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.