Post by: Bill S on 23/07/2015 22:28:54
1.) How does gravity get out of a black hole?
It doesn’t. Gravity is not a force, it is a feature of the geometry of spacetime. Spacetime is influenced by the presence of the mass of the black hole; not by anything that has to escape from it.
2.) Why is the speed of gravity restricted to the speed of light?
Gravity does not travel. What travels is information about the presence/nature of the mass in question. Exchange of information is limited to “c”.
3.) What is opposing the black hole so that the black hole gravity does not go to infinity (What limits the collapse)?
If the centre of a black hole is a singularity, this is defined as a point where spacetime curvature is infinite, so gravity is infinite. Obviously, spacetime could not become more curved, and the area of infinite curvature must be infinitesimally small, so it is self limiting.
I feel sure these attempted answers are by no means the "last word", and I would appreciate comments.
Post by: PmbPhy on 24/07/2015 03:40:42
Recently I came across three questions about black holes and gravity. Below are the questions and my initial attempts at answers.Have you read the FAQ on this: http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html
1.) How does gravity get out of a black hole?
It doesn’t. Gravity is not a force, it is a feature of the geometry of spacetime. Spacetime is influenced by the presence of the mass of the black hole; not by anything that has to escape from it.
2.) Why is the speed of gravity restricted to the speed of light?
Gravity does not travel. What travels is information about the presence/nature of the mass in question. Exchange of information is limited to “c”.
3.) What is opposing the black hole so that the black hole gravity does not go to infinity (What limits the collapse)?
If the centre of a black hole is a singularity, this is defined as a point where spacetime curvature is infinite, so gravity is infinite. Obviously, spacetime could not become more curved, and the area of infinite curvature must be infinitesimally small, so it is self limiting.
I feel sure these attempted answers are by no means the "last word", and I would appreciate comments.
It will address most of your questions.
Post by: jeffreyH on 24/07/2015 08:18:22
Recently I came across three questions about black holes and gravity. Below are the questions and my initial attempts at answers.
1.) How does gravity get out of a black hole?
It doesn’t. Gravity is not a force, it is a feature of the geometry of spacetime. Spacetime is influenced by the presence of the mass of the black hole; not by anything that has to escape from it.
2.) Why is the speed of gravity restricted to the speed of light?
Gravity does not travel. What travels is information about the presence/nature of the mass in question. Exchange of information is limited to “c”.
3.) What is opposing the black hole so that the black hole gravity does not go to infinity (What limits the collapse)?
If the centre of a black hole is a singularity, this is defined as a point where spacetime curvature is infinite, so gravity is infinite. Obviously, spacetime could not become more curved, and the area of infinite curvature must be infinitesimally small, so it is self limiting.
I feel sure these attempted answers are by no means the "last word", and I would appreciate comments.
I think Pete's link summed it up. It is a very complex situation to try to understand. We have no experimental evidence for any of this. However, GR predicts various things that have been confirmed by evidence. So I would say that there should be a high level of confidence in current views on the matter.
Post by: Bill S on 24/07/2015 12:31:49
How much is the thinking about these things likely to have changed over the past 20 years?
Post by: David Cooper on 24/07/2015 18:35:05
(With LET this is not such a problem as there is no curvature creating extra space for light to travel through, so light is moving slower than normal in a gravity well instead and gravity is allowed to propogate at the unslowed speed of light through the fabric of space.)
Post by: Bill S on 24/07/2015 22:56:06
Post by: David Cooper on 24/07/2015 23:43:37
Post by: jeffreyH on 25/07/2015 17:08:20
Post by: David Cooper on 26/07/2015 01:19:16
If gravity is a curvature of space caused by a mass and all the mass is at the centre of a black hole, when you move that black hole along you have to have a mechanism to adjust the curvature of the space ahead of the black hole. If you don't want any signals to go faster than light, you'd have to have the leading part of the "dent" somehow take up the shape of the part of the dent ahead of it and pass its own shape to the part just behind it without any guidance from the centre, but if you tried to implement a mechanism of that kind, it would go wrong when two black holes collide because the dents would just keep going (looking like normal black holes but with no content) while the singularies merge and stop.
Post by: Bill S on 29/07/2015 17:53:01
https://en.wikipedia.org/wiki/Vacuum_solution_%28general_relativity%29
“The fact that the gravitational field itself possesses energy yields a way to understand the nonlinearity of the Einstein field equation: this gravitational field energy itself produces more gravity.”
Why would this not result in “run-away” increase in gravity?
