What effect does mass-energy density have on gravitational field strength?

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Offline jeffreyH

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Does the mass-energy density affect gravity at higher densities? Does any amplification of field strength occur? How could we test if this were true?

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Offline Supercryptid

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The gravitational force experienced between two bodies is related to the mass and distance between them. In some sense, density doesn't matter. If you replaced the Sun with a high-density object like a black hole of equal mass, the planets would retain their normal orbits because the mass/distance factor has remained unchanged. The difference with a black hole is that, since all of the mass is concentrated in the center, you can get very close to it (thus experiencing a much higher attractive force). If you got the same distance from the center of the Sun, the gravitational attraction would not be nearly so strong because much of the Sun's mass would be above you as well as below you (effectively cancelling out some of the attractive force).
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Offline jeffreyH

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Can this ever be confirmed experimentally? The mathematics may point in this direction but does the experimental situation agree.

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Offline jeffreyH

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In this situation if we have a mass M1 with radius R1 and a smaller mass M2 with radius R2 then the gravitational field strengths at the respective surfaces would be G1 and G2.
If we compress M1 to the same radius as R2 then length contraction and time dilation should increase. However by the reasoning above the change in the dimension of R1 should alter G1 at the surface (if we wish to preserve orbits around the mass) even though the mass has not increased and is simply compressed. Therefore G1 must increase with compression if the above statements are true. Meaning that the same mass exhibits higher field strength at its surface under compression. I.E. Gravitational amplification.
« Last Edit: 29/09/2013 00:01:36 by jeffreyH »

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Offline jeffreyH

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I am working on an equation for mass-energy density for gravity. I will post a graph of the results when done.

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Offline Supercryptid

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In this situation if we have a mass M1 with radius R1 and a smaller mass M2 with radius R2 then the gravitational field strengths at the respective surfaces would be G1 and G2.
If we compress M1 to the same radius as R2 then length contraction and time dilation should increase. However by the reasoning above the change in the dimension of R1 should alter G1 at the surface (if we wish to preserve orbits around the mass) even though the mass has not increased and is simply compressed. Therefore G1 must increase with compression if the above statements are true. Meaning that the same mass exhibits higher field strength at its surface under compression. I.E. Gravitational amplification.
The field strength at the surface has increased because you have decreased the distance to the center. Shell theorem predicts that the strength of gravity at the surface of a sphere is dependent only upon the mass of the sphere and its radius, not its density: http://en.wikipedia.org/wiki/Shell_theorem

For example, imagine if the Earth was a giant, hollow shell only one foot thick, but that this shell was super-dense such that it had the same mass as the real Earth. Shell theorem predicts that the gravitational force at the surface of this "shell Earth" is the same as that of "real Earth". You could do the same by positing that the Earth is hollow and 99% of its mass is tied up inside of a black hole at its center. This "black hole Earth" still exhibits the 1G force at its surface. The density and arrangement of mass within a sphere does not affect its surface gravity (assuming that the mass is distributed in a spherically-symmetrical manner).
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Offline jeffreyH

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In this situation if we have a mass M1 with radius R1 and a smaller mass M2 with radius R2 then the gravitational field strengths at the respective surfaces would be G1 and G2.
If we compress M1 to the same radius as R2 then length contraction and time dilation should increase. However by the reasoning above the change in the dimension of R1 should alter G1 at the surface (if we wish to preserve orbits around the mass) even though the mass has not increased and is simply compressed. Therefore G1 must increase with compression if the above statements are true. Meaning that the same mass exhibits higher field strength at its surface under compression. I.E. Gravitational amplification.
The field strength at the surface has increased because you have decreased the distance to the center. Shell theorem predicts that the strength of gravity at the surface of a sphere is dependent only upon the mass of the sphere and its radius, not its density: http://en.wikipedia.org/wiki/Shell_theorem

For example, imagine if the Earth was a giant, hollow shell only one foot thick, but that this shell was super-dense such that it had the same mass as the real Earth. Shell theorem predicts that the gravitational force at the surface of this "shell Earth" is the same as that of "real Earth". You could do the same by positing that the Earth is hollow and 99% of its mass is tied up inside of a black hole at its center. This "black hole Earth" still exhibits the 1G force at its surface. The density and arrangement of mass within a sphere does not affect its surface gravity (assuming that the mass is distributed in a spherically-symmetrical manner).

