Andrew Strominger

a black hole is still a matter of some debate.

And one explanation

which perhaps Einstein might have given, is that light carries energy.

You know it carries energy because we have photo cells and we can take the light from the sun and collect it, turn it into electricity.

So there's energy in light.

And anything that carries energy is subject to a gravitational pull.

Gravity will pull at anything with energy.

Now it turns out that the gravitational pull exerted by an object is proportional to its mass.

And so if you get enough mass in a small enough region,

you can prevent light from escaping.

And let me flesh that out a little more.

If you're on the Earth,

and you're on a rocket ship leaving the surface of the Earth, and if we ignore the friction from the air, if your rocket accelerates up to 11 kilometers per second, that's escape velocity.

And if there were no friction, it could just continue forever to the next galaxy.

On the Moon, which has less mass, it's only 7 kilometers per second.

But going in the other direction, if you have enough mass in one place, the escape velocity can become the speed of light.

If you shine light straight up away from the Earth, it doesn't have too much trouble.

It's going way above the escape velocity.

But if you have enough mass there, even light can't escape the escape velocity.

And according to Einstein's theory of relativity, there is an absolute speed limit in the universe, the speed of light, and nothing...