black holes do not have infinite gravity!

[alpha to omega]

if black holes had infinite gravity, since the force of gravity is limitless, wouldn,t it have infinite gravity throughout the entire universe?

 

Edited August 27, 2018 by alpha to omega
spelling and grammer

[Strange]

1 minute ago, alpha to omega said:

if black holes had infinite gravity ,since the force of gravity is limitless ,wouldnt it have infinite gravity troughout the entire universe?

They don't.

But I suppose you are right, if they did then it would be infinite at any distance.

[alpha to omega]

it's just that so many people think that because their gravity is strong enough to stop light

 

[dimreepr]

4 minutes ago, Strange said:

They don't.

But I suppose you are right, if they did then it would be infinite at any distance.

And we'd all be living in flatland.

[Strange]

4 minutes ago, alpha to omega said:

it's just that so many people think that because their gravity is strong enough to stop light

I doubt many people think that the gravity is infinite. (If they do, they are just confused.)

But it is strong enough to stop light escaping. That is why they are called "black holes".

[Janus]

28 minutes ago, alpha to omega said:

it's just that so many people think that because their gravity is strong enough to stop light

 

That's due to a concept known as "escape velocity".   It the velocity something would have to be moving in order never fall back to the object.

For example, the escape velocity from the surface of the Earth is ~11 km/sec.  If you were able to launch something from the surface of the Earth at that speed, and didn't have to worry about air resistance, it would never fall back to Earth. 

Escape velocity can be found by the formula  V_esc = sqrt(2GM/d)

Where G is the universal gravitational constant, M is the mass of the body, and d is the distance from the center of the body you are starting from.

For comparison, the acceleration due to gravity, which at the surface of the Earth is ~9.8m/sec²,  is found by

a₈= GM/d²

So for example, if you were starting from a point 1/2 again as far away from the center of the Earth than the surface is, the escape velocity from that point would be ~82% of that at the surface and  would be ~9 km/sec.  the acceleration due to gravity would fall to ~44% of the Earth surface value

So what if you moved closer to the center of the Earth?   That becomes a bit more complicated.  As you move inward, more and more of the Earth's body is overhead and no longer contributes to the gravity you feel.  So even though d is getting smaller,  M decreases even faster.  If we were to assume that the Earth was of uniform density, the acceleration due to gravity would steadily decrease to zero, while the escape velocity would continue to slowly increase until it reaches a value some bit less than twice that at the surface.

But what if, instead of boring into the Earth, we jut kept compacting it into a smaller and smaller size. That way, we can get closer to its center, while still keeping all of its mass "under" us.

Compacting the Earth by to 1/2 its radius raises the escape velocity by a factor of 1.414 and the surface gravity by a factor of 4.

Compacting it to 1/10 it present radius raises the escape velocity by ~3.2 and surface gravity by 10.

Decrease the radius to just under 9mm, and the escape velocity exceeds c, the speed of light.  Meaning even light can't escape this small object, and we've created a black hole.

But this has no effect on the gravity felt at other distances from the black hole.   At the original Earth radius distance, the acceleration due to gravity is still 9.8m/s² , and the escape velocity would still be ~11 km/sec.

If the Earth was a thin hollow shell surrounding an Earth-mass black hole at its center, in terms of the gravity we experience on the surface, we would not notice a difference.

A black hole does not need infinite mass to prevent the escape of light.

And that's basically what a black hole is; a mass that is so compact that its surface is closer to its center than the distance at which the escape velocity for that mass equals c. (While we actually don't know what happens inside this radius (marked out by an imaginary surface of a sphere called the event horizon.) We also don't know of anything that will stop any object inside it from continuing to compact under the crushing local force of gravity until it becomes a mathematical point called a singularity.  This is one of the, as of yet, unsolved questions in science. ) 

 

 

[t686]

A black hole would be something that has no friction with surrounding fields.  If light can't penetrate its outer layer, then that means the black hole has no friction with its surroundings.  I solved RedBarron's gravity device in that he should have a bendable metal that is connected to the solid rod connected to the motor.  The device spins clockwise, and having a bendable rod  would remove any preferred orientation, it absorbs the change.  The bendable rod might also set up oscillations that reduce the residual friction, because he grooved mountains side by side in the spinning cylinder wheel, and the mountains reduce the friction by and large.