It is not simply having decreased the distance to the centre. I can decrease the distance to the centre be digging down into the earth to the same point. However the gravitational effect would decrease and not increase. The radius makes no sense inside the mass itself as it becomes a fraction. In the inverse square nature of gravitation this will not explain compression without a functional adjustment of mass to radius.

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Offline jeffreyH

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Here is a plot I have done as density increases/radius decreases. Does anyone know at what percentage of the radius of a mass that we get the Schwarzschild radius? I would like to see if it falls correctly on this graph. The y-axis is gravity strength and the x-axis percentage of the uncompressed radius.


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Offline jeffreyH

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Oops. That wasn't the density plot.


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Offline Pmb

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Quote from: jeffreyH
Does the mass-energy density affect gravity at higher densities?
Yes.

Quote from: jeffreyH
Does any amplification of field strength occur?
I don’t understand what this means so I’ll just say no.

Quote from: jeffreyH
How could we test if this were true?
When you tell me what amplification of field strength occur[/I] means I’ll let you know.

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Offline Supercryptid

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It is not simply having decreased the distance to the centre. I can decrease the distance to the centre be digging down into the earth to the same point. However the gravitational effect would decrease and not increase.
I explained this before with the Sun example. The reason that the gravity decreases when you dig into the Earth is because some of the mass of the planet is now above your head and therefore pulling on you in a direction away from the center. At the center of the Earth, gravity is at its weakest because all of the mass is around you, not below you. It pulls on you in all directions roughly equally, cancelling out the attractive force. In a black hole, the center is where gravity is the strongest because that's where all of its mass is concentrated.
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Offline jeffreyH

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Quote from: jeffreyH
Does the mass-energy density affect gravity at higher densities?
Yes.

Quote from: jeffreyH
Does any amplification of field strength occur?
I don’t understand what this means so I’ll just say no.

Quote from: jeffreyH
How could we test if this were true?
When you tell me what amplification of field strength occur[/I] means I’ll let you know.

The way I am looking at it the compression of mass produces tighter gravitational flux lines. The reason why photons get trapped is that they are outnumbered by a denser graviton field. It is like a laser beam for photons except this is an intensification of gravitons much like a laser beam. That is the 'amplification' I am exploring.

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Offline Supercryptid

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The way I am looking at it the compression of mass produces tighter gravitational flux lines. The reason why photons get trapped is that they are outnumbered by a denser graviton field. It is like a laser beam for photons except this is an intensification of gravitons much like a laser beam. That is the 'amplification' I am exploring.
The more proper analogy would be to compare a gravitational field with an electromagnetic field, not a laser. Although both laser beams and EM fields are made up of photons, they have rather different properties (virtual vs. real photons). The best equivalent to a laser would be a uniform beam of gravitational waves.
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Offline jeffreyH

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It is not simply having decreased the distance to the centre. I can decrease the distance to the centre be digging down into the earth to the same point. However the gravitational effect would decrease and not increase.
I explained this before with the Sun example. The reason that the gravity decreases when you dig into the Earth is because some of the mass of the planet is now above your head and therefore pulling on you in a direction away from the center. At the center of the Earth, gravity is at its weakest because all of the mass is around you, not below you. It pulls on you in all directions roughly equally, cancelling out the attractive force. In a black hole, the center is where gravity is the strongest because that's where all of its mass is concentrated.

You appear to be talking about a mass at normal density. I am dealing with collapsing masses. The radius here is related to an isolated mass and not a two mass interaction.

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Offline jeffreyH

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The way I am looking at it the compression of mass produces tighter gravitational flux lines. The reason why photons get trapped is that they are outnumbered by a denser graviton field. It is like a laser beam for photons except this is an intensification of gravitons much like a laser beam. That is the 'amplification' I am exploring.
The more proper analogy would be to compare a gravitational field with an electromagnetic field, not a laser. Although both laser beams and EM fields are made up of photons, they have rather different properties (virtual vs. real photons). The best equivalent to a laser would be a uniform beam of gravitational waves.

