PDA

View Full Version : Black hole question

Uclock
2006-Oct-18, 05:30 PM
Known physics tells us that at the centre of a black hole there is a singularity, if this is true then why do different black holes have a different mass?
Surely a singularity is a singularity, how can they have a different mass?

antoniseb
2006-Oct-18, 05:49 PM
Known physics tells us that at the centre of a black hole there is a singularity, if this is true then why do different black holes have a different mass?
Surely a singularity is a singularity, how can they have a different mass?
Hi Uclock, welcome to the BAUT forum.

To answer your first question, I'm not sure what you ean by "known physics". Some models predict that there is a singularity, and other don't. The fact is that very little can be said with certainty about what happens inside an event horizon.

We can say for relatively certain that black holes CAN have different masses, as we have observed stellar mass black holes orbiting other objects (or vice versa), and stars and other luminous material orbiting super-massive black holes in the centers of galaxies.

We may also be able to say that we've observed that black holes have angular momentum. It is not clear whether they can have charge or magnetic moment. If they do, that will say something about whether they have singularities.

Blob
2006-Oct-18, 06:00 PM
Hum,
is this like asking why a cup of water weighs more than an empty cup?
(ie. How can things of the same size have different gravity wells?)

Uclock
2006-Oct-18, 06:37 PM
Thanks for the welcome Antoniseb.

Hi Blob

What I am trying to say is if gravity becomes infinite inside a black hole then that should happen to all black holes, how can they have a different mass if they contain a singularity at the centre? or is it just that our models of black holes are wrong because the physics break down?

Hi Antoniseb
When I mean known physics I mean the models we use to provide us with answers.

So in the end we know nothing about the inside of black holes because the physics used to model them break down. Interesting!

antoniseb
2006-Oct-18, 06:52 PM
if gravity becomes infinite inside a black hole...
Sometimes people get confused by this idea. The gravitational force may be infinite between two test masses in a singularity (because R=0, not because M=infinity), but the gravitational force experienced by a mass outside the singularity is not infinite because the distance from the center of gravity is not zero.

Uclock
2006-Oct-18, 07:36 PM
Yes, but the gravitational force inside a black hole always become infinite as you move towards the centre.

Is this because our physics breaks down and we have something wrong.

Might it be we have an incorrect understanding of gravity?

antoniseb
2006-Oct-18, 07:43 PM
the gravitational force inside a black hole always become infinite as you move towards the centre.
Might it be we have an incorrect understanding of gravity?
How do you know that it always becomes infinite? Also, it doesn't become infinite until the test mass is in the center, not as it moves toward it. And again, that is only if there are singularities.
Is it possible that we have "an incorrect understanding" of gravity? I think a better way to express it is that our models for gravity have not been tested accurately under the most extreme physical circumstances, and so for special situations, our models may prove to be making simplifying asumptions that are no longer valid. Our model of gravity works very well for things we've been able to measure so far.

publius
2006-Oct-18, 07:53 PM
Uclock,

First, the singularity is a singularity in density, ie mass per volume. A finite mass M is thought to be concentrated at a point of zero volume. The density is what is infinite, not the total mass.

Second, the notion of "gravitational force" does not exist in the Newtonian sense per General Relativity. The force required to remain stationary in the classical Schwarzchild black hole actually becomes infinite at the event horizon. Past that, there is no such thing as remaining stationary. While tidal forces are real and felt by free fallers, there is no net "gravitational force" felt by any point test mass that free falls into the black hole.

Per the rather complicated math of General Relativity, space and time actually "flip" at the event horizon. The singularity exists in the future of all paths that cross the horizon.

-Richard

Uclock
2006-Oct-18, 08:54 PM
The test mass would experience a stronger force at the end nearest the singularity than the end farthest away from the singularity or am I mistaken?

It seems you might be missing my point. Does known physics break down at the singularity?

If not what comes after the singularity?

antoniseb
2006-Oct-18, 11:09 PM
Does known physics break down at the singularity?
Known Physics does not extend inside the event horizon, so it can't really break down at the singularity if there is one.

Ihopeso
2006-Oct-19, 12:57 AM
The test mass would experience a stronger force at the end nearest the singularity than the end farthest away from the singularity or am I mistaken?

It seems you might be missing my point. Does known physics break down at the singularity?

If not what comes after the singularity?

Ludicrous is it not, a singularity is a mathmatical entity, I mean it gives me open slather on what a black hole can and can not do! :shifty:

Now your using your head Uclock, what with physics "breaking down" and redshift equals distance under serious threat, looks like the 'mainstream' model is in serious trouble.

Question how long before TPTB eat a little humble pie, step back re-examine the data and branch out from there :whistle:

There are a couple of ATM theories out there :shhh: that explain nearly all observed phenomena extremely well, but you get banned if you dare mention them :naughty: to inquisitive minds like yours. :think: So good luck in your hunt for answers!

antoniseb
2006-Oct-19, 01:16 AM
you get banned if you dare mention them
Actually it takes a little more than mentioning them to get banned. We do, however, ban sockpuppets when we find them, for little more than just posting.

