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JohnLisk

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Gravity pressure
« on: 11/02/2008 15:37:55 »
I am not a Physicist and I am unused to the terminology you use, however I have an interest in this Universe and often consider areas where knowledge is limited.
I wondered how an object such as a singularity could have such massive gravity if it is such a small object.
I may well be wrong but I was under the impression that Total Gravity is relative to total mass so if you have a sphere of a specific size technically the pressure of gravity experienced on the surface would be x and as long as the surface area remained constant x would be constant, but if the mass remains the same and the area changes larger or smaller then the pressure of gravity would also change.
Gravity Pressure = GForce      
                     MassArea
In short Gp=Gf                       
               Ma
Gravity pressure equals the force of gravity divided by the area of the mass producing the gravity
In so much as Gravity is a constant relative to the total mass of an object, this will not change.
However the pressure gravity excerpts on an object considered being in a low kinetic state will change as the area of the object changes.
E.g. take a planet and reduce the space that exists between the particles of matter so as the total surface area is reduced to a small percentage of the original object.
The mass remains constant as does the force of gravity, however the pressure that gravity applies to the remaining surface area will increase.
I suspect that it is this variable pressure that is present at the collapse of a star and contributes to the formation of a singularity.
The same may well be applied to the Magnetic field strength of an object in a state of stellar collapse
Perhaps an expert could revue this for me


 

Offline lightarrow

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Gravity pressure
« Reply #1 on: 11/02/2008 18:32:41 »
I am not a Physicist and I am unused to the terminology you use, however I have an interest in this Universe and often consider areas where knowledge is limited.
I wondered how an object such as a singularity could have such massive gravity if it is such a small object.
I may well be wrong but I was under the impression that Total Gravity is relative to total mass so if you have a sphere of a specific size technically the pressure of gravity experienced on the surface would be x and as long as the surface area remained constant x would be constant, but if the mass remains the same and the area changes larger or smaller then the pressure of gravity would also change.
Gravity Pressure = GForce      
                     MassArea
In short Gp=Gf                       
               Ma
Gravity pressure equals the force of gravity divided by the area of the mass producing the gravity
In so much as Gravity is a constant relative to the total mass of an object, this will not change.
However the pressure gravity excerpts on an object considered being in a low kinetic state will change as the area of the object changes.
E.g. take a planet and reduce the space that exists between the particles of matter so as the total surface area is reduced to a small percentage of the original object.
The mass remains constant as does the force of gravity, however the pressure that gravity applies to the remaining surface area will increase.
I suspect that it is this variable pressure that is present at the collapse of a star and contributes to the formation of a singularity.
The same may well be applied to the Magnetic field strength of an object in a state of stellar collapse
Perhaps an expert could revue this for me


Welcome on this Forum!

About your question, if I've understood correctly, you say: the gravitational force stay constant while the objects contracts, but its surface decreases, so the pressure, which is force/surface, increases.
If that was your question, then you don't consider the fact that the gravitational force don't stay the same, but it increases:

F = -G m1m2/r2

So, if r decreases, F increases.
 

Offline DoctorBeaver

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Gravity pressure
« Reply #2 on: 11/02/2008 18:36:18 »
When speaking of gravity, you should refer to volume, not area, as all matter is 3-dimensional (or, rather, at least 3-dimensional). To all intents and purposes, gravity can be taken as acting from the centre of an object. Therefore, as the diameter decreases the force at the surface will increase.

For instance, if an object has an original radius of 4,000m and, somehow, gets shrunk to a radius of 2,000m, the force exerted 4,000m from the centre will be the same as it was when the surface was there. At the new surface, 2,000m from the centre, the force will be 4 times that at the old surface (double the distance = half the strength).

In the case of a star, as it contracts, more of its mass gets closer to the centre and, hence, feels a greater gravitational force. It will therefore collapse further causing more mass to get closer to the centre, and so on.

As far as I am aware, black holes do not have magnetic fields. So, somewhere between being a star & being a black hole, the magnetic field disappears. I'm not sure at what point that happens.
 

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« Reply #3 on: 11/02/2008 18:41:34 »
Alberto - as you are well aware, if the mass remains constant then the force of gravity remains the same at any given distance from the centre of mass regardless any change in size of the mass. Your answer does not make that clear as the words contradict the formula. You should have said that "...you don't consider the fact that the gravitational force at the surface don't stay the same, but it increases"
 

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« Reply #4 on: 11/02/2008 18:43:28 »
Alberto - as you are well aware, if the mass remains constant then the force of gravity remains the same at any given distance from the centre of mass regardless any change in size of the mass. Your answer does not make that clear as the words contradict the formula. You should have said that "...you don't consider the fact that the gravitational force at the surface don't stay the same, but it increases"

Yes, you're right!  :)
 

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« Reply #5 on: 11/02/2008 18:52:08 »
As far as I am aware, black holes do not have magnetic fields. So, somewhere between being a star & being a black hole, the magnetic field disappears. I'm not sure at what point that happens.