Post by: jeffreyH on 30/07/2015 00:15:32
Post by: Bill S on 31/07/2015 21:10:28
Quote from: Jeffrey
If you think in terms of a decreasing trend where less and less gravity results due to the decreasing amounts of energy in each successive generation there would be no runaway effect.
That makes sense to me, but are we sure that energy would decrease in successive generations, and if it did, would that necessarily bring the process to a halt?
Post by: jeffreyH on 01/08/2015 18:38:49
Quote from: JeffreyIf you think in terms of a decreasing trend where less and less gravity results due to the decreasing amounts of energy in each successive generation there would be no runaway effect.
That makes sense to me, but are we sure that energy would decrease in successive generations, and if it did, would that necessarily bring the process to a halt?
I can't answer that question at the moment. What I can say is that we do not see an infinite amount of gravitational energy from sources in the universe around us. I am also sure that conservation laws must prevent this.
Post by: evan_au on 03/08/2015 10:32:13
Quote from: BillS
In the case of a black hole that forms from the collapse of a star, is the gravity at the event horizon greater than the gravity at the surface of the original star?The gravitational acceleration at the event horizon will be far greater than the gravitational acceleration at the surface of the original star.
My reasoning:
- Let's take a star with radius R m
- If we call the gravitational acceleration at the surface of this star A m/s2.
- If the entire mass of that star collapses into a black hole, the gravitational acceleration at distance R from the center of the black hole will still be A m/s2
- Let's call the Schwarzchild radius of the black hole S m.
- The gravitational acceleration at the surface of the black hole will be A(R/S)2.
- ...And the tidal forces grow even faster than an inverse square law...
Quote
how does a situation in which two black holes are approaching each other differ from one in which two stars are approaching each other?Stars in close binary orbits often suck loosely-held gas from the less-dense star, and dump it onto the denser star. This atmospheric stripping starts well before their distance is equal to the sum of their radii. This frequently causes a Type 1a supernova (https://en.wikipedia.org/wiki/White_dwarf#Type_Ia_supernovae_2). But the stars are still at a fairly large distance when their stellar atmospheres merge.
Black holes have a much smaller radius than their parent stars, so they can approach to much closer distances, with much faster orbital velocities, but with no transfer of gas between them. They merge when their event horizons touch, which is at a relatively small distance.
Post by: jeffreyH on 03/08/2015 18:50:50
Quote from: BillSIn the case of a black hole that forms from the collapse of a star, is the gravity at the event horizon greater than the gravity at the surface of the original star?The gravitational acceleration at the event horizon will be far greater than the gravitational acceleration at the surface of the original star.
My reasoning:
- Let's take a star with radius R m
- If we call the gravitational acceleration at the surface of this star A m/s2.
- If the entire mass of that star collapses into a black hole, the gravitational acceleration at distance R from the center of the black hole will still be A m/s2
- Let's call the Schwarzchild radius of the black hole S m.
- The gravitational acceleration at the surface of the black hole will be A(R/S)2.
- ...And the tidal forces grow even faster than an inverse square law...
Quotehow does a situation in which two black holes are approaching each other differ from one in which two stars are approaching each other?Stars in close binary orbits often suck loosely-held gas from the less-dense star, and dump it onto the denser star. This atmospheric stripping starts well before their distance is equal to the sum of their radii. This frequently causes a Type 1a supernova (https://en.wikipedia.org/wiki/White_dwarf#Type_Ia_supernovae_2). But the stars are still at a fairly large distance when their stellar atmospheres merge.
Black holes have a much smaller radius than their parent stars, so they can approach to much closer distances, with much faster orbital velocities, but with no transfer of gas between them. They merge when their event horizons touch, which is at a relatively small distance.
If we take
then as r approached rs the result will tends towards 1. If we take the maximum value of A to be equal to an instantaneous velocity of c at r=rs then that states in a simple way the relationship between escape velocity and and g. However this cannot be true since g depends upon distance from source and the density of that source. Since density drops with increase of mass we can arrive at a situation where g < 1 at the event horizons of very large black holes. To balance the books there is something missing in your line of reasoning.NOTE: Also as initially r > rs then would't that give superluminal speed outside the horizon?
Post by: evan_au on 04/08/2015 11:50:45
Quote from: jeffreyH
Since density drops with increase of mass we can arrive at a situation where g < 1 at the event horizons of very large black holes.Spaghettification (ie acceleration due to gravity differing significantly between your head and your feet) is inevitable when approaching a stellar-mass black hole.
However, the rate of change of gravitational acceleration is much gentler near a supermassive black hole, and I understand that reaching the event horizon in one piece is theoretically possible (provided that you don't get minced and/or fried by an accretion disk)....