A uniform beam works for me. Bearing in mind that like light the waves are not parallel and disperse with distance. If we could ever produce a directed graviton field we may be able to create a graviton uniform beam that would act laser like..

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Offline jeffreyH

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Interestingly with this sort of technology it would be possible to tow quite large object in space.

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Offline jeffreyH

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A clarification on the two graphs above. They are both in natural units. Graph 1 is the increase in gravitational strength as the radius contracts. Graph 2 is the mass density increase as the radius contracts. There is a proportionality as would be expected but what ties then together? That is my next mission.

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Offline Supercryptid

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You appear to be talking about a mass at normal density. I am dealing with collapsing masses. The radius here is related to an isolated mass and not a two mass interaction.

What's the fundamental difference between "normal" densities and collapsing masses? It's just a matter of degree, really. Besides, if a higher density object has even slightly more gravity than an object of similar mass with a lower density, then that would mean that the Sun (hypothetically) collapsing into a black hole would affect the orbits in the Solar System. This is in contradiction to current physics knowledge. Look what NASA has to say on the subject: http://spaceplace.nasa.gov/review/dr-marc-sun/black-hole-sun.html

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If the Sun were somehow compressed enough to become a black hole, it would be less than 6 kilometers (well under 4 miles) across. It would exert no more gravitational force on Earth or the other planets in the solar system than it does now. Why? Because it would contain no more matter than it does now and it would be no closer to the planets than it is now.
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Offline jeffreyH

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You appear to be talking about a mass at normal density. I am dealing with collapsing masses. The radius here is related to an isolated mass and not a two mass interaction.

What's the fundamental difference between "normal" densities and collapsing masses? It's just a matter of degree, really. Besides, if a higher density object has even slightly more gravity than an object of similar mass with a lower density, then that would mean that the Sun (hypothetically) collapsing into a black hole would affect the orbits in the Solar System. This is in contradiction to current physics knowledge. Look what NASA has to say on the subject: http://spaceplace.nasa.gov/review/dr-marc-sun/black-hole-sun.html

Quote
If the Sun were somehow compressed enough to become a black hole, it would be less than 6 kilometers (well under 4 miles) across. It would exert no more gravitational force on Earth or the other planets in the solar system than it does now. Why? Because it would contain no more matter than it does now and it would be no closer to the planets than it is now.

You misunderstand. I agree with you. Yes the orbits would be consistent outside of the collapse. Within the collapsing region density prevails. Working out towards the original radius you arrive at the same values as you had before collapse.
« Last Edit: 30/09/2013 07:57:39 by jeffreyH »

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Offline jeffreyH

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I have attached a plot of standard gravity plot against density. Where these cross at a value of 1 represents normal solid matter at an ideal density. To the right series 1 represents densities above melting point, first liquid and then gas. To the left is solid matter compression which will ultimately lead to a singularity. Series 2 is the inverse square plot for gravity without reference to density. I am thinking through some conclusions at the moment.


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Offline jeffreyH

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The graph above needs some correctional factor to converge the plot of high densities with normal mass density. This will reflect the distortion in gravitation experienced by collapsing objects. This distortion would result in the theoretical frame dragging experienced around the event horizon of a black hole. This effect will lesson and become 'normal' at the point where the original uncompressed mass surface would have been. I currently have no idea what this factor would be but I am looking at Hooke's law, the theory of elasticity and the linear mapping of tensors. I need to relate this to the stress energy tensor. As this relates to densities it could get very interesting.

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Offline jeffreyH

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I have just been looking into the  Lense–Thirring effect and the equations are just a little beyond me. Is there any background on how this is put together?

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Offline jeffreyH

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If I am correct and there is a loss of field strength at the equator around a massive collapsed object, could this gravitational field explain the relativistic jets as gravitational waves spiral up and away from the mass. Thus pulling matter away with them? This factor could explain a weakening gravitational field in the plane of the equator.

