Black Holes In Question

[zanzibarjones 1]

How can you bring Gravity into this?Can you tell me 100% by what magical circumstance is gravity created and in charge of this?For decades people have said that gravity is created by forces spinning, then it was said that gravity was like a ball on a rubber mat. A bend in space. Now gravity in and around a black hole defies all of that be being sucked into the center of an object?So which is it? Is a black hole using gravity to pull things in, or some other force?If light can not escape a black hole, would you not agree then that gravity has nothing to do with it? Because I don't believe light is affected by gravity."According to Einstein's General Relativity Theory,light will be affected in the same way matter is affected by gravity. This is because under this theory, we should think of gravity not in terms of vector like forces, but as a consequence of the "shape" of the universe."

[Bikerman 2]

How can you bring Gravity into this?Can you tell me 100% by what magical circumstance is gravity created and in charge of this?
For decades people have said that gravity is created by forces spinning, then it was said that gravity was like a ball on a rubber mat. A bend in space. Now gravity in and around a black hole defies all of that be being sucked into the center of an object?

So which is it? Is a black hole using gravity to pull things in, or some other force?

If light can not escape a black hole, would you not agree then that gravity has nothing to do with it? Because I don't believe light is affected by gravity.

"According to Einstein's General Relativity Theory,light will be affected in the same way matter is affected by gravity. This is because under this theory, we should think of gravity not in terms of vector like forces, but as a consequence of the "shape" of the universe."

Right - stick with the rubber mat. It isn't perfect but it will do.
There is no 'force' of gravity in relativity. Things move in straight lines, but the straight lines themselves are bent by mass. This is where you drop a weight onto the rubber mat. There is no force acting on a particle that goes past the weight - it is just that its normal straight-line path has a curve in it, so it follows the curve.
Now, curve enough and the particle (small ball is normally used) will go into orbit - spinning around the weight.
Add more and more weight and the rubber stretches with steeper and steeper sides until once the small ball has gone too far, it can never get out, no matter how fast it spins....
This applies to everything - mass AND light - they all follow the curved path.

[zanzibarjones 1]

Maybe I'm un-educated, oh wait I am.... I don't understand how light can be affected by this? Light has no mass... can you explain why light follows this rule too? I understand why mass does.By the way I love causing trouble with questions like these

Edited August 24, 2010 by zanzibarjones (see edit history)

[Bikerman 2]

Maybe I'm un-educated, oh wait I am.... I don't understand how light can be affected by this? Light has no mass... can you explain why light follows this rule too? I understand why mass does.

OK. Light has no rest mass (ie if you could stop light it would disappear). Now this doesn't matter because in Relativity nothing is 'pulling' the mass. Gravity isn't a force, it is just the bend in spacetime (the rubber mat). Let's simplify it and leave the 'time' bit out and just deal with space.
So the mat is space and the weight is mass causing gravity. Does it matter how much mass the things you roll past the weight have? Yes. Good answer. BUT what if you imagine something with no mass? It will still follow the curved path of the rubber, not an absolute straight line. That is how light is - it follows a path (we call it a 'geodesic') which without any weight distorting the mat is a straight line, but when weight is added the path becomes curved.
Edited August 24, 2010 by Bikerman (see edit history)

[Bikerman 2]

If you really want to boil your noggin then try getting to grips with some of the rest of the theory of Relativity. One common source of bafflement is something called the 'Twin's Paradox'.
This is relevent to black holes because extreme gravity causes time dilation - ie different clocks go at different speeds depending on the gravity - and you age at a different rate. Essentially time goes at different rates in different gravity fields.
It also happens when you go very fast. The Global Positioning System has to account for both of these factors - the satellites are wizzing around pretty quick, that means they have a slower time than on the ground. But the gravity is much less up there which speeds their time up again. You have to calculate both effects, subtract one from the other, and then you can make the clocks tick at the correct speed. This is vital because they pulse information down to you at the speed of light (nearly) and by measuring the time, you work out the distance. Even a very small error (0.00001s) would mean you were way out when you calculate the distance, because light travels so quickly.

If you want to try to understand this, the following is a really nice site for novices in Relativity
Click Here

[zanzibarjones 1]

If you really want to boil your noggin then try getting to grips with some of the rest of the theory of Relativity. One common source of bafflement is something called the 'Twin's Paradox'.This is relevent to black holes because extreme gravity causes time dilation - ie different clocks go at different speeds depending on the gravity - and you age at a different rate. Essentially time goes at different rates in different gravity fields.
It also happens when you go very fast. The Global Positioning System has to account for both of these factors - the satellites are wizzing around pretty quick, that means they have a slower time than on the ground. But the gravity is much less up there which speeds their time up again. You have to calculate both effects, subtract one from the other, and then you can make the clocks tick at the correct speed. This is vital because they pulse information down to you at the speed of light (nearly) and by measuring the time, you work out the distance. Even a very small error (0.00001s) would mean you were way out when you calculate the distance, because light travels so quickly.

