A few astronomy questions...

Yes, black holes are thought to spin very fast. Their spin somehow is responsible for twin jets of x-ray emissions from the axis of their spin that astronomers look for in order to find a suspected black hole.

Gravity is one of the (theorized) four fundamental forces of the universe created at the big bang. Mass attracts other mass. I may be wrong, but I don't belive any exchange of energy is involved. Repelling gravity takes a great deal of energy, however.

Equus, I thought the jets came from the crunching down of mass which is being sucked into the black hole. Maybe they're located at the poles due to the spinning, but the spinning doesn't cause the ejection - or does it?

Are x-rays unaffected by gravity? How do they escape the pull?

I just had an astronomy class where the professor said the rate of expansion of the universe is accelerating, and this is considered odd because there is no energy to support the accelleration, so she didn't want to expound on it too much. I don't see how that is different than gravity.

SCoates - if you're taking astronomy - why are you asking US?

I waste too much of the classes time with my questions. If it won't be on the test, they don't care.

Second question first, since it can be answered using classical Newtonian physics.

As I am sure you know, the law of conservation of energy states that "energy can neither be created nor destroyed". So if the Earth (for example), accelerates an object towards it, the increased kinetic energy must means a decreased of some other kind of energy.

This does not mean that the earth itself must lose energy.

The kinetic energy simply comes from "potential energy". This energy is "stored" in the position of the Earth and some other object.

If I take a heavy rock up a tower, I am adding energy to the Earth-Rock system (you could say I am converting chemical energy from my into potential energy). This added energy doesn't change anything about the rock or the Earth. Only the positions of the rock and the Earth relative to each other change.

It is incorrect (or at least unhelpful) to say that you are adding energy to the rock. You are adding energy to the Earth rock system (i.e. the combination of earth and rock in their relative positions).

Now when I drop this rock, it will accelerate gaining kinetic energy. Now the rock has gained energy (which will in turn be converted into heat and sound). But the Earth hasn't changed just because the rock is going faster (except the Earth does accelerate a very small amount). The Earth didn't lose any energy.

It is the Earth-Rock system, the fact that they are now closer that has changed. Earth and Rock far apart has more energy than Earth and Rock near. The energy is in the position. That is it.

(edit: fix spelling error)

Some black holes are thought to spin and some are not. Seems like the bigger ones maybe don't. The spinning does have an effect on things but not anything major.

Gravity is thought to have a force carrier but nobody has found much evidence for it yet. Some M theories do without a carrier but most need it.

The x-rays that are detected from black holes don't eminate from inside the event horizon. They are created as matter speeds up towards the EH. Thats why some escape and can be detected.

The part about the expansion of the universe accelerating is just a fledgling theory at the moment. There's not a lot to support it yet.

Lots of if's, but's and maybe's about all this stuff.

Black holes have what is called an "acretion disk". This is a ring of matter that is orbitting just outside the event horizon. It orbits very fast because the gravity of the black hole is very strong.

This matter emits X-rays (since it is moving so fast). Since it is outside the event horizon, these X-rays can escape. This is one way we detect black holes.

I'm not an expert, so I could be wrong. But I believe the spin of the b.h. is why there are two opposing jets of x-rays instead of a general emission all over. It may have something to do with the matter falling in, but even x-rays shouldn't be able to escape from the event horizon, the edge of the b.h. I think (correct me) it's released by distressed matter as it approaches the black hole, but the x-rays can only leave the area at the poles.

What about a meteor coming towards earth from far off in space. Is potential energy inherent in its system? Obviously it can be accelerated towards any planet. Is it's potential energy something which has stuck with it from its origin (wherever that may be), and may be expended on any planet? If that is the case, seeing that some planets have potential to accelerate it more powerfully than others, what balances the system?

I don't think I understand energy very well.

First part of your question - the spin. Well outside of the event horizon the answer is very likely in most cases. Think of water going down the drain in a bath - you get a violent whirlpool as matter gets sucked down. The black hole may be moving so as matter or energy gets sucked in towards a moving target it both maintains its original angular momentum and gets a hell of alot more.

Inside a blackhole's event horizon must leave the rather well understood framework of classical physics and Einstein's relativity. Instead you have competing theoretical physics frameworks describing what is happening. Some believe in singularities at the centre of the black hole - some believe angular momentum of matter or energy is conserved so you get an ultra dense, ultra fast spinning gravstar core - but not a singularity - as angular momentum counterbalances gravitational contraction inside a black hole's event horizon.

