Black hole question

[Guest]

I recently read that time varies wrt gravity and that at/near the event horizon it pretty much stops. If this is the case I can't understand a couple of things:

1) How does the black hole swallow captured mass as it seems to me to sit and accumulate on the event horizon.

2) I'm probably looking at this whole issue simplisticaly (make that definately) but as far as I'm aware a black hole isn't a singularity. Given this view and that gravity in the center of a sphere is zero, wouldn't that make a black hole a very thin hollow ball of varible radius?

That kinda makes some sense to my simple mind but I know I'm missing something.

Here's one to really laugh at: If all the above are to some degree valid then couldn't it be that we are inside a black hole (our universe) and hence explain the universe we can see as expanding and accelerating as well?

Simple answers in laymans terms would be greatly appreciated.

Thanks in advance.

[maxim]

1) How does the black hole swallow captured mass as it seems to me to sit and accumulate on the event horizon.

Because only an external viewer would see this - he sees the last information available about an object falling into a BH 'frozen' at the EH. A traveller passing the event horizont wouldn't notice something special. You may read some popular sience books on that (i.e. Stephen Hawking).

maxim

[ajtribick]

Further question: doesn't the outside observer also see the black hole evaporate through Hawking radiation in finite time.

Therefore if from an outside observer's point of view, it takes an infinite time to cross the event horizon, but a finite time for the black hole to evaporate, surely if you are falling into the black hole you never cross the event horizon but instead get fried by an almighty burst of gamma radiation when the black hole evaporates?

[Guest]

I can see how the external observer see's this. For the one going into the black hole time slows to a crawl, this means a lot more things come into play. Getting fried by is radiation is one. Being blown apart as the black hole's mass evaporates below its critical limit to stay as a black hole is another- Big quess from me based on logic, but, alas my logic may not have enough information to be correct in it's answer or how it derives an answer.

The one I'm wondering is if the mass does indeed concentrate as a shell as it implies a lot of things. To me this seems logical as this is what is observed from the external viewer. The "victim" would has his time slowed down so slow the universe may as well end before a single second passes for him and is therefore irrelevant.

The other question is if gravity gets affected near the event horizon. eg, if the mass is slowly getting deposited on the outside and time is slowed so much, then what is left of its gravity influence? Things I read state gravity is as fast as light, if this is the case then surely whats left is a gravitational echo of what was once there? If it's not the case then gravity must be faster than light????

All verrry confusing. Looks like I have much to learn.

[eburacum45]

Several things to remember-

the mass in a black hole is supposed to be concentrated in a dimensionless point at the centre; not at the event horizon.

Apart from being torn apart by the difference in gravitational pull between your head and feet, you would not experience anything special at the event horizon; for you time would pass normally. (if the black hole were very big, you would not even get torn apart, as the gravity gravient would be less steep.)
If it took a minute to fall in that is all you would experience- slightly less if you achieved relativistic speed on the way in.

Only from an external viewpoint would your time appear to slow; this is because the light reflected by you or emitted by any torch or transmitter you were holding would be stretched by gravity as it struggles to escape- this redshifting would become infinite at the event horizon, but would not affect you personally.

[maxim]

Thats correct.
The traveller wouldn't watch anything special while passing the EH. You may take the model of going supersonic and may speculate if everything in front of the traveller is becoming invisible black, but not very much more would happen.

This situations where discussed in popular sience book ten years ago. You may take a look into you local bib.

maxim

[Mikeydude750]

Well...if you want to be technical...if the person being sucked in were to look down, he would see weird things happening, but overall, he wouldn't notice.

[Guest]

wow, now I'm really confused.
So let me get this straight, what I gather now is that the ingoing observer will fall into the event horizon quickly from his point of view but slowly from the external observers view. So far so good as this much I figured as time is slowed for the in going observer.

Then it gets messy for me.
From what you're saying the in going observer actually enters the EH and quickly wrt to the external observer but the external observer can't actually observe that it's happened! For the in going observer this too happens quickly.

If this is the case then the internal observer has gone through the EV and entered the singularity(?) already even though the external observer is still watching him fall. Since theory states nothing travels faster than light then from an external point of veiw he is still at or near the event horizon by any measurable means we can check. E.g. His gravity interaction with the outside space is as if he was still at the EH even though he is part of the singularity. Inside the EH this gravity "echo" wouldn't be felt for if it was then we come back to the black hole accumulating mass on the event horizon. If it isn't the there is a discontinuity in space time at the EH. I hope I'm using the correct terminology here, please forgive me if I'm not.

To avoid the discontinuity it's effect must be felt inside the EV but this makes no sense either unless the in going observer doesn't actually ever reach the singularity.

