COLLAPSED STARS

Stars are balanced by its' own gravity which pushes inward on the star and pressure that comes from its burning of nuclear fuel which pushes outwards on the star. When the star has burned all its' nuclear fuel the gravity takes over and squeezes the star. Then there are three things that can happen to the star depending on how massive it is. First it will blow of a lot of unstable matter from it, but the rest of it will have one of three faiths.

PAULI EXCLUSION PRINCIPAL & DEGENERACY PRESSURE

Paulis exclusion principal says that 2 fermions(matter particles) can't be at the same place at the same time. When a particle from the fermion family gets cornered, like if you where to trap it in an ever shrinking box(the box is made up by Fermions so the particle and the particles making up the box can't be at the same place at the same time), it would start to move fast and in an unpredictable fashion. This is because the wave length(which corresponds to the energy of the particle, the lower wave length the higher energy and higher energy corresponds to faster motion) has to be a whole number of waves between the two walls of the box(meaning that it can't have a 2.2, 3.3 wave waves, only 2, 3 e.t.c), and when the box shrinks the wave length also has to shrink and the particle gets a higher energy. The pressure form this motion in confined space is called degeneracy pressure. The same thing is true for the core of a star. If the star starts to shrink because of the gravitational Pressure, the particles inside it will react similar to the particles in a shrinking box and start to move around furiously. Thereby creating a degeneracy pressure which can actually hold the star up from collapsing under its gravitational pressure.

WHITE DWARFS

White dwarfs are what happen to the lightest kinds of stars. Here the pressure is produced by the electrons degeneracy pressure, which is enough to hold the star up.

NEUTRON STARS

When a star is more massive then 2 solar masses, when it starts to collapse the electrons degeneracy pressure will not be enough. The electrons will move more and more furiously until the power produced by their motion will overwhelm the electromagnetic force, which is the force the keeps the electrons around the atom. Now when the electrons are no longer attached to the nucleus of the atom and moves around freely they will collide with protons since positive attracts negative charges. Then the electrons negative charge and the protons positive charge takes each other out and creates a neutral charge, a neutron. Now the star starts too fill up with neutrons and since the neutrons are more numerous then the electrons where(the neutrons that were already there and the newly created out of the electrons and protons), the neutron degeneracy pressure will be enough to stop the collapse of stars heavier then 2 solar masses.


BLACK HOLES

Particles can't move faster then light, infact if they have mass they can't even reach the speed of light. This means that degeneracy preassure can't grow infinetly high in a star, since it depends of the speed of the particles. So when a star more massive then 3 solar masses collapse, degeneracy pressure can't no longer hold up the massive gravitational pressure produced by the stars mass. The star implodes on itself to zero size and forms a singularity. A singularity is a place with zero volume and an infinetly high density. At the singularity space time is infinitely warped and therefore gravity is infinitely strong. In fact, at the singularity spacetime is so severely warped thatit cease to exist which is a very serious thing since then, time ends and space ends. The singularity is like the edge of the universe.

But string theory makes away with singularities and instead of creating a singularity it should stop contract at the Planck length(10-33cm). I will still use the term 'singularity' since string theory isn't proven right.

And when the star collapse and forms the black hole, all information about the star and all the irregularities(i.e mountains e.t.c) gets destroyed and it's impossible to from the black hole deduce what the star which created it looked like. It's said that a black hole has "no hair".
To get away from the gravitational pull of earth, e.g when a space ship is going out into space, you need to accelerate yourself up to some velovity. This is called the escape velocity(actually, since gravity is a force which acts over an infinte distance you can never really get out of an objects gravitational pull. But you can get as far away so that the pull become insignificant, so better stated is that escape velocity is the velocity you need to get so far away so that you don't notice a planets gravity). But the singularity creates such a immense gravity around it so that not even light can escape from it, it has a escape velocity higher then that of light. And since light has a constant velocity and nothing can move faster then light, then if light can't escape from the black hole then neither can anything else. The distance from which light cannot escape doesn't lay exactly at the singularity itself. It lays some distance away from it, and how far away or how big the black hole is, is determined by how strong its gravitational pull is, and so on how massive the star which collapsed and formed the black hole was. The border from which not even light can escape is called the 'event horizon'.

The singularity disserves a more in-depth look. What happens when you fall into a black hole and its event horizon?
Lets say you throw youself toward the hole and its horizon. And you buddy(who is happy to get rid of you) stays at a constant position somewere outside the event horizon.
In Einsteins theory of relativivty, gravity causes time to slow down but in the same theory time isn't anything constant, time is measured differently by different people or 'observers' it's said that they are in different reference frames(a reference frame depends on how big gravitational field the observer is in). As you plunge towards the hole you have a little radio with you, and every second it sends out a little 'peep' to your friend. Your friend will notice that the closer you get to the event horizon the interval between the peeps will get longer and longer, it will take longer time then one second between them. And when you are right at the event horizon your buddy will no longer hear any peeps. From his reference frame you will fall untill you hit the event horizon. But then you will freeze at the event horizon, to him it will look like time has stoped(because of the gravitational field you're into).

But to you, time continues as normal. you pass the event horizon. Then you would first get stretched more and more until you reached the singularity and gets crushed into zero size.
But a black hole doesn't have to bee the final end station for everything which falls into it. If the black hole is rotating(if the star which formed the black hole rotated the black hole will also rotate) there is a chance that you can 'miss' the singularity and fall out of a white hole(see below). There's also a couple of proposals in which the implosion might change into an explosion. But not an explosion into our universe, but into another. In fact a black hole might create a new universe. When it collapse from our universe it would start to inflate spacetime into another universe. This new universe would be connected to ours only by the black hole. So our universe might be a black hole in another universe.

