Astronomy/Black hole and space
Sorry for my English but it is not my natural language. I am a computer engineer sorry for maybe "stupid" question, it is not my field.
I wish ask about black holes.
The black hole is very dense and the light can't escape.
The light is without mass but the gravity can deviate the light folding the space (relativity theory), so I think the space/time is fold inside black hole... very fold until it is folded like a circle and the light can't escape.
Problem, if the space/time is folded like a closed circle this is a separate space from our world nothing can escape, but nothing can enter or... if it is circle folded and connected with our space, the light can enter and also escape... Can you help me to resolve my doubt?
Il tuo inglese è molto buono! But I think I'll stick with English - my Italian would probably not be understood by you.
This is a very interesting answer, and I'll give you the best understanding that I (and our current understanding of the physics of black holes) have. But there is still debate and no one really "knows" the true answer.
First of all, since light can't escape a black hole, we have no knowledge of what is happening there. And we never will. Although some information does "leak out" (you can look up Hawking Radiation), most information is forever hidden. So our "understanding" is based on current theory alone. This is not normally how science operates. In physics, a theory or model is proposed, tested by experiment or observation, and, by the results, we determine whether our theory or model is a good one. With the interior of black holes, no tests or observations can be done. So it's really all untestable conjecture.
From what we can best determine by theory, within a black hole all matter is infinitely compressed - into a singularity or mathematical point. An infinitely dense "object". But how is that possible? Well, when a star losses its nuclear fuel, it is no longer producing energetic photons and elementary particles (mainly electrons) which interact (collide) with the gas and counteract the strong gravity of the star. So the star has no choice but to collapse under its own gravity. Eventually, something tries to stop the collapse. Atoms and molecules try, but fail as electrons are stripped away through collisions. Eventually, one is left with bare atomic nuclei and free electrons, forming an electron degenerate gas. If the star isn't too massive, the collapse stops there. A white dwarf is formed, which is a very hot core of atomic nuclei (largely carbon). But if the star's core (after an initial explosion as it tries to stay alive) is more massive (between about 1.5 and 3 times the mass of the sun), then nuclei are not strong enough to support the great gravity. So the star collapses further, and this time the basic constituents of nuclei (protons and neutrons) try to support the star. But in the dense matter, the free electrons combine with the protons to produce more neutrons. So we're left with a very compact ball of neutrons (called neutron degeneracy) which try and support the great gravity. This is a neutron star. But what if the mass of the final core is even greater - say, 5 to 20 times the mass of the sun. Then, even the mighty neutron breaks down - that ball of neutrons simply can't support the great weight of the star's material. So it continues to collapse. What can stop the collapse next? Nothing that we know of!
Remember that Newton said the force of gravity is inversely proportional to distance squared. The collapsing star becomes very small (its radius is less than a few kilometers), so its surface gravity is very high. As the massive star (which is still emitting photons) continues to collapse, its surface gravity will exceed the point where photons can escape from the surface. The energy of a photon is proportional to its frequency. As Einstein predicted, in a strong gravitational field (like on the surface of our collapsing star), photons lose energy (they're red-shifted, or the frequency of light is lower). In a strong enough field, they're infinitely red-shifted, which means they lose all energy. Light can't escape. The photons (which are packets of energy) are actually absorbed by the gravitational field. Our collapsing star becomes a black hole.
But I said that the star continues to collapse to a singularity. How do we know that? Think of it this way: Say we have a rocket ship that wants to take off from the moon. The moon doesn't have as much surface gravity as earth (about 1/6), so it doesn't need much energy to take off and even accelerate away from the moon. But if we now wanted to take off from the earth, we'd need more energy (since earth's gravity is higher). And if we wanted to take off from Jupiter, we'd need even more energy (a bigger rocket) because Jupiter's gravity is much greater than earth's.
Now, say you were in a rocket and tried to take off from a collapsing star. The gravity would be so high that you need a huge amount of energy in your rocket. Another way of thinking of gravity is to say that space-time is curved. The more curvature, the more gravity. When your rocket feels the curvature, it causes it to move (be attracted to the star). If you wanted to stay still or accelerate away from the star, you'd have to counteract the curvature by adding energy. At the event horizon of a black hole, you'd have to add an INFINITE amount of energy just to stand still. Since no matter we know of contains an infinite amount of energy, all matter must continue to collapse. Nothing can stop it. Matter becomes a mathematical point.
For exactly the same reason, Einstein said that the speed of light was a limiting speed. If we were in a very fast rocket, as we approached the speed of light, space-time would become more curved and we would need more and more energy. To actually reach the speed of light requires an infinite amount of energy!
So that's the reason a rocket (or anything else) can't escape from inside a black hole. The strong gravity would require the rocket to have an infinite amount of energy just to stand still. It MUST fall to the center. And a photon (quantum of light) losses all its energy to the gravitational field. It simply can't exist in the strong gravitational field.
Of course, whether or not all matter condenses to a singularity is open to question. There may be quantum gravitational fluctuations (which current theory can't explain) which prevents a true singularity. We'll never know for sure, but theories may be developed which try to explain this.
Now to get to the actual question you asked. Yes, space-time is "folded" (as you say) or strongly curved. Some would say it's the maximum curvature space-time can have. Relativity predicts that time would "stand still" in such a curvature, but that's time as measured by a distant observer. And we can't ever observe inside a black hole! So it's a theoretical idea only.
But I wouldn't say that space-time is folded like a circle. A circle is 2-dimensional and space-time is 4 (or more) dimensional, so it's difficult to visualize. The reason that light can't escape isn't because it's trapped in a circle inside a black hole. Rather, photons can't be produced in a black hole. Photons are produced by a change in energy (for example, an electron changing its orbit). But in the condensed core (singularity) of a black hole, everything must be in its lowest energy state. Even if a quantum fluctuation were to produce a photon and it tried to escape, it would IMMEDIATELY lose all its energy back to the gravitational field. It would cease to exist.
But a photon (or matter) could enter a black hole, in spite of the strong space-time curvature. In the case of matter, it would accelerate to the velocity of light and reach the singularity in almost no time. A photon would simply lose its energy to the gravitational field and increase the total energy/mass of the black hole.
So I hope that helps. The curvature of space-time is not folded like a two-way gate to keep everything either in or out. Rather, it's more like a one-way gate. It can accept things (and accelerate them to the speed of light) but it prevents things from escaping (unless we figure out a way to beat Einstein and travel faster than light!).
Prof. James Gort