# Why can't light escape black holes?

Ven
Hello, I am aware that this question was the subject of a topic on this site, but it did not fully answer my question.
My question is, why can't light escape from a black hole's gravitational pull, when, according to general relativity, the speed of gravity is equal to the speed of light?
My question may seem a tad elementary, but I'm in the ninth grade, and my school doesn't offer physic's classes until next year, so I'm a bit of an autodidact.

n1person
The presence of a large mass in a small area, the black hole, deforms space time itself a huge amount. It would take an infinite amount of energy to "overcome" this deformation, so all paths leading out of it end up back at the mass. Imagine a rubber sheet with a tiny little heavy marble, it'll deform the sheet. Now take the limit as the weight gets big and the size gets small and it will deform the sheet considerably at a point (a singularity).

It has nothing to do with the "speed" of the interaction.

Hello, I am aware that this question was the subject of a topic on this site, but it did not fully answer my question.
My question is, why can't light escape from a black hole's gravitational pull, when, according to general relativity, the speed of gravity is equal to the speed of light?
My question may seem a tad elementary, but I'm in the ninth grade, and my school doesn't offer physic's classes until next year, so I'm a bit of an autodidact.

The "speed of gravity" is only in reference to a changing gravitational field. The black hole solution is basically static, so no changes in the gravitational field need to propagate.

As the above notes, the geometry inside the event horizon of a black hole is such that even if you shine a flashlight directly outwards, the beam ends up traveling inwards towards the hole!

superpaul3000
why can't light escape from a black hole's gravitational pull, when, according to general relativity, the speed of gravity is equal to the speed of light?

There are a couple separate things you're mixing up here. First, gravitational waves are what travel at the speed of light. If you have an area of flat space and suddenly introduce a large mass you create a disturbance that propagates away as a gravitational wave. This is like having a long rope, taking one end, and whipping the rope up and down once. You will see a wave going down the rope with some speed. Light is also a wave but a different type of wave. Swinging the rope side to side and seeing a horizontal wave go down the rope can represent light (note that light is actually an oscillation of the EM field, I'm just trying to convey the idea of two different types of waves with the whole vertical/horizontal wave on a string picture).

Secondly, you can have a mass that is just sitting in space without moving or disappearing and reappearing. In such a case the space around it is bent instead of flat. The space around a black hole is very bent. It is similar to pulling the string towards you. If you pull the string towards you fast enough, you'll be pulling it faster than waves can move down it. No matter how hard you swing the rope the waves will always travel at the same speed. Similarly no matter how energetic the light is it won't be able to travel across space faster than space itself is being sucked into the black hole.

Analogy:

Gravitational wave: Vertical wave on string
Light wave: Horizontal wave on string
No gravity (flat space): String is not moving
Gravity (bent space): Pulling the string towards you
Black hole's gravity (really bent space): Pulling the string towards you faster than any wave can move down the string.

GrayGhost
Hello, I am aware that this question was the subject of a topic on this site, but it did not fully answer my question.
My question is, why can't light escape from a black hole's gravitational pull, when, according to general relativity, the speed of gravity is equal to the speed of light?
My question may seem a tad elementary, but I'm in the ninth grade, and my school doesn't offer physic's classes until next year, so I'm a bit of an autodidact.

Here's another way to look at it ... Once inside the event horizon, the direction of time is always toward the singularity at its center. All entities travel forward in time. Since nothing travels backward in time, everything including light must continue on toward the singularity, never escaping the black hole.

GrayGhost

Naty1
My question may seem a tad elementary, but I'm in the ninth grade

Welcome Ven!

I think the above answers cover your question to the extent it can be. It is NOT elementary...consider that Einstein "proved" black holes could not even exist...so he was way, way further off...

Another analogy I like: a flat lake and sinkhole suddenly opens underneath and water begins running down into it in a rapid spiral...A bug floating on that water (that is light) can maybe swim fast enough to get away at first...that would be outside the event horizon of a black hole...but once the bug passes that theoretical point of no return, the event horizon, he just can't swim fast enough...neither can anything else.

A great book on this area of Physics is Kip Thorne's BLACK HOLES AND TIME WARPS...you should be able to find inexpensive used paperback copies online...like AMAZON...that's what I did...very little math, lots of detailed explanations of physical phenomen.