Blackholes have always fascinated me, and I am always hungry to know more about them, and always itching to talk about them. So, lets discuss.
What are blackholes?
A blackhole is a region of space-time that once entered cannot be exited. It means if you fall into a blackhole, you will never get out of it.
Why are blackholes one way portals?
Because of gravity. The escape velocity required to leave a blackhole is greater than the speed of light, and since nothing moves greater than the speed of light, anything inside a blackhole is doomed to stay inside - even light.
Why is the escape velocity so high and gravity so high for a blackhole?
Escape velocity is high if gravity is high, and gravity is high when space-time curvature or mass density is high.
What causes high mass density?
Death of a star.
What does death of a star mean?
It means nuclear fusion within a star stops, and the atoms of a star start collapsing and falling inward towards the center of the mass.
What is nuclear fusion?
In the interiors of a star, hydrogen atoms combine to form helium atoms, and in the process they emit radiation. The helium atoms combine to form carbon, oxygen, and these combine to form higher elements like iron. The thermal radiation creates pressure that balances the inward pull of gravity, and prevents the star from collapsing inward. However, when a star runs of hydrogen atoms, the pressure and radiation needed to stop gravity doesn't exist anymore, and the atoms start rushing towards the center of mass, and the star begins to enter its death throes.
What else is there to know about blackholes?
A blackhole has a singularity at its center. A singularity has infinite mass density, and laws of physics break down in the face of a singularity. The boundary of a blackhole is called event horizon, and once anything crosses the event horizon, it is pretty much doomed to hit the singularity. If a spaceship were to observe your journey towards a blackhole, it would only ever see you approaching and hovering near the event horizon, it will never be able to see your rocket cross the event horizon as time will slows down, and light is unable to escape.
The event horizon in a way is a time machine. Lets say if I was to hover near the event horizon for ten earth years, I would have aged only by a few minutes or days depending upon the size of the blackhole. That is because time near a high gravity object slows down, and so does light. The singularity of a blackhole could be an entry point into another universe as per one scientific hypothesis.
Cosmic censorship hypothesis by Roger Penrose states that there are no naked singularities in our universe and that nature conceals every singularity with an event horizon. This helps preserve the deterministic nature, and causality in the universe, since a singularity that can be observed will alter causality and the ability to determine space-time behavior.
The Hawking radiation, named after Stephen Hawking, is the only radiation that leaks out of a blackhole. As per quantum mechanics, particle-antiparticle duo are generated in space-time, and when this combination is generated in the gravitational field of event horizon, one of the virtual particles might fall into the blackhole, causing the other particle to become a “real” particle and escape from the blackhole as a radiation. This is Hawking radiation but it is different from thermal radiation in the sense that it does not give any information about the object that emitted it. The only information available about the objects that made the blackhole is its mass, charge and angular momentum. This indicates that information within a blackhole is forever destroyed, and this is called as the no-hair theorem.