your hoover and a piece of lint. Place the lint on the ground and turn on the hoover, slowly edge the hoover closer and closer. The event horizon is the point at which the lint is pulled into the hoover and has no chance of escaping the suction.

Scale this up to the black hole and not even the fastest things in the universe can escape.

I just had to explain this to an actual 5 year old a few months ago.

Told him that a black hole sucks up everything, including light, which is why it’s all black. The event horizon is the point of no return. Once you cross that point, there’s no escape. You’ll stretch like spaghetti and time will slow down and then you’re gone.

Think about the surface of a trampoline. When you roll a ball on a trampoline, it goes in a straight line. But if you sit in the middle and somebody tries to roll the ball across, it will instead curve inward towards the low point. The ball didn’t change, but the shape of the surface changed. From the point of view of the ball, what is “straight ahead” looks like a curve to us.

Mass does the same thing to three-dimensional space that you do sitting on a trampoline: it creates curves. Things that look like they’re traveling in a straight line start to take a curvy path when they’re moving near mass, because space is curved.

Back to the trampoline. With no weight on the trampoline, it doesn’t take a lot of effort to make the ball reach the other side. Just a little bit of a push and the ball will roll the rest of the way. But as we add mass to the center, there’s a “pull” for the ball to “fall” into the center instead of reaching the other side. If you want the ball to not get stuck in the middle, you have to roll it harder. More mass means you have to add more energy to the ball to stop it from falling into the center. At some point you’ve added so much mass to the center, and the curve has become so steep, that there’s basically no amount of energy you can give the ball so it can reach the other side (assuming you don’t break the trampoline, and don’t just throw the ball over the hole). In other words, from the point of view of the ball, *all straight lines lead to the center of the trampoline*. No matter what angle or how much force, the ball will go into the middle.

This is a black hole. A black hole is a place in space where there’s so much mass that all straight lines go into the center. Everything that’s inside cannot get out. The point of no return, the place where all those lines start bending inward, is called the Event Horizon. Outside the Event Horizon you can still be on a bad line and fall in, but there are some options to go elsewhere. Once you cross the Event Horizon, however, you’re out of options. At that point you are going into the middle.

The best explanation of an event horizon I’ve ever heard is this analogy.

Let’s say you are in a canoe on a river. A little way down river is a waterfall. Right now you are safe and can paddle in either direction. As you get closer to the waterfall the river picks up speed. For a while you will be able to paddle against the current and get to safety. But at some point the current is going to be moving faster than you can paddle and you are going to get pulled over the waterfall. That point is call the event horizon.

Same thing applies with black holes. It’s the point at which there is no escape and you’re going to be sucked into it.