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| Stars and planets warp space time |
A day doesn't pass without something about another Super Massive Black Hole that has been spotted out in this galaxy or that galaxy.
Every suprise that the universe throws at us are something to do with black holes. But here is a thing, I don't think they exist.
What is the evidence for black holes? what are the theories? do we even need them to exist? Well that is what I am going to write about in this blog and the next probably, but first a quick summary of the state of play.
Newton - published "Principia" a work in three book on 5 July 1687. Rightly regarded as one of the most important works in the history of science. It contains a law of Gravity. summary...
The force of Gravity is related to the mass of an object, the more mass, the greater gravitational force.
Why is this so? He didn't say. Who knows, well… no one actually, still don’t. Moving on.
1783, geologist John Michell writes to the royal society to point out that a really big sun would have a really big gravity, so big in fact that not even light could escape, should light fall under the rules of gravity.
1796 Laplace promotes the same idea in the first couple of editions of one of his books and then chickened out and had it removed from later editions. If you’re going to put forward a daft idea, have the bottle to see it through eh?
Not much happens in the 18th and 19th century and then comes along Albert Einstein.
Einstein and General Relativity
1915 Albert Einstein develops his theory of General Relativity, a Geometric theory of gravitation.
Einstein realised that it may be possible to describe the universe in a rather odd way using equally odd mathematics.These turn out to be mindnumbingly complex. Fortunately the ideas themselves are not really that complex and so we can understand the physics without being able to do the maths. Which is as it should be.
Space time (an idea also dreamed up by Albert Einstein) can actually be considered to behave a little like a giant rubber sheet pulled tight. There’s not much in the universe, it is mostly nothing, so the sheet is flat. When you get near a star the sheet gets a little warped, imagine placing a weight on a trampoline, it distorts the surface, same idea. Why does it behave like this? I'm not sure anyone really knows.
Years earlier a really clever mathematician called Riemann has spent some time working out the maths of geometric surfaces, Einstein applied this work to his idea and bingo, you have General Relativity. In other words, General Relativity is a mathematical model of how surfaces change their shape when you distort them. This can also be applied to the universe.
Einstein was able to develop a series of 10 equations, now known as the Einstein’s field equations (EFE) which describe how gravity can actually come about from the distortion of spacetime by matter and energy (energy being equivalent to mass thanks to E=mc2, who came up with that? Einstein, he was definitely on a roll!)
It wasn’t long before people began to realise that these equations had the potential to do some strange things and they began to wonder what this may actually mean in the physical world.
This turned out to be difficult, partly because the maths was difficult, but also because there was no chance of testing any of this in a lab, at least not in those days.
Take gravity waves, Einstein argued in 1916 that it should exist. These are waves that travel at the speed of light and would be produced in a binary star system for example. A binary star system being two stars very close together that orbit each other (such things do actually exist in nature). For more than 40 years people where not sure if Einstein was even right about his interpretation. It was only in 1957 when Feynman put forward a thought experiment that people figured that it may be right. This also had something to do with the fact that Feynman was one of the best theoretical physicists of his day.
But here is the thing, do gravity waves exist? As of 2011 we have NOT detected a single gravity wave. No experiment has so far proved their existence. So it could still be wrong!
Back to black holes…
The next thing we need to think about is escape velocity,
Back on earth, we realise that we want to send someone to the moon, the problem is that gravity is determined to stop us from leaving. We throw up a stone and it comes back down, what goes up must come down right? But what happens if you throw it really fast, it goes higher and takes longer to come back down. So throw it really really fast, so fast that it gets high enough that gravity can no longer pull it back! The speed we need to throw it so that gravity doesn’t pull it back is known as the escape velocity.
We find that escape velocity is related to gravity, the greater the gravity the faster we have to go to escape. It’s easier to escape from the moons gravitational pull than the earths. Although an atmosphere complicates escape velocity, to escape the earth required a massive Saturn 5 rocket. The rocket that blasted off from the moon was tiny in comparison! That’s because the moon as far less mass, far less gravity! The sun on the other hand had a large escape velocity, about 620 km/s (over 200,000 km/hr).
Now Newton told us that has we get farther away from something the gravity gets smaller, double the distance the gravity is only a quarter, make it five times the distance and the gravity is only 1/25th. Keep that in mind.
Imagine taking something like the sun and pushing other things, like large planets and things into it, more and more and more. So its starts to get more massive and this according to Einstein, warps space time which increase the gravity which crushes the sun. But the sun is full of hot gases which push back against the gravity.
We keep going more and more, so the space time gets more and more warped and gravity gets greater and greater crushing the sun even more. The escape velocity starts to clime 1000km/s, 10000 km/s, 100000 km/s. This goes on until you've got a Mr Creosote of a sun with an escape velocity of 300000 km/s. The gases keep pushing back but eventually they haven't got enough fuel to continue the struggle.
At 300000 km/s something interesting happens. We have managed to get a gravity so big that the escape velocity would have to be faster than the speed of light! So even light can't escape because it can't go fast enough!
What would happen then? What what would happen to a star with that much mass? Can such a thing exist? These questions will be discussed in the next post on Black Holes.
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