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Offline jeffreyH

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The wikipedia article for the Lense-Thirring effect notes that it need further explanation.

http://en.wikipedia.org/wiki/Lense%E2%80%93Thirring_precession

Some of the symbols have no explanation. I don't believe this will lead to a gravitational explanation for the jets anyway but would like some help with clarification of the symbols.

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Offline jeffreyH

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The gravitational mass-density convergence may well explain relativistic jets and the speed of matter expulsion. Internal to the event horizon this convergence may cause a bow shaped gravity at the poles perpendicular to the equator. This bow may converge at the poles to a point-like gravitational beam and outwardly produce the conical shape of the jets. This is where the gravitational field strength would be lost from the equatorial plane and elsewhere. The origin of the bow would be the singularity and induced by the angular momentum of rotation. The intensification of the gravitational field, rather than drawing matter in, pulls matter outward in its wake. The field strength may make it more mass-like than normal. As the field reduces in strength it leaves a bulge of matter at the extremities of the jet. This gas cloud is then held together by its own gravitation.

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Offline jeffreyH

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Here is a quick sketch of the concept. Comments on this would be appreciated. How wrong am I?


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Offline Pmb

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The gravitational force experienced between two bodies is related to the mass and distance between them. In some sense, density doesn't matter.
That is incorrect. The greater the mass density is then the greater the mass that occupies the volume the

It's accurate in some sense to think of mass density in gravity as you would charge density in electrodynamics.

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Offline Pmb

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Does the mass-energy density affect gravity at higher densities? Does any amplification of field strength occur? How could we test if this were true?
Mass-energy is only one source of gravity. Momentum and stress also contribute. You need to take all of these into account to determine what happens. I'm not sure what you mean by amplification effects though. The geometric object which acts as the source of gravity is called the stress-energy-momentum tensor. It's definition is given here http://home.comcast.net/~peter.m.brown/sr/energy_momentum_tensor.htm

I've been playing with different distributions of mass to show how mass-energy comes into play as do momentum and stress. They're at http://home.comcast.net/~peter.m.brown/gr/gr.htm

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Offline jeffreyH

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Does the mass-energy density affect gravity at higher densities? Does any amplification of field strength occur? How could we test if this were true?
Mass-energy is only one source of gravity. Momentum and stress also contribute. You need to take all of these into account to determine what happens. I'm not sure what you mean by amplification effects though. The geometric object which acts as the source of gravity is called the stress-energy-momentum tensor. It's definition is given here http://home.comcast.net/~peter.m.brown/sr/energy_momentum_tensor.htm

I've been playing with different distributions of mass to show how mass-energy comes into play as do momentum and stress. They're at http://home.comcast.net/~peter.m.brown/gr/gr.htm

It is so good to have someone tell you WHY you are wrong. I will look at both your links and work through any equations. I have just ordered "The Absolute Differential Calculus (Calculus of Tensors) Levi-Civita, Tullio". That should be a fun read!

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Offline jeffreyH

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Just a thought. Could the zero point energy be an effect of gravity propagating through matter? Could the definition of phonon be equivalent to graviton?
« Last Edit: 04/10/2013 06:42:53 by jeffreyH »

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Offline Supercryptid

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That is incorrect. The greater the mass density is then the greater the mass that occupies the volume the

It's accurate in some sense to think of mass density in gravity as you would charge density in electrodynamics.

The gravity is only greater because the higher density causes there to be more mass (assuming a constant volume). If distance from the mass and the mass itself are kept constant, then gravity stays the same (regardless of what changes in radius and density may occur).

Just a thought. Could the zero point energy be an effect of gravity propagating through matter? Could the definition of phonon be equivalent to graviton?

Zero-point Energy has to do with Heisenberg's Uncertainty Principle and quantum vacuum fluctuations. Gravity isn't required to explain it.
« Last Edit: 04/10/2013 21:40:13 by Supercryptid »
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Offline jeffreyH

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That is incorrect. The greater the mass density is then the greater the mass that occupies the volume the

It's accurate in some sense to think of mass density in gravity as you would charge density in electrodynamics.