Edited August 27, 2018 by t686

[Strange]

5 minutes ago, t686 said:

A black hole would be something that has no friction with surrounding fields.  If light can't penetrate its outer layer, then that means the black hole has no friction with its surroundings.  I solved RedBarron's gravity device in that he should have a bendable metal that is connected to the solid rod connected to the motor.  The device spins clockwise, and having a bendable rod  would remove any preferred orientation, it absorbs the change.  The bendable rod might also set up oscillations that reduce the residual friction, because he grooved mountains side by side in the spinning cylinder wheel, and the mountains reduce the friction by and large.

Please don't post this sort of nonsense as an answer to a serious question.

[Phi for All]

5 hours ago, t686 said:

A black hole would be something that has no friction with surrounding fields.  If light can't penetrate its outer layer, then that means the black hole has no friction with its surroundings.  I solved RedBarron's gravity device in that he should have a bendable metal that is connected to the solid rod connected to the motor.  The device spins clockwise, and having a bendable rod  would remove any preferred orientation, it absorbs the change.  The bendable rod might also set up oscillations that reduce the residual friction, because he grooved mountains side by side in the spinning cylinder wheel, and the mountains reduce the friction by and large.

!

Moderator Note

This is a mainstream section. Please don't EVER make up garbage like this in the mainstream sections. We have students who would like to give the right answers on their tests. If you don't know the answer, ask questions.

 

[koti]

7 hours ago, Strange said:

They don't.

But I suppose you are right, if they did then it would be infinite at any distance.

Wouldn't infinite curvature render the concept of distance irrelevant?

[beecee]

10 minutes ago, koti said:

Wouldn't infinite curvature render the concept of distance irrelevant?

As informed by a noted astronomer at one time, a singularity need not be infinite, simply may lead to infinity, with the great percentage of beliefs being that the "infinite singularity" and as a result infinite spacetime curvature and density, stop somewhere at the quantum/Planck level. 

[koti]

6 hours ago, Janus said:

That's due to a concept known as "escape velocity".   It the velocity something would have to be moving in order never fall back to the object.

For example, the escape velocity from the surface of the Earth is ~11 km/sec.  If you were able to launch something from the surface of the Earth at that speed, and didn't have to worry about air resistance, it would never fall back to Earth. 

Escape velocity can be found by the formula  V_esc = sqrt(2GM/d)

Where G is the universal gravitational constant, M is the mass of the body, and d is the distance from the center of the body you are starting from.

For comparison, the acceleration due to gravity, which at the surface of the Earth is ~9.8m/sec²,  is found by

a₈= GM/d²

So for example, if you were starting from a point 1/2 again as far away from the center of the Earth than the surface is, the escape velocity from that point would be ~82% of that at the surface and  would be ~9 km/sec.  the acceleration due to gravity would fall to ~44% of the Earth surface value

So what if you moved closer to the center of the Earth?   That becomes a bit more complicated.  As you move inward, more and more of the Earth's body is overhead and no longer contributes to the gravity you feel.  So even though d is getting smaller,  M decreases even faster.  If we were to assume that the Earth was of uniform density, the acceleration due to gravity would steadily decrease to zero, while the escape velocity would continue to slowly increase until it reaches a value some bit less than twice that at the surface.

But what if, instead of boring into the Earth, we jut kept compacting it into a smaller and smaller size. That way, we can get closer to its center, while still keeping all of its mass "under" us.

Compacting the Earth by to 1/2 its radius raises the escape velocity by a factor of 1.414 and the surface gravity by a factor of 4.

Compacting it to 1/10 it present radius raises the escape velocity by ~3.2 and surface gravity by 10.

Decrease the radius to just under 9mm, and the escape velocity exceeds c, the speed of light.  Meaning even light can't escape this small object, and we've created a black hole.

But this has no effect on the gravity felt at other distances from the black hole.   At the original Earth radius distance, the acceleration due to gravity is still 9.8m/s² , and the escape velocity would still be ~11 km/sec.

If the Earth was a thin hollow shell surrounding an Earth-mass black hole at its center, in terms of the gravity we experience on the surface, we would not notice a difference.

A black hole does not need infinite mass to prevent the escape of light.

And that's basically what a black hole is; a mass that is so compact that its surface is closer to its center than the distance at which the escape velocity for that mass equals c. (While we actually don't know what happens inside this radius (marked out by an imaginary surface of a sphere called the event horizon.) We also don't know of anything that will stop any object inside it from continuing to compact under the crushing local force of gravity until it becomes a mathematical point called a singularity.  This is one of the, as of yet, unsolved questions in science. ) 

 

 

Thank you for your clear and backed by examples explanations Janus. I enjoy your posts and I’m learning from them, please keep doing what youre doing.