This makes no sense to me, eg. two black holes colliding. This would never be observed by an external observer as they will never pass each others EH??? But they actually have come together as a singularity???

Best I get to a library and read up on all this as you suggest. The only problem is that most books that discuss such topics are written by physisists or mathematicians and are generaly written for them too. Need to find one of the gifted few that can explain things at a lower level, ie describe what happens not the maths that explains what happens.

Any suggestions are very welcome.

[maxim]

Traversing the EH does NOT mean entering the singularity. The EH is not the singularity. Actually it is ways off.
It is obvious that the EH can't be the singularity because it has a surface, and therefore exists in three dimensions, whereas the singularity has no dimension. The EH is only the border at which all light is trapped under gravity influence of the black hole.

Here are some books that are quite easy to read:

Martin Rees, 'Before the Beginning. Our Universe and Others'
1997, Simon & Schuster Ltd, London

John D. Barrow, 'Theories of Everything: The Quest for Ultimate Explanation'
1991, Oxford University Press, Oxford/New York

James Trefil, 'The Dark Side of the Universe'
1988, Charles Scribner's Sons/Macmillan Puplishing Company, New York

Stephen W. Hawking, 'A Brief History of Time: From the Big Bang to Black Holes'
1988, Bantam Books, New York


maxim

[Guest]

If this is the case then the internal observer has gone through the EV and entered the singularity(?) already even though the external observer is still watching him fall.

There's no "already" about it - from the external observer's point of view, our falling victim really never crosses the event horizon - he approaches it asymptotically. But the falling person experiences time differently, and perceives he arrives in a finite time and then progresses inexorably into the singularity.

Since theory states nothing travels faster than light then from an external point of veiw he is still at or near the event horizon by any measurable means we can check. E.g. His gravity interaction with the outside space is as if he was still at the EH even though he is part of the singularity.

But all we can see as external observers is the gravitational effect of the black hole outside its event horizon - we can't ever sneak into the hole and observe our victim apparently plastered to the event horizon ... as soon as we approach, we're caught up in the local time frame and find ourselves falling behind him into the black hole. So to an external observer the hole has gained the victim's mass, and if we approach we'll encounter the event horizon exactly where it would be expected for that new, higher mass (because he has preceded us into the hole).[/quote]

This makes no sense to me, eg. two black holes colliding. This would never be observed by an external observer as they will never pass each others EH??? But they actually have come together as a singularity???

You're treating the event horizon as if it were some observable surface with an associated time rate. It's just a mathematical surface that accommodates itself to the internal mass. So it's going to behave differently from a falling person with a clock - the event horizons aren't going to appear frozen in time relative to each other, and the merging of the singularities (which have no meaningful intrinisic time rate at all) is shielded from our view.

Grant

[Matt McIrvin]

Here's a FAQ that deals with many of these questions:

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html

[ElPelado]

1) How does the black hole swallow captured mass as it seems to me to sit and accumulate on the event horizon.

Why a satellite around Earth may fall? Because even at that attitude there is still gas from the atmosphere that produces friction with the satellite and slows it down, making his orbit lower and lower till the it reenters teh atmosphere.
The same happens in a Black hole: all the matter and gass around it revolves at different speeds. That produces the same effect: a friction between things revolving in different velocities slows down the matter that is nearer to the black hole, and that makes it fall to the black hole...

[julesstoop]

That's not his question. From an outside observer matter never seems to pass the event horizon. Then: how is it possible for a black hole to become more massive measured from the same outside vantage point(?)
Measuring a suposedly growing mass is as much an observation as not seeing any matter contributing to the growth cross the EH -> paradox.

[Evil Dr Ganymede]

That's not his question. From an outside observer matter never seems to pass the event horizon. Then: how is it possible for a black hole to become more massive measured from the same outside vantage point(?)
Measuring a suposedly growing mass is as much an observation as not seeing any matter contributing to the growth cross the EH -> paradox.

*blink, blink*

Damn. That's a really good point...! A BH shouldn't be able to increase in mass while we observe it because anything we see approaching the event horizon will slow down and stop before it crosses it - technically, we'd only see the mass cross it if we waited til the end of the universe!

Wow. That's something I'd definitely be interested in hearing an explanation for!

[eburacum45]

There is an explanation here, Oh Evil one;
http://www.astronomycafe.net/qadir/q756.html

to muddy this explanation up a bit, I think that if you consider gravitons as the particles which carry gravity, they are able to cross an event horizon because they are virtual particles, so can get out of black holes. Electromagnetic fields are also carried by virtual particles so they can get out too, which is why charge can be extracted from a charged black hole.

But the real maths is much more complex than that oversimplification, or so I am led to believe.

[cpotting]

Hmmm...

I know this is an old thread, but I just got around to looking at this forum.

