# Black Hole Theory

Discussion in 'Physics & Math' started by livingin360, Jan 15, 2012.

1. ### rohITRegistered Senior Member

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76
i feel that particles, stars, "television sets" or whatever you throw into a black hole will never reach its surface. According to Relativity, as we move faster, our speed in time slows down.
Moreover, escape velocity of black holes is greater than the speed of light and, since relativity rules out the possibility of anything traveling at the speed of light, the particle can never reach the surface (unless it reaches a terminal velocity somehow...i wonder how that will take place!)
So, as the particles move closer to the surface, they will slow down and never reach the surface. Again, that is what i think will happen. Plz do tell if you have reasons to think otherwise.

3. ### James RJust this guy, you know?Staff Member

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38,312
rohIT:

There's no need for something to travel at the speed of light in order to fall into a black hole. It's only getting out of a black hole that requires an escape velocity faster than light.

What actually happens is this:

A distant observer watching a TV set fall into a black hole will see the TV falling. As it falls, the light from the set becomes more and more red-shifted. The set also appears to slow down as it approaches the event horizon. In fact, it never quite reaches the event horizon before it "fades to black".

A person riding on the same TV set sees something quite different. They see the outside universe behind them becoming more blue-shifted as they fall towards the event horizon. As they cross the horizon, they don't notice anything special. Then, a while after that, they start to feel immense tidal forces which gradually pull them apart (spaghettification!). The person and TV set then are ripped apart into constituent atoms and sub-atomic particles. When they reach the singularity, we don't know what happens, exactly.

5. ### waitedavid137Registered Senior Member

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90
Why do so many people insist on trying physics with their feelings? This is never valid.
As long as the stuff dropped in can be treated as a test mass, that is as long as you can neglect the effect on spacetime of whats infalling then what you have is a static state Kerr solution really but most people will answer this in reference to the Schwarzschild solution and then yes it would take forever according to a remote observer for anything to cross the event horizon. Thats the typical answer you'll get, but in reality the metric won't actually be one of those solutions, but will be dynamic including the effects on spacetime of both the mass of the black hole and whatever is falling into it. The real answer is that its a nonlinear problem where the event horizon does grow to consume the matter around the hole as the stuff is infalling and this does happen in finite remote time.

7. ### rohITRegistered Senior Member

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76
@James R
What i am saying is that when the particles "land" on the surface of a black-hole (need not and, can't acc to String Theory, be a singularity), they must have velocity greater than the speed of light. Consider an object being thrown up from the surface of the earth. In absence of any fluid friction from atmosphere, the object must reach the surface of the earth with the same velocity it was thrown up in order to conserve energy. Now, the same thing must apply for BHs. If an object from outside is falling into a balk hole, its velocity on its surface needs to be equal to the escape velocity unless a terminal velocity is reached by the body. Moreover, as the object speeds up, its time slows down. So, for the object, it can as well "feel" as though it is stationary in time and thus does not move at all.

8. ### rohITRegistered Senior Member

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76
I understand the difficulty in trying to bring feelings into what fact should say, but, you cant predict anything inside the black hole. All you have are a set of equations, and this set, you cant verify to be true inside a black hole as you cannot observe what is happening to things as they fall inside. I think, in situations like this, intuition could be given a chance and then whether it is correct or not, can be verified theoretically.
Again, what i suggested, wasn't baseless

A black hole cannot have an atmosphere and so, there is very little probability of viscous forces making the particles to reach a terminal velocity. Basically, what i typed previously

9. ### OnlyMeValued Senior Member

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3,914
You begin here by saying that what occurs within a black hole is essentially limited to theory, this much is true...

The problem then seems to be that though you have defined a black hole as theoretical you add an absolute statement that a black hole cannot have an atmosphere.

The question to you then becomes why can't a black hole have an atmosphere? And what other than theory, your own in this case, dictates it to be so?

If we follow the logic of experience and observation, any gravitating object of sufficient mass should have an atmosphere. A black hole certainly seems to have sufficient mass. And you have already established that most of what goes on in a black hole is theoretical and unconfirmed.

10. ### rohITRegistered Senior Member

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76
@OnlyMe
The reason why i wrote that is a black hole's immense gravity. It will pull to its surface anything that is near. But, that's only when the black hole is formed (Neutron stars are said to have few centimeters of atmosphere). Now, when a particle enters the event horizon with any velocity, its potential energy loss as it falls inside is compensated by increase in kinetic energy. So, if a particle entered the event horizon with 0 velocity, it will reach with as much velocity on its surface as is required for it to escape from the black hole with 0 velocity if it is projected from the same point. It implies that a particle has to reach a velocity greater than speed of light, which relativity rules out. So, maybe, these particles are suspended above the surface (this is only a speculation) and when sufficient particles are suspended, probably those can provide enough resistance to slow down the falling particles.
So, if this secondarily acquired (if at all it is) atmosphere is what you are talking about, it could (theoretical and unconfirmed) but, it cannot have an atmosphere right from its birth.