If you want to try to understand this, the following is a really nice site for novices in Relativity
Click Here

Yeah and ya know Stephen Hawking says that if you were able to travel at the near speed of light speed, you would be travelling into the future. The thought that if you could travel at that speed, leave earth and return in what you think is 25 years would actually be 100 years (or somewhere around there). I think that is cheating by saying you've traveled through time. I mean at that speed things slow down. So technically you are just not aging as fast as some who stayed on earth.

I know, white rabbit, sorry. Just had to get that in there. time travelling.... whole different subject.

[Bikerman 2]

Yeah and ya know Stephen Hawking says that if you were able to travel at the near speed of light speed, you would be travelling into the future. The thought that if you could travel at that speed, leave earth and return in what you think is 25 years would actually be 100 years (or somewhere around there). I think that is cheating by saying you've traveled through time. I mean at that speed things slow down. So technically you are just not aging as fast as some who stayed on earth.

 

I know, white rabbit, sorry. Just had to get that in there. time travelling.... whole different subject.

 

Well, not really - all part of the same.

 

Why is it cheating? Surely if you can travel for a year and go 20 years into the future (and you can do better than that if you go fast enough) then that must count as time travel?

Yes, you are only slowing the clocks down, but that is still time travel - you are travelling at a different rate in the time dimension.

The way I imagined this to get started is quite a good little mental picture - you might find it useful.

 

You travel in spacetime. Spacetime is space and time all wrapped up into one. Your speed through spacetime is always the same - the speed of light. We spend all our time doing nearly all of that travel through time (we don't move fast enough to really count when it comes to moving through space. The fastest human speed achieved is about 25,000mph in Apollo 10. Compare that to 187,000 miles per second, and you see how slow it is.

 

Since the speed through spacetime is always the same, the faster you move through space, the slower you move through time (although to you it is no different because your life is measured against spacetime, as is everyone else). The difference is only obvious when someone else doesn't move through space as fast as you. They are then moving through time much quicker than you.

 

Get it? It is a handy little mental model to explain what is going on.

 

This is where the maths is useful, and the maths for time dilation is easy peasy - watch and I bet you can follow it no bother.

 

Let V be the speed through space of you. Let V₁ be my speed through space. (I need to use velocity rather than speed, because otherwise we might not be moving apart as fast as we are moving - we might be moving at angles to each other).

 

So your velocity is V and mine is V₁.

We want to know the speed that our clocks are ticking. Another way of saying that is we want the rate of change of time. Rate of change, in maths, is written as a greek letter Delta (Δ), so the rate that your clock ticks can be written as ΔT and mine can be written as ΔT₁. That's all we need. The formula for calculating the different rate is:

ΔT₁ = ΔT/(square root(1 - V²/c²)

 

If I has a square root sign that would be perfect. Still, you can see it isn't very hard. The extra term ( c ) stands for the speed of light (our speed through spacetime).(I can actually show you how to get this forumla just using pythagoras and a right triangle, but that is another time).

So you might see from this formula that the important bit is the V² - your velocity squared (we could swap, it doesn't matter, since this will work whether you are V or V₁)

 

As your velocity gets closer to the speed of light - c - then that last bit of the formula gets closer and closer to c²/c² which is 1. (anything divided by itself is 1). And look what happens then. You get ΔT/sqrt(1-1) in other words ΔT/0 which is not possible - or we can say it is infinite - you cannot divide anything by zero and get a sensible answer. Theoretically, if you could move at the speed of light then no time would pass for you at all. You could fly right round the universe and no time would have elapsed for you. Now you can't, because it isn't only time that goes wacky, length and mass also go wacky and they both use the same formula. So if you use L for length instead of T for time then the forumla is the same. Likewise, use M for mass and the formula is the same. That means as you get close to the speed of light, your mass goes up and up towards infinity and your length gets longer and longer towards infinity. Remember that this is from my viewpoint - you don't feel any change at all. So I see you stretch out in length, I see your clock go slower and s l o w e r and I see you getting heavier and heavier (or I would, if i could weigh you).

 

See, it's much better with the maths

Edited August 25, 2010 by Bikerman (see edit history)