The gravity is only greater because the higher density causes there to be more mass (assuming a constant volume). If distance from the mass and the mass itself are kept constant, then gravity stays the same (regardless of what changes in radius and density may occur).

Just a thought. Could the zero point energy be an effect of gravity propagating through matter? Could the definition of phonon be equivalent to graviton?

Zero-point Energy has to do with Heisenberg's Uncertainty Principle and quantum vacuum fluctuations. Gravity isn't required to explain it.

Firstly gravity over square surface area must increase if gravity is a particle based field and we take mass-density into account. This causes problems with the inverse square law for reasons I won't go into now.

Secondly I agree the zero point energy is not gravity related after having read up on it a bit more. Also it is not a multi-directional effect.

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Offline Pmb

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Quote from: Supercryptid
The gravity is only greater because the higher density causes there to be more mass (assuming a constant volume). If distance from the mass and the mass itself are kept constant, then gravity stays the same (regardless of what changes in radius and density may occur).
Not in general relativity. If you have more mass packed into a smaller space and there is more pressure due to this compactness then the pressure also contributes to the gravitational field. GR is much different than Newtonian gravity. It's much more complicated.

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Offline Pmb

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Just a thought. Could the zero point energy be an effect of gravity propagating through matter? Could the definition of phonon be equivalent to graviton?
Zero-point energy contributes to the gravitational field just like all other sources of energy. But no. The rest is wrong. There is no reason to make such assumptions either. BTW a phonon is a classical entity and the graviton a quantum mechanical one.

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Offline Pmb

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Quote from: jeffreyH
It is so good to have someone tell you WHY you are wrong.
And its so good to know I can still be useful even though I'm disabled.

There are many good books on geometry such as that of Bernhard F. Schutz. Also Differential Forms and Connections by R.W.R Daring comes highly recommended by a GR expert friend of mine. I myself will be studying it when I'm better (still ill due to lengthy recovery from surgery). Introduction to Vectors and Tensors by Bowen and Wang looks good. I bought a copy a couple of months ago and haven't been well enough to study that either. :(

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Offline Pmb

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Quote from: Supercryptid
The gravity is only greater because the higher density causes there to be more mass (assuming a constant volume). If distance from the mass and the mass itself are kept constant, then gravity stays the same (regardless of what changes in radius and density may occur).
As I said, that is wrong. At least in general.

I'd like to point out that what was said above only applies to the special situation of a point particle or something of that nature. When its something like a long line of mass then its different. Suppose we wish to determine the gravitational field a distance d from the z axis where along the z-axis is a long line of of constant mass density. The gravitational field at a distance r from the z-axis is proportional to the mass density, not the mass, and inversely proportional to r. So there really are situations where the field is proportional to mass density and not to the entire mass. This will hold for a very long line of matter when we want to look at the field close to the line.

The GR calculation is found here
http://home.comcast.net/~peter.m.brown/gr/grav_field_rod.htm

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Offline Supercryptid

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Interesting...can the effect of added pressure increase gravity significantly? Does that make the "replace the Sun with a black hole" analogy I mentioned earlier wrong then?
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Offline jeffreyH

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Interesting...can the effect of added pressure increase gravity significantly? Does that make the "replace the Sun with a black hole" analogy I mentioned earlier wrong then?

I don't think it would but this is only a hunch at the moment.

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Offline Pmb

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Interesting...can the effect of added pressure increase gravity significantly? Does that make the "replace the Sun with a black hole" analogy I mentioned earlier wrong then?
No. That remains correct.

Each case must really be analzed individually. It's dangerous to form general conclusions. Let me give you an example; you've heard of cosmic strings, right? A straight cosmic string has an enormous linear mass density. It's extremely thin, less than the width of an atom but never ends in an open universe. It has an equally large tension too. Tesion is like pressure but is negative. They contribute equally in the case of the cosmic string. The end effect is that you could be standing right next to a cosmic string and not know it from its gravitational field. The only gravitational effect a cosmic string has is to change tghe topology of the surrounding space from planar to conical. Amazing stuff, isn't it? :)

A vacuum domain wall is another example. In this case the wall is a two dimensional object rather than a one dimensional object like the string. The tension contributes twice as much so the wall has a replusive gravitational field. Interesting thing about the vacuum domain wall is that the gravitaitonal field it generates has zero spacetime curvature.