There seems to be a problem with black holes and thermodynamics. Thermodynamics is a part of science where you study how large groups of particles works. Thermodynamics has a set of laws. The second law of thermodynamics state that entropy in a closed system always must rise.
Entropy is a measurement of distortion or chaos. To see why it should always increase, we could take a normal example of your untidy room(I know you got one). An untidy room is a room in chaos, things lay around everywhere. Lets say hypothetically that you do decide to tidy it(an action forbidden by the laws of physics). Then it might seems like the entropy is gone, and everything is neat and ordered. But in the process of cleaning you developed heat by the effort and also started to breath faster. All those things cause the molecules in the air to move around in a chaotic fashion and in fact the total entropy has increased. (CONCLUSION: Never tidy your room. You're wasting entropy).
But a black hole could get rid of entropy. Since everything inside the event horizon is cut off from our universe, you could throw the entropy down into the hole. Then the total entropy in our universe would have decreased and the second law of thermodynamics would be violated.
A thought would be that the black holes horizon was a measurement of its entropy, the larger horizon the more entropy. This would make sense, since the more you throw into a black hole the more it grows. There's just one problem, entropy also means heat. Think of one litre petrol, in it the molecules lay orderly, it has low entropy. But the if you burn it, the molecules starts to fly around in a chaotic pattern, it now has high entropy. So motion equals entropy, and motion equals heat. So the black hole should radiate.
So if a black hole has entropy should radiate heat. Which seems ridicules since a black hole only sucks things into it, right?

HAWKING RADIATION
Actually no, that isn't completely true. Black holes do actually radiate, this radiation is dubbed hawking radiation after its discoverer Stephen Hawking. The Hawking radiation was discovered when you incorporate the ideas of quantum mechanics into how a black hole would work. To understand hawking radiation, you must first understand two things:
1. Quantum vacuum fluctuations
Vacuum is normally described as something completely empty, with no particles what so ever in it. But In quantum mechanics, there's the heisenbergs uncertainty principal which says that at particle small levels you can't know every thing to a 100%, the more you know about one thing the less you can know about another thing. This apply for example to energy and time. If you want to know the energy level better you have to measure it under a longer time. The vacuum, which isn't supposed to have any energy, has particles constantly being created and destroyed. This is because of the above mentioned uncertainty principal. According to it, energy may fluctuate and the higher the energy level of the fluctuations have, the less time they can exist. This makes it possible for a virtual particle/anti-particle pair to be created, move around a bit and then meet and annihilate each other so that the energy borrowed in the creation is returned. The particles created in these fluctuations are said to be virtual, because you can't detect them directly but you can measure their effect on other things such as the energy levels in atoms. But the virtual particles can become real, detectable, particles if they are able to take some energy from a field of some kind. the problem is that under their short life time they simply haven't got enough time to become real particles.
2. Tidal gravity.
The longer away you get from earth the weaker its gravitational pull will be, e.g. you'll fell the pull of gravity stronger on the surface of earth then if you where out in the atmosphere. This is called tidal gravity. For a black holes the tidal gravity near the horizon can be incredibly strong, there may be an immense difference between the pull on your feet then the pull on your head, if your feets are closer to the horizon then your head. But as it turns out for a bigger black hole the difference in tidal gravity is far less then a small black hole. We'll talk about this later.

If a virtual particle/anti-particle pair is created near the event horizon of a black hole, the tidal gravity can be so strong that the pair gets separated for a long enough time so that the gravitational field give the particles energy and then they can become real particles. Then one of the particles might be sucked into the hole so that it can't annihilate the other. While the other one escapes and moves away from the hole. So now the hole will have lost half the energy it put into making the two virtual particles real particles.
The amount of radiation should also increase. To see this, we first have to understand something about the intensity of tidal gravity: I said that the pull on your feet and the pull on your head might be very different in the vicinity of a black hole. But if you increased the power of the gravitational field, so the pull would be larger. Then your head and your feet would be pulled faster towards the hole, but since the pull would be stronger you would fell less difference between the pull on your feet and the pull on your head. They would be pulled more in unison.

And the higher tidal gravity the more efficiently are for example the particle pair torn apart. Now as said, the more energy the particles have the less time they can be apart . So for high energy particles the gravity has only a little while to manage to pull them apart and make them into real particles i.e we need a strong tidal gravity. So big black holes can only make virtual particles of low energy into real particle, since it has a very low tidal gravity and needs the particles to exist a longer time in order to be able to give them enough energy so they can become real particles. And then small black holes can radiate particles of both low and high energies because of their high tidal gravity which doesn't require the particles to exist for such a long time, but instead it can tear them apart efficiently. Therefore as the hole radiates away its mass and gets smaller the intensity of the radiation should increase.

You can also explain Hawking radiation by another quantum mechanical properties called quantum tunnelling. In quantum mechanics a particle doesn't have a specific state but has a certain probabilities of existing a little bit every were. So even a particle falling into the horizon has a chance of being out side it. And can therefore also suddenly appear out side it, and can escape the hole. Here the amount of radiation would also increase as the black hole decreased, since the smaller hole the less barrier the particle has to tunnel through, the particle has a larger chance of being somewhere outside the horizon.

So the black hole actually have entropy and the amount of entropy is shown be the size of its event horizon.
WHITE HOLES

In the world of mathematic you can reverse everything, and that is what white holes are based on if we "reverse" a black hole that sucks matter in to it we get a white hole that pour out matter.




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