atyy said:Juan Maldacena gave a talk at this year's Marcel Grossman Meeting:
http://www.icra.it/MG/mg12/en/..\talks_plenary\Maldacena.pdf
What is the meaning of Maldacena's second slide?
A black hole is a region in space where the gravitational force is so strong that nothing, including light, can escape from it. It is created when a massive star collapses in on itself.
The size of a black hole can vary greatly, but it is typically measured by its event horizon, which is the point of no return for anything that enters the black hole. The event horizon can range from a few kilometers to billions of kilometers in diameter.
No, we cannot see a black hole directly as it does not emit or reflect any light. However, we can observe the effects of a black hole's gravity on its surroundings, such as the distortion of light from stars or gas being pulled into the black hole.
A black hole is formed when a massive star runs out of fuel and collapses in on itself, causing its core to become incredibly dense. This collapse creates a strong gravitational force that continues to pull in more matter, making the black hole grow in size.
If you were to fall into a black hole, you would be stretched and pulled apart by its intense gravitational force. This process is known as "spaghettification." Once you pass the event horizon, the pull of the black hole is so strong that you cannot escape and would eventually be crushed into the singularity at its center.