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

Question

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In: Physics

Answer 1

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.