In three dimensions there is even more repulsion and this is how the accelerating expansion of the universe works.

On the other hand the effective active gravitational mass density of radiation is that the (positive) contribution of the raditation pressure contributes a significant amount to the active-grav-mass.

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Offline jeffreyH

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Is then gravitation expressed at the event horizon of the black hole as a result of the in falling matter. If so then what part would the original mass play? Or is this looking at it wrong?

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Offline jeffreyH

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To collapse below the Schwarzchild radius some of the mass is already contained within this volume. Mass outside this region, when collapsing inwards, will approach the radius with escape velocity increasing proportionally. Has anyone tried working this through during the collapse event to calculate the effects on gravity as the process evolves?

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Offline Pmb

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Is then gravitation expressed at the event horizon of the black hole as a result of the in falling matter. If so then what part would the original mass play? Or is this looking at it wrong?
I'm sorry but I don't understand this question. Can you rephrase it for me please?

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Offline Pmb

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Quote from: jeffreyH
To collapse below the Schwarzchild radius some of the mass is already contained within this volume.
As observed from outside the event horizon, nothing can pass through the event horizon and go inside.

Quote from: jeffreyH
Mass outside this region, when collapsing inwards, will approach the radius with escape velocity increasing proportionally.
Actually, as matter approaches the event horizon it slows down and comes to a stop at the event horison and never crosses it.

Quote from: jeffreyH
Has anyone tried working this through during the collapse event to calculate the effects on gravity as the process evolves?
I don't understand what you mean by "the effects on gravity." Can you clarify this for me please?

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Offline jeffreyH

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Quote from: jeffreyH
To collapse below the Schwarzchild radius some of the mass is already contained within this volume.
As observed from outside the event horizon, nothing can pass through the event horizon and go inside.

Quote from: jeffreyH
Mass outside this region, when collapsing inwards, will approach the radius with escape velocity increasing proportionally.
Actually, as matter approaches the event horizon it slows down and comes to a stop at the event horison and never crosses it.

Quote from: jeffreyH
Has anyone tried working this through during the collapse event to calculate the effects on gravity as the process evolves?
I don't understand what you mean by "the effects on gravity." Can you clarify this for me please?

The effects on gravitational field strength. Does it increase or decrease overall? This is assuming a Kerr black hole.
« Last Edit: 05/10/2013 08:50:51 by jeffreyH »

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Offline jeffreyH

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Is then gravitation expressed at the event horizon of the black hole as a result of the in falling matter. If so then what part would the original mass play? Or is this looking at it wrong?
I'm sorry but I don't understand this question. Can you rephrase it for me please?

I would forget this one. I need to know what I mean myself first. :-)

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Offline Pmb

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Quote from: jeffreyH
The effects on gravitational field strength. Does it increase or decrease overall? This is assuming a Kerr black hole.
Not sure. I'm not an expert on black holes so I'd rather remain silent on questions requiring great detail like this.

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Offline Supercryptid

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Interesting...can the effect of added pressure increase gravity significantly? Does that make the "replace the Sun with a black hole" analogy I mentioned earlier wrong then?
No. That remains correct.

Each case must really be analzed individually. It's dangerous to form general conclusions. Let me give you an example; you've heard of cosmic strings, right? A straight cosmic string has an enormous linear mass density. It's extremely thin, less than the width of an atom but never ends in an open universe. It has an equally large tension too. Tesion is like pressure but is negative. They contribute equally in the case of the cosmic string. The end effect is that you could be standing right next to a cosmic string and not know it from its gravitational field. The only gravitational effect a cosmic string has is to change tghe topology of the surrounding space from planar to conical. Amazing stuff, isn't it? :)

A vacuum domain wall is another example. In this case the wall is a two dimensional object rather than a one dimensional object like the string. The tension contributes twice as much so the wall has a replusive gravitational field. Interesting thing about the vacuum domain wall is that the gravitaitonal field it generates has zero spacetime curvature.