One of the largest known black hole candidates is in the center of NGC4889 (https://en.wikipedia.org/wiki/Schwarzschild_radius#Supermassive_black_hole), with an estimated mass of 21 billion solar masses, and a Shwarzchild radius rs≈6x1013m. If my quick calculation is correct, the acceleration due to gravity at the Schwarzchild radius is about 10 times Earth-normal (as seen by a distant observer).
So I suggest that it is unlikely that you can find a black hole in our part of the universe that has a gravitational attraction as low as Earth's surface 10m/s at rs.
Post by: jeffreyH on 04/08/2015 23:04:52
Gravity is an interaction between waves of light in the infrred spectrum, its all around us everywhere in space, There is a big mystery around what light is and how it travels, I explain that under TOE fun. Also this also clears up how light waves interact, The slit experiment caused a lot of confusion my theory explains it clearly and simply [;)]
Prove it.
Post by: jeffreyH on 04/08/2015 23:10:59
Quote from: jeffreyHSince density drops with increase of mass we can arrive at a situation where g < 1 at the event horizons of very large black holes.Spaghettification (ie acceleration due to gravity differing significantly between your head and your feet) is inevitable when approaching a stellar-mass black hole.
However, the rate of change of gravitational acceleration is much gentler near a supermassive black hole, and I understand that reaching the event horizon in one piece is theoretically possible (provided that you don't get minced and/or fried by an accretion disk)....
One of the largest known black hole candidates is in the center of NGC4889 (https://en.wikipedia.org/wiki/Schwarzschild_radius#Supermassive_black_hole), with an estimated mass of 21 billion solar masses, and a Shwarzchild radius rs≈6x1013m. If my quick calculation is correct, the acceleration due to gravity at the Schwarzchild radius is about 10 times Earth-normal (as seen by a distant observer).
So I suggest that it is unlikely that you can find a black hole in our part of the universe that has a gravitational attraction as low as Earth's surface 10m/s at rs.
The interesting region appears to be very near the horizon of a black hole using the Schwarzschild metric. Although the Kerr metric is more realistic it is also a lot harder to work with. I am talking in Planck lengths here and applicable to any size of black hole.
Post by: Bill S on 05/08/2015 23:14:12
Post by: Mordeth on 09/08/2015 14:39:22
Comparisons between Schwarzschild and Kerr black holes is interesting, but is there any evidence for non-rotating BHs anywhere in the Universe? Is it even possible, under the constraints of gravity, for a body to form by accretion without rotating?
Black holes do not form by accretion. They form due to the collapse of a massive star. Accretion is how the black hole grows. There is no evidence of a non-rotating black hole, although it is not forbidden. Even one photon hitting a non-rotating black hole would cause it to spin due to conservation of angular momentum. It is also assumed that a black hole retains the angular momentum of the star that formed it. Therefore, one would expect most, if not all actual black holes are Kerr black holes ie., rotating. The math used to derive a Schwarzschild black hole is simply far easier to deal with and the Schwarzschild solution is a useful teaching aid.
Post by: Bill S on 09/08/2015 23:49:23
I should have said:
Is it even possible, under the constraints of gravity, for a body that may later collapse to a BH, to form by accretion without rotating?
Post by: Mordeth on 10/08/2015 02:06:58
Hi Mordeth, good to meet a fellow nit-picker. [:)]Hi Bill,
I should have said:
Is it even possible, under the constraints of gravity, for a body that may later collapse to a BH, to form by accretion without rotating?
Highly unlikely but theoretically possible. The issue is not with gravity but with the conservation of angular momentum and the distribution and spin of particles. Consider a cloud of dust. If all particles of the cloud were evenly distributed and the cloud was a perfect sphere with no angular momentum, then it could collapse via accretion into something that is non-rotating. This object could form a non-rotating black hole if it's mass collapses to within its Schwarzchild radius.
Post by: Bill S on 10/08/2015 14:56:48
Quote from: Mordeth
Consider a cloud of dust. If all particles of the cloud were evenly distributed and the cloud was a perfect sphere with no angular momentum, then it could collapse via accretion into something that is non-rotating.
As you say; very unlikely, I don't know enough about the physics of this sort of situation to know if it is even possible, but it raises some interesting thoughts.
Could there be a perfectly spherical cloud of dust if accretion was not already acting on it?
Would there have to be a particle that was absolutely central to avoid rotational movement?
If gravity = spacetime curvature, would that curvature initiate rotation?
That's just a start, there would then be questions about the initiation/conservation of angular momentum, but let's not go there (yet). [:)]