They can have:
http://filer.case.edu/sjr16/advanced/stars_blackhole.html
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The simplest black hole has no spin and no magnetic field. This is called a Schwarzschild black hole. A black hole that has a field but no spin is called a Reissner-NordstrÝm black hole. One that has both a magnetic field and spin is called a Kerr black hole.
 

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« Reply #6 on: 11/02/2008 18:57:42 »
How can that be? The EM force is mediated by photons, but they cannot escape the black hole. Surely, any magnetic field is generated by the accretion disk, not the black hole itself.
 

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« Reply #7 on: 11/02/2008 19:21:26 »
How can that be? The EM force is mediated by photons, but they cannot escape the black hole. Surely, any magnetic field is generated by the accretion disk, not the black hole itself.
There's no need of accretion disk (even if an accretion disk can certainly create a mag field), because a charged and rotating BH generates a mag field by itself. This is because a rotating BH drags spacetime around itself so, in its proximities, it's as if you weren't actually stationary with respect to it and so with respect to that charge, and in a ref frame where a charge is seen as moving, there is a mag field. In other terms: if you can feel the coulombian force out of the BH, then, if it rotates, this force transforms partially in mag field.

So, your question boils down to this: how it's possible to feel the coulombian force out of a charged BH?
Don't know. Maybe charge is similar to mass in its property of bending spacetime, so the coulombian force you feel here outside the BH can be nothing else than the effect of (in other dimensions) bending of spacetime here, exactly as in the case of mass and gravitational force. Recall the "elastic net" paradigm.
« Last Edit: 11/02/2008 19:29:31 by lightarrow »
 

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« Reply #8 on: 11/02/2008 19:30:40 »
Thank you, Alberto. I've read about charged & rotating BHs, but I always assumed that any EM  was generated outside of the BH itself for the reason I gave.
 

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Gravity pressure
« Reply #9 on: 11/02/2008 19:33:23 »
I must admit I hadn't considered the magnetic field of a blck hole! but as they can have charge and say a neutron star just before it collapses into a black hole can have a very strong magnetic field.  It is clear that a black hole is quite likely to have a magnetic field as it almost certainly has a lot of angular momentum.
 

lyner

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Gravity pressure
« Reply #10 on: 12/02/2008 10:20:53 »

.....
If the mass of an object remains the same its gravity must remain the same or if you prefer constant, since gravity is relative to mass.
.......
My notion is that gravity is a force and therefore must act upon an objects total surface area, since you have both force and area through transposition you gain pressure, this being the result when you divide force by area.
Now since gravity must remain relative to mass the only way an increase in the effect of gravity can be gained is if the area of an object changes and as i initially explained this will happen as the distance between the particles of matter either reduces or becomes greater.
When you say "the gravity remains the same" there is your basic problem. At a large distance, the gravitational field will be the same for all objects of equal mass but that's as far as it goes.
Also, gravity acts throughout the whole volume - not over the surface area.

I think you need to look at this from a different direction.
The reason for pressure inside a star is that the material on the surface is attracted to the rest of the material of the star. This is an inward force (of course). The more mass underneath the surface, the greater the force AND the smaller the diameter, the greater the force (inverse square law applies). As you go deeper, each 'shell' you go through is attracted by the material beneath, adding to the pressure. However, the effect of material in the outer shells cancels out, so the central force decreases as you get nearer the centre. The total pressure is due to the sum total of all these forces.
If a star of given mass decreases in radius, then it becomes more dense, then everything is closer together and the forces  (hence, pressure) are greater.
What we are really talking about, here is volume and density; of course, area is there, too but it's not a part of the argument. Transfer of energy to mass is not a part of the basic argument - any change in mass is miniscule. Avoid using 'modern' physics ideas until the classical ideas stop working for you; too many pitfall there!

In the case of a black hole, the density is very high and the forces and pressures get out of hand. The situation is no longer 'classical' and General Relativity comes into play. Nevertheless the same general idea applies.
 

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Gravity pressure
« Reply #11 on: 12/02/2008 10:33:06 »
I think that as ever with this kind of discussion we are tying ourselves in knots over definitions, here are some which are used scientifically.