3 minutes ago, beecee said:

As informed by a noted astronomer at one time, a singularity need not be infinite, simply may lead to infinity, with the great percentage of beliefs being that the "infinite singularity" and as a result infinite spacetime curvature and density, stop somewhere at the quantum/Planck level. 

A singularity is as far as I know by definition a theoretical point which has no spacial size so I guess we could say its infinitely small. A black hole doesn’t have to be infinitely small though, it depends on its mass, there are black holes which have ridiculously large size like the ones in the center of galaxies. I was thinking that if we  dealt with a hypothetical universe in which an infinite mass would occupy an infinitely small spacial region, the concept of size would stop to have any meaning. This doesn’t apply to black holes ofcourse, if it did we’d all be affected by the infinite curvature at arbitrary distances which we aren’t. Infinite gravity does not require infinite mass, just enough density for the curvature to become infinite. Singularities are still a mystery and frankly I can’t wrap my head around them (if they even exist) 

[beecee]

3 hours ago, koti said:

A singularity is as far as I know by definition a theoretical point which has no spacial size so I guess we could say its infinitely small. 

I'm ready to have the experts jump down my throat, but that as far as I know, is the mathematical singularity...The singularity also can be defined as where our theories, models and laws break down, and that's at the quantum/Planck level, which is not the theoretical point singularity as defined mathematically. I believe most physicists do not believe the mathematical singularity point is ever reached...rather a surface of sorts at or "below" the quantum/Planck level.

Edited August 28, 2018 by beecee

[Markus Hanke]

In very/overly simple terms, a physical (as opposed to coordinate) singularity in GR is defined as a region of geodesic incompleteness. This means that time-like geodesics cannot be extended into the future past the singularity region. This definition does not assume any specific topology or geometry for such regions; for example, you can also have ring singularities, and naked singularities (whether these exist in the real universe is a different question, of course).

[geordief]

In very/overly simple terms, a physical (as opposed to coordinate) singularity in GR is defined as a region of geodesic incompleteness. This means that time-like geodesics cannot be extended into the future past the singularity region. This definition does not assume any specific topology or geometry for such regions; for example, you can also have ring singularities, and naked singularities (whether these exist in the real universe is a different question, of course).

Is it considered that the quantum foam  (which I have been told elsewhere  doesn't exist physically but is a model)might act on curvature in such a way as to prevent a collapse into a mathematical singularity?

A bit like gravitational lensing at the quantum  level...... 

Edited August 28, 2018 by geordief

[John Cuthber]

18 hours ago, dimreepr said:

And we'd all be living  dead  in flatland.

 

[geordief]

 

5 minutes ago, John Cuthber said:

And we'd all be   dead  in flatland.

Planck life?

Edited August 28, 2018 by geordief

[Markus Hanke]

4 hours ago, geordief said:

Is it considered that the quantum foam  (which I have been told elsewhere  doesn't exist physically but is a model)might act on curvature in such a way as to prevent a collapse into a mathematical singularity?

I don’t know whether quantum foam (of spacetime) would suffice as a countermeasure to gravitational collapse. I guess one would have to bite the bullet and do the math on this, which most certainly wouldn’t be easy.

What I do know though is that some models that quantise spacetime itself (i.e. where space and time cannot be infinitely subdivided) naturally and explicitly lead to such mechanisms. Case in hand is Loop Quantum Gravity (I’m picking this because the spin foam of LQG is mathematically related to Wheeler’s original quantum foam idea) - in this model, there is a smallest unit of area and volume, and if you do the math, you find that no singularity forms during gravitational collapse. Instead, the discreteness of space counteracts the collapse, and infalling matter “rebounds” in that non-classical region, and begins to move outwards again (presumably in the form of black body radiation at the horizon). So essentially, a black hole is a cyclical process - infalling matter, then a bouncing outward. The bouncing out bit, however, happens in the far future, due to the extreme time dilation in the non-classical region. We are talking very large time scales here, many times the current age of the universe - which is why we don’t see a bunch of exploding black holes around us.

The same mechanism would apply for the universe as a whole - there is no singularity at the BB, but rather a “Big Bounce”, meaning we’d be the result of a “previous” (a dangerous term to use in this context, but you get the drift) universe collapsing.