You only have a spin continued within the event horizon if time, matter and energy and space still exist in some form within the part of the black hole.

I tend to think inside a black hole's event horizon you night find at least 4 layers corresponding to transitions in reality (particles, energy carriers and universe's coorditante system - spacetime) based on the ever rising energy density as you move towards the epicentre. Inside and close to the black hole's event horizon I presume everything is pure energy - the energy density is too high even close to the event horizon for matter to exist as a particle - it should e=mc^2 into its equivalent energy component to be crushed easier.

As matter or energy falls into a black hole its converts a fantastic amount of gravitational potential energy into kinetic energy so for matter it gets hotter. Basically after you heat any matter to well over 5 million Kelvin expect it to very quickly transition from solid -> liquid -> gas -> plasma -> quarks / bosons. Progress this energy then inwards past the event horizon and the temperatures / energy density will rapidly rise as space gets ever more compacted by the increasing gravity.

From 4 forces down to 3

Now at a set point or layer the energy density will exceed 120 GeV - at this point strong and weak nuclear forces combine - so the forces carriers change and hence the rules of physics change.

3 becomes 2

Go further in and eventually you reach 10 ^ 16 GeV (the hierarchy problem - its too large to study) and then combined Nuclear forces and electromagnetism merge - once again the force carriers are different so physics changes again.

Last man standing - unified Grand Physics

Continue inwards and eventually you exceed 10 ^ 19 GeV energy density - gravity and combined nuclear / electromagentic forces merge into quantum gravity and welcome to a new reality again. We don't know how spacetime itself theoretcially transitions as the force carriers combine. Once all the four forces are combined does spacetime even exist or is reality governed by other principles? So it could be a very meaningful or totally meaningless question to ask is this reality spinning.

* * *

Gravity - the source is one of the four fundamental force carriers birthed first of all four forces only moments after the big bang itself. Force carrier unknown - thought to be graviton (if quantised around 119 GeV) or Higgs Boson / Higgs Field (possible if not quantised). Large Haldron Collider in 2007 might shed more light at these energy densities. CERN thought they had 5 possible readings of Higgs boson at 118 GeV late last year before it was switched off to be upgraded to LHC.

Gravity - least well understood force of all the force carriers e.g. why is it so weak - does it leak across membranes of existence? Its hard to pull a supermagnet off a fridge - so why can someone lift a brick off surface gravity of an entire planet? Does gravity propogate at lightspeed (IMHO) or trillions of times faster (doubt it)? Why does gravity interacts stongly with spacetime - space tells matter / energy how to move - but matter / energy tell space how to warp. So much is unknown! Why is the expansion of space accelerating - dark energy? hubble spheres and disconnected casuallity over the universe limiting gravity's propogation? other facts? Unknown.

You have asked a great question - the answers are still being considered!

Any two objects have potential energy between them. Even a distant meteor and the Earth. If you ignore the interaction of all other forces, any object will lose potential energy and accelerate toward the Earth.

I am not sure I understand your question. The meteor doesn't have potential energy by itself (think if the meteor was the only object in the Universe). It is only the positioning of the meteor with the Earth that has potential energy.

If there is another object that affects the meteor besides the Earth, you need to include the three objects in the system. But no single object has (gravitational) potential energy.

The question about potential energy at its origin is not relevant. If two objects with mass poofed into existance with some distance between them, they (that is the system) would start out with potential energy and would accelerate towards each other. The Big Bang theory implies there was no distance between objects when the universe started, but a whole lot of energy. The inital energy can be seen as being converted into the potential energy that exists now.

Are you interested in going into the mathematics?

<head's spinning>

Mine too

Try this instead...



Yes, many do.



Not necessarily. It depends on how the hole formed.



It doesn't matter how much mass is spinning. The Black hole "jets" are a result of the orientation of the accretion disk which tends to match the orientation of spin.



No energy is required to make things fall toward each other. Falling is their natural state. Energy is required to *prevent* them from falling toward each other.

No Rosborne. I must respectfully disagree. Work is a force acting over a distance. If there is "not falling" there is no work. Saying that energy is required to prevent work from being done is patently incorrect.

My coffee cup is sitting on my table in a very pleasant state of not falling. This happy state of affairs does not require energy from the table or anything else in the Universe.

Thanks ROsborne, that does help. As does your fine-tuning EBrown.

... furthermore the state of falling "requires" energy by every definition of the term.

Gravity is doing work on the object (exerting a force over a distance)
The object is gaining kinetic energy.
The system is losing potential energy.

I think that the two jets that you wrote about are not in all black holes but only in qusars