Sunday, 14 September 2014

Black holes

So I thought I'd make a separate blog from my debugging blog as obviously, there are basically no similarities.

When I was at school I wanted to do something with Astrophysics but I didn't know what, I really enjoyed it and would even spend my own time researching into things. One of my favourite topics where black holes, they were just so intriguing due to their mysterious behaviour and how little we knew about them.
The fact that their gravity was so strong it absorbed light makes it very difficult to see, never mind examine up close and so most knowledge of black holes are just theories.

So what exactly is a black hole?

Well a black hole is an object which is so dense that its gravity is so strong where not even light can escape it, giving it the black colour. Keep in mind that a black hole can have less mass than a star, it doesn't need to contain a lot of mass to become a black hole, it just needs to be very dense.

So I bet you're wondering: "how can we know a black hole is there if we can't see it?"
Although we cannot actually see a black hole directly we can observe the behaviour in the region of space surrounding it. An example if this is light, when observing a black hole the light around it in the distance bends because the gravity is so strong creating disk shapes around the edge of the black hole.



Sometimes if an object is being consumed by the black hole like a large star then the friction can create what is known as an accretion disk which is a large disk of light produced as it spirals into the black hole.


So anything can theoretically become a black hole if it is compressed enough to become so dense that its gravity pulls in light.
Using a formula we can actually calculate the diameter of an object when compressed enough needed to form a black hole, this measurement is called the Schwarzschild Radius and here's the formula:

This means Schwarzschild Radius =  2 x Gravitational Constant (Newtonian Gravitational Constant) x Mass (Object we're calculating) / Speed Of Light (Squared).

So lets try and calculate the Schwarzschild radius of Neptune seen as nobody has done it.

2 x 6.67 x 10^-11 x 1.02 x 10^26 / (3 x 10^8)^2 = 0.1511866667

This result is in metres so in centimetres we would get 15 centimetres which is very small.
To transform Neptune into a black hole we need to compress Neptune to a diameter of 15 centimeters.

As well as a Schwarzschild Radius a black hole also has an event horizon which is a diameter around the black hole where events beyond the event horizon cannot be seen by somebody outside the black hole as that is commonly referred to as the 'point of no return' where nothing can escape the black hole after that point. As you fall into the black hole you are presented with extreme stretching which would tear your atoms inside your body apart resulting in your inevitable death. You would them fall closer towards the singularity and then, well I'm not really sure in a Schwarzschild black hole, may be you (or the billions of atoms that were you) are absorbed then slowly released through Hawking Radiation.

To an outside observer you fall into the black hole slowing down until you reach the event horizon when you suddenly stop and become red shifted until you vanish, you on the other hand keep falling and the distant space around you gets smaller and smaller until you can't see anything.

Charged Black Holes

All of the above is about static black holes which contain no charge, charged black holes use different geometry all together called Reissner-Nordstrøm.
They both developedthe theory of a black hole gaining a charge, a charged black hole has two event horizons an inner horizon (I believe it's called the cauchy horizon) and an outer horizon (the outer horizon being the event horizon that is present in the Schwarzschild black hole).

According to the mathermatical theorm the space within the inner horizon is a wormhole although on paper this is correct as it uses Einstein's theory of E=MC^2 where all energy is converted and none is destroyed so if all this light and energy is consumed it must be released and hence the wormhole theory. The problem is the mass inflation within the black hole which prevents a wormhole forming so in reality it can't happen therefore the Kerr-Newman geometry for a rotating black hole is considered true more so than the Reissner-Nordstrøm geometry for a charged black hole as it is unlikely a charged black hole would ever be present but most black holes are rotating.
It's essentially saying with the wormhole theory that you fall down a waterfall and then must be slowly pushed back up a different water fall, it doesn't work in reality.
In a charged black hole you don't have to reach the singularity as you can be forced back out of the hole if you have no charge.

I think that's it for this topic (so far), I may go further into detail about the different black holes and even look into rotating black holes in the future.

Thanks for reading.

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