NOTE: This is not a proven theory, just an example amongst many different hypotheses that are currently under investigation. I’m not claiming that this is a valid description of what really happens - we don’t know this just yet.

[geordief]

15 minutes ago, Markus Hanke said:

I don’t know whether quantum foam (of spacetime) would suffice as a countermeasure to gravitational collapse. I guess one would have to bite the bullet and do the math on this, which most certainly wouldn’t be easy.

What I do know though is that some models that quantise spacetime itself (i.e. where space and time cannot be infinitely subdivided) naturally and explicitly lead to such mechanisms. Case in hand is Loop Quantum Gravity (I’m picking this because the spin foam of LQG is mathematically related to Wheeler’s original quantum foam idea) - in this model, there is a smallest unit of area and volume, and if you do the math, you find that no singularity forms during gravitational collapse. Instead, the discreteness of space counteracts the collapse, and infalling matter “rebounds” in that non-classical region, and begins to move outwards again (presumably in the form of black body radiation at the horizon). So essentially, a black hole is a cyclical process - infalling matter, then a bouncing outward. The bouncing out bit, however, happens in the far future, due to the extreme time dilation in the non-classical region. We are talking very large time scales here, many times the current age of the universe - which is why we don’t see a bunch of exploding black holes around us.

The same mechanism would apply for the universe as a whole - there is no singularity at the BB, but rather a “Big Bounce”, meaning we’d be the result of a “previous” (a dangerous term to use in this context, but you get the drift) universe collapsing.

NOTE: This is not a proven theory, just an example amongst many different hypotheses that are currently under investigation. I’m not claiming that this is a valid description of what really happens - we don’t know this just yet.

I feel lucky now  I have no skills to investigate this personally. It could become an obsession  and when the scales are as extreme as this it feels as if  ,with all the application and abilities in the world there could well be no light at the end of the tunnel and never a means to test whether any particular theory was valid.

I see the search for patterns in the cosmic background radiation  has run into the difficulty that almost any pattern can be divined  with enough "good will" .

Edited August 28, 2018 by geordief

[Markus Hanke]

16 hours ago, geordief said:

I see the search for patterns in the cosmic background radiation  has run into the difficulty that almost any pattern can be divined  with enough "good will" .

To some degree, yes. Nonetheless, whatever the outcome, it must fit into all the rest of what we already know about physics, so it is by no means an arbitrary process. 

[DimaMazin]

On 27.08.2018 at 6:05 PM, Strange said:

I doubt many people think that the gravity is infinite. (If they do, they are just confused.)

But it is strong enough to stop light escaping. That is why they are called "black holes".

If speed of escape is c , then do you think that light has less speed than c ?

[geordief]

It just occurred to me ,what would be a verified measurement of gravity at the greatest distance from the source?

 

I suppose BBH mergers' gravitational waves would count.

Is there any possibility that an extremely faint such detection might exhibit  signs of discontinuity as the signal fades?

[DimaMazin]

On 27.08.2018 at 7:43 PM, Janus said:

That's due to a concept known as "escape velocity".   It the velocity something would have to be moving in order never fall back to the object.

For example, the escape velocity from the surface of the Earth is ~11 km/sec.  If you were able to launch something from the surface of the Earth at that speed, and didn't have to worry about air resistance, it would never fall back to Earth. 

Escape velocity can be found by the formula  V_esc = sqrt(2GM/d)

Where G is theIs d a quantity of contracted meters? Is vesc a velosity wich is unlimited by ultimate speed? universal gravitational constant, M is the mass of the body, and d is the distance from the center of the body you are starting from.

 

 

Is d a quantity of contracted meters? Is v_esc  a velosity wich is unlimited by ultimate speed?

[Strange]

3 hours ago, DimaMazin said:

If speed of escape is c , then do you think that light has less speed than c ?

What an odd question  

Why would anyone think that light speed is less than the speed of light??? 

[swansont]

3 hours ago, DimaMazin said:

If speed of escape is c , then do you think that light has less speed than c ?

 

2 hours ago, geordief said:

It just occurred to me ,what would be a verified measurement of gravity at the greatest distance from the source?

 

I suppose BBH mergers' gravitational waves would count.

Is there any possibility that an extremely faint such detection might exhibit  signs of discontinuity as the signal fades?

!

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