Mass - this is the property which appears to give rise to gravitational attractions, assuming nothing is lost the mass will remain constant as a star collapses.

gravitational field - this is the force a kg of mass will feel at a point. If the mass is constant at a fixed distance the field will be constant, but as the star collapses you can get closer to the mass so the field increases. - I think this is what you mean by gravitational pressure. The sum of this field at any radius will be a constant for a constant  mass.

gravitational potential - this is 0 minus the amount of energy it would take to move 1kg from a point to infinity. The potential at the surface of the start will get lower as the star gets smaller.
 

lyner

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Gravity pressure
« Reply #12 on: 12/02/2008 13:56:42 »
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total force of gravity an object can produce cannot exceed its Total mass
This is just plain wrong; the distance between the masses is a major factor.
The force is F=G.m1.m2/D^2
Whatever the value of the masses, by putting them close enough together, you can increase the gravitational force.
Don't be sidetracked by the fact that Newton's law of gravitation falls down in extreme circumstances, the closer you get, the higher the field strength. It's only once you are inside the object that this stops working. If you increase the density of a star (shrink it), Newton's law works  and the surface gravity goes up and up.
 

lyner

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Gravity pressure
« Reply #13 on: 12/02/2008 22:22:29 »
Quote
sophiecentaur Please explain which masses you are talking about?
Google "Newton's Law of Gravitation".
Could be you and the Earth. Could be a couple of steel blocks. Could be just two atoms; the total result would be the sum of all the gravitational forces in the system due to each of the atoms. It is quite OK to treat the problem as one of integration but, to start with, 'point' masses get you thinking correctly.
The point I was trying to make was that 'gravity' is not a function of a single variable; it is the general term we use for the force between  two objects which depends on their masses and their separation; its value is not just a property of one of the objects.
Classically, there is no limit to this force, if you 'concentrate the masses' into a small enough volume and put them close enough together.
This needs to be considered before launching into more modern ideas.

 

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« Reply #14 on: 13/02/2008 08:16:48 »
If I may poke my little beaver nose in for a moment.

I think the confusion is arising because you are talking about different aspects of gravity. John is referring to gravity as a distortion of spacetime whereas sophiecentaur is describing the interaction between 2 masses.

You are both right in what you are saying; although John's terminology is a bit unusual.

John is right in saying that if the volume of a mass decreases while the mass itself remains constant then the gravitational force at its surface increases proportionally to the reduction in the radius. If you regard gravity as a distortion of spacetime then it stands to reason that the stronger the gravitational field, the greater the distortion. Therefore, if the mass is concentrated into a point, you will get a larger distortion of spacetime in that region - although the region will be smaller in volume.

Sophiecentaur, however, is also correct in saying that gravity is an interaction between 2 masses. Therefore, it doesn't matter how large or small those masses are, if their mass and the distance between them remains constant then the gravitational attraction between them also remains constant.

Newton's law, F=G.m1.m2/D2, only makes sense if there is more than 1 mass. It is, therefore, not an equation of gravity. Rather, it is an equation of gravitational interaction.
« Last Edit: 13/02/2008 08:20:30 by DoctorBeaver »
 

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« Reply #15 on: 13/02/2008 08:41:13 »
As the star contracts, its mass will become more concentrated. That means the gravitational interactions between the particles will increase and there will be more force acting on the surface pulling it inwards more strongly. This will cause a greater concentration of matter which means more force pulling the surface inwards and so on until SCHLOOP - the whole thing collapses to a point.
 

lyner

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Gravity pressure
« Reply #16 on: 13/02/2008 09:00:48 »
Quote
I am saying is that in an object which has a set amount of matter and therefore a fixed mass, you cannot have more non magnetic attractive force than is available due to the ratio of mass to non magnetic attractive force.
Thanks to Dr B, I think I see where you're coming from but if you want to use a term like 'force' then, unless you re-define it, you must mean the same thing as everyone else. Force is an effect on an object; it will accelerate it. The 'force' of gravity, or, more accurately, the Field (which refers to the effect on a unit of mass) varies with distance. If you have a distributed object (your star or black hole or whatever) then the forces within are caused by interaction between each of the constituent parts. Whether you want to introduce GR into the situation or not, this is true. The field (or indeed the gravitational potential, which represents the Energy involved in the situation) is position dependent. You seem to be ignoring this and, if you are having a problem with reconciling your ideas, perhaps this is why.
I keep repeating in these forums, that you can't throw out conventional Physics, wholesale when you introduce new ideas; they have to include the old. I have never read anything (of repute) 'new' about gravity which ignores the concepts of density, potential, distance etc..
This may sound Chauvinistic but, if you are not a Physicist and you want to discuss Physical concepts, you need to use the accepted language and formulae or you can come to erroneous conclusions. If you were to discuss arithmetic and suddenly introduced the notion of "2+2=5, occasionally", people might have difficulty with what you have to say.
Your original statements involving area and total gravity do not really fit the normal language of the subject.