In three dimensions there is even more repulsion and this is how the accelerating expansion of the universe works.

On the other hand the effective active gravitational mass density of radiation is that the (positive) contribution of the raditation pressure contributes a significant amount to the active-grav-mass.

Forgive me for taking this a bit off topic, but I find these conclusions rather fascinating. I've heard of domain walls before, but never that they were gravitationally-repulsive. If only we could prove their existence and duplicate them on a tiny, controlled scale. Then antigravity machines may prove plausible. Not that I expect this to happen any time soon.

You sound fairly confident about the gravitational repulsion that causes the Universe's expansion. Yet I've never heard of that explanation before. It does sound like a nice model, as it doesn't invoke a mysterious "dark energy" to explain it. Is this a mainstream theory?
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Offline Pmb

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Quote from: Supercryptid
Forgive me for taking this a bit off topic, but I find these conclusions rather fascinating. I've heard of domain walls before, but never that they were gravitationally-repulsive.
 If only we could prove their existence and duplicate them on a tiny, controlled scale.
If we could create that kind of matter and control it then we could try it. That's way off in the future if you ask me though.

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You sound fairly confident about the gravitational repulsion that causes the Universe's expansion.
That is what the entire cosmology community believes. Who am I to differ?

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Yet I've never heard of that explanation before. It does sound like a nice model, as it doesn't invoke a mysterious "dark energy" to explain it. Is this a mainstream theory?
Yes. Please understand that the term "dark energy" is simply the name given to the cause of gravitational repulsion. See
http://news.nationalgeographic.com/news/2012/02/120215-dark-energy-antimatter-physics-alternate-space-science/# - Is Dark Energy Really "Repulsive Gravity"?

See also http://en.wikipedia.org/wiki/Dark_energy
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This accelerating expansion effect is sometimes labeled "gravitational repulsion", which is a colorful but possibly confusing expression. In fact a negative pressure does not influence the gravitational interaction between masses—which remains attractive—but rather alters the overall evolution of the universe at the cosmological scale, typically resulting in the accelerating expansion of the universe despite the attraction among the masses present in the universe.

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Offline jeffreyH

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I have often thought that collapsing systems such as black holes and neutron stars can only exist in an expanding system. If the system were collapsing we should see white holes. This is why white holes are unstable but in the collapsing system it would be black holes that were unstable.

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Offline yor_on

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That would make traveling the astronomical 'void' a very dangerous experience me thinks :) when matter and antimatter meet you get radiation, does you not? Shouldn't that be measurable, or is the assumption that this only took place in some beginning never more happening? also it makes me think of a  twisted 'mirror universe' in where half of its existence finds the other half non-existing, as non measurable. A very weird concept which reminds me of Bakers SF (think it was him?) depicting a ultimate 'war of resources',  between beings of matter and anti matter, arranging the universe to a anti matter state, ultimately making it inhabitable for us of matter.
==

It is a nice idea though in the sense of explaining why we then could have a equal amount of anti matter matter, possibly? From a big Bang without those annihilating each other at the moment they emerge. But if they exist in equal proportions, and they should if they do not 'naturally meet', due to gravity/anti gravity, what exactly would make the universe expand, and accelerate? Reminds me of my old idea (my joke actually) about 'virtual particles', although I doubt that one too, it's just too simplistic. You can as easily argue that it then need a equal amount of virtual 'photons' of mass to come into existence, and then you should have no expansion I think, neither a acceleration.

I think geometry holds the answer myself, with probably more degrees of freedom, or less? :) Depending on how you look at it. It all depends on if we got that one right I guess? I can alternatively imagine another sort of topology, solely defined from local measurements, in where the real question becomes what allows a 'local point' to in any way being able to measure another point (connect to). That universe does have a topology (dimensions) defined by our measurements, but its consistency is ultimately a local experience. And ultimately a question of pure logic.

As if what define a universes geometry is a question of some principle allowing local points to measure on other points, a 'point like' universe if one like :) Such a universe, should allow for entanglements easily as our definitions of distance, and motion, would need to be retranslated to fit such a concept.
« Last Edit: 06/10/2013 16:17:45 by yor_on »
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