It seems to me that several people have mixed up the visual effects that gravity has with what is actually happening.
To illustrate the difference: just because you see a star in the sky does not mean that is actually there. The star moved several AU over the years and then went supernova before your grandfather was born. But you still see a point it in the sky (where it was).

Similarly, as the outside observer sees the ingoing subject approach the event horizon, the light from the subject will be progressively slowed by the gravity of the black hole. The observer will see light that originated at progressively smaller increments in time arriving at (for the observer) constant increments in time. This gives the illusion that time is slowing down for the ingoing subject.

However, the inbound subject does fall through the event horizon (which is NOT the same as the singularity). He will (very shortly) meet a very nasty end. But the light emitted from the microsecond before he crossed event horizon has finally struggled out of the gravity well and reached the observer.

A moment later the inbound subject is finding very difficult to hang on to his consistuent atomic particles (and his lunch) - and at that moment the light from the nanosecond before he hit the EH reaches the observer.

A moment after this the inbound subject has changed is mind and is screaming about a raise in pay... And the light from the 10 attoseconds before he hit the EV has reached the observer.

Now, the inbound subject hits the singularity and Hawkins-knows-what happens to him. But his mass is now part of the singularity (which now weighs in a 20 billion solar masses + 94 kgs). And the light of from 2 attoseconds before he hit the EV struggles free to reach the observer.

Jump ahead 10 years. The inbound observer is now floating towards Earth cleverly disguised as an unborn baby, and listening to Wagner. His widow is happily re-married to an insurance salesman, and gasoline now costs $14.35 a litre........... And the light from .004 attoseconds from before the observer hit the EV has finally made it to a very bored, and severly underpaid observer.

The observer will NEVER see when the inbound observer reaches the EV. But that does not mean that it didn't happen!

Hope that helps.

Oh, and by-the-way, remember that black holes and singularities are not the same thing. A black hole can be anything that has enough gravity to generate an event horizon. A dense neutron star can do this. As soon a the escape velocity at the surface exceeds the speed of light, you have a "visible" event horizon and therefore a black hole.

Clive Pottinger.

I

[Guest]

...black holes and singularities are not the same thing. A black hole can be anything that has enough gravity to generate an event horizon. A dense neutron star can do this. As soon a the escape velocity at the surface exceeds the speed of light, you have a "visible" event horizon and therefore a black hole.

That is news to me;
a dense neutron star?

I thought they collapsed into singularities as soon as they became black holes...
not that there would be much difference from the outside, I suppose, though that too might be wrong.

[granthutchison]

Similarly, as the outside observer sees the ingoing subject approach the event horizon, the light from the subject will be progressively slowed by the gravity of the black hole.

Remember that light travels at the speed of light - although each photon loses energy as it climbs out of a gravity well, its velocity is undiminished. So you can't explain things by suggesting that the light just takes longer to make the journey. The signals from the falling astronaut do arrive farther and farther apart in time, but this is because his time rate really does get slower (relative to a distant observer) as he moves deeper into the gravity well - he seems to approach the event horizon ever more slowly, emitting ever more red-shifted photons. As in all of relativity, though, the two observers disagree about what events are simultaneous - the distant observer sees the falling astronaut never reaching the event horizon, but for the falling astronaut time passes briskly and he gets there and passes through quite swiftly.

Oh, and by-the-way, remember that black holes and singularities are not the same thing. A black hole can be anything that has enough gravity to generate an event horizon. A dense neutron star can do this.

I'm interested to know more about this - what's a "dense" neutron star? The escape velocity from the surface of a conventional neutron star is only about a third of the speed of light, so it won't form an event horizon.
And everything inside a black hole must end up at a central singularity, because spacetime is so distorted that the time and radial space dimensions are swapped inside the event horizon - the radial direction becomes "time-like" and you cannot help but travel along it. There's no escape from singularity formation.

Grant

[Art_Vandelay]

Could an external viewer even see a black hole? I am probably wrong, but as the star collapses to form the black hole, the star's material will fall into itself at an increasing speed after the supernova, right? It falls into itself faster and faster, eventually reaching the speed of light an instant before the singularity has actually formed. This means that, to an external viewer, the matter will appear to slow down and stop an instant before the singularity has formed. No singularity, no black hole. It's just a stray thought I had, but I'm certainly not a physicist. Am I wrong about this?

[granthutchison]

Well, that's what a black hole is - it's just a region of space you can't get any information from (that's the meaning of "event horizon" - the place beyond which you are ignorant of all events). So what you see is a patch of nothing, surrounded by an area of distorted vision and extreme redshifts because of the effects of the black hole's gravity.
The singularity is therefore always (and mercifully) invisible, because it is hidden by the event horizon.

Grant