Blob
2006-Oct-19, 01:53 AM
Known Physics does not extend inside the event horizon, so it can't really break down at the singularity if there is one.

Hum,

Tensor
2006-Oct-19, 02:41 AM
Known Physics does not extend inside the event horizon, so it can't really break down at the singularity if there is one.

Actually antoniseb, that's not true. Using GR, we can model down almost to the singularity. It depends on the choice of coordinates. After all, the singularity is nothing more than where the equations of GR don't work. And, there are two kinds of singularities near a black hole. One, can be avoided by changing coordinates. Such as the singularity at the EH, in spherical coordinates. Switch to Kruskal coordinates and you can model right down to the singularity. Be aware that Kruskal coordinates are more useful inside the EH, and spherical are more useful outside the EH. The other type of singularity cannot be transformed away in any coordinates, which describes the singularity at the center of a black hole. As Publius pointed out, within the EH, spacelike and timelike spaces change into the other. For GR, known physics doesn't extend to singularity, but is valid everywhere else. This all supposes that GR is a valid model. And we really don't have any positive evidence otherwise, except at the singularity inside a black hole or the singularity at the begining of the big bang.

As for Uclock's question about what comes after the singularity, we don't know. We don't know of anything that can model it.

antoniseb
2006-Oct-19, 10:35 AM
Actually antoniseb, that's not true. Using GR, we can model down almost to the singularity.

I didn't say that our current models don't make predictions. I said that known physics does not extend beyond the event horizon. After that we are relying on very untested parts of GR, if you take the GR approach.

Tensor
2006-Oct-19, 11:25 AM
I didn't say that our current models don't make predictions. I said that known physics does not extend beyond the event horizon. After that we are relying on very untested parts of GR, if you take the GR approach.

Ahhhhhhh, you are quite correct. It's a subtle difference that I missed. Sorry.

Uclock
2006-Oct-19, 11:34 AM
So GR flips time and space at the event horizon and the shortly after begins to break down but it describes space as we see it very well outside a black hole although there is no disription of time in GR.

I take it then that GR also breaks down near the big bang so at all the extremes of this Universe GR breaks down but it is still considered the best theory of gravity that mainstream physics has.

Is everyone happy with the fact that GR breaks down at the extremes of this Universe?

antoniseb
2006-Oct-19, 11:50 AM
Is everyone happy with the fact that GR breaks down at the extremes of this Universe?

Happy, no, content for the time being, yes.
The frontiers of science are very interesting. Exploring the realms where GR is untested is very exciting. In the rest of this century, and probably beyond, we will increase the detail with which we can observe near the event horizon of Sgr A*, and some nearby stellar mass black holes, as well as some nearby giant SMBHs. We will also make higher energy observations in the laboratories that will tell us more about the small scale of matter, and the area where GR and Quantum Mechanics don't really work together.

Uclock
2006-Oct-19, 05:02 PM
Happy, no, content for the time being, yes.
The frontiers of science are very interesting. Exploring the realms where GR is untested is very exciting. In the rest of this century, and probably beyond, we will increase the detail with which we can observe near the event horizon of Sgr A*, and some nearby stellar mass black holes, as well as some nearby giant SMBHs. We will also make higher energy observations in the laboratories that will tell us more about the small scale of matter, and the area where GR and Quantum Mechanics don't really work together.

I wish I could be content with a theory of gravity that breaks down but unfortunately it is my nature to question things until I find the right answer.

antoniseb
2006-Oct-19, 05:23 PM
I wish I could be content with a theory of gravity that breaks down but unfortunately it is my nature to question things until I find the right answer.

Roll up your sleeves and dive in. Learn the math, and what we have observed and can observe, and build experiments that get to the core of the problem. If you are willing and able to advance our understanding, then you are a valuable commodity in the world of science.

If on the other hand all you are going to do is take a position that you are not satisfied, and hope that someone else will hand you the answer, you have a long disappointing wait ahead of you.

You have a lot of reading to do. I'd start here (http://arxiv.org/). If you bump into things you don't understand, try Wiki and Google to help get answers.

Argos
2006-Oct-19, 05:40 PM
Originally Posted by Uclock:
if gravity becomes infinite inside a black hole...

Sometimes people get confused by this idea. The gravitational force may be infinite between two test masses in a singularity (because R=0, not because M=infinity), but the gravitational force experienced by a mass outside the singularity is not infinite because the distance from the center of gravity is not zero.

I think what Antoniseb is saying is that a finite mass divided by a zero volume yields an infinite density. Everything becomes infinite when divided by zero, and that&#180;s what singularities are about.

RussT
2006-Oct-19, 09:21 PM
(because R=0, not because M=infinity),

What it seems that everyone is forgetting, is that 'singularity' is just a placeholder for what is really happening there!

R does NOT = 0 and M does NOT = infinity!!!

R = Planck length/size and M = the smallest mass that a particle can be compressed to, which = Planck mass.

Then you get into the differences that could exist between stellar black holes and Massive Black Holes.