 

Offline lightarrow

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« Reply #17 on: 13/02/2008 12:58:16 »
Humankindís need to extend his boundaries and encounter and conquer the unknown is both his greatest strength and his ultimate weakness.
By moving outside of the comfort zone man has gained new territory and new knowledge.
However, in doing, this man has impinged on the boundaries of his neighbours and because of differences in custom and language many wars have been fought.
Yet without humankinds need to look over the horizon and gain new knowledge and new territory.
We would still be ignorant savages sitting on our arses in mud.
I believe that the greatest joy is to exchange information with other human beings, learning from them as they learn from me, and if I have to learn a new language to do this, I will endeavour to do so. However, no matter how intelligent I may be, it will still take me some time to achieve fluency up until then I will no doubt amuse with pigeon tongue.
Perhaps this is more expressive:
Potential gravity of a mass in terms of spatial distortion
                 Volume of the mass in stasis
Will produce X it terms of pressure  or perhaps force
Perhaps written as X=Gm                     
                         Vm


Your intuition makes you correctly think that mass density  is related (not equal) to spacetime curvature. In general, however, it's energy density, not only mass density, that you have to consider.
 

lyner

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Gravity pressure
« Reply #18 on: 13/02/2008 13:15:49 »
I am fascinated by other peoples' ideas but I am very reluctant to take them on board without applying a certain amount of 'rigour'. New ideas must pass the acid test.
I will just ask you one question about your idea of 'potential gravity'  to see how well it fits with observation and conventional thought.
Where does this spatial distortion occur? It cannot occur everywhere but must vary. Its biggest effect would, no doubt, be somewhere very near the object and drop off with distance.
In the region very near the object, there would be a rapid change of this function and you head towards a 'singularity' as the object approaches zero size and if you are right next to it.
Modern theories attempt to deal with what happens when the 'simple', mathematical model / function goes 'to infinity, such as will happen when distances approach zero. Is your idea to deal with this problem?
My comment must be that it is very brave to start at this point rather than to become conversant with what goes on before this situation is reached.
Remember that conventional Physics only breaks down under very extreme conditions and any new, viable, theory should allow for standard theory to 'take over' when ordinary circumstances prevail.
 

lyner

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Gravity pressure
« Reply #19 on: 13/02/2008 19:21:49 »
OK but WHAT do you mean by this 'potential force of gravity'?
What you are talking about is a reasonable scenario but your terminology and explanation do not seem to connect with my theoretical knowledge.
The actual pressure will be zero at the surface because there is nothing on top of it; there will still be a gravitational field though.
Where is your force acting? The field (force on a unit mass) will be different at all distances away from the centre.
What is volume/area? If it is a sphere, then only one is needed and volume would seem to be the relevant one, but why not just talk of radius? The pressure, defined in conventional terms, will be different throughout the object; greatest in the middle (the bottom of the pile of stuff). This is fundamental physics, at heart, and you can't just change it unless you are working in an entirely different paradigm, in which case you have to make up your own Physics from scratch. Also it has to be experimentally verifiable; this is a tall order; are you up to it?
btw, look at my profile.
« Last Edit: 13/02/2008 19:24:03 by sophiecentaur »
 

lyner

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Gravity pressure
« Reply #20 on: 13/02/2008 22:56:17 »
My view of gravity, and of most of Science is what it 'does' rather than what it 'is'. The reason for this is that I do not believe we can ever truly 'know' or 'understand' anything. There will always be another layer to peel away, however well we think we know about the World.
I therefore look upon things pragmatically. I see Science as a set of rules by which we can predict what will happen next and, naturally, the received wisdom about Science tends to get my vote because the overwhelming majority of conventional Science (despite what so-called free thinking students seem to claim) makes a pretty good job of doing this.
In a question such as this, there are two approaches; one approach strikes out in a brand new direction, uses a subjective view and comes to conclusions that may or may not be testable. It does not really value the work or authority of past figures because it is basically looking for some magic key which will solve things at a stroke.
The other approach is to start at the bottom and work upwards, learning what is accepted wisdom (accepted, mostly because vast swathes of it have been experimentally verified by some very bright people). Every now and then, there is an error or short-coming which can be identified or a brand new phenomenon which doesn't fit the existing structure. Someone who makes a leap in thought at this stage has a chance of making the next big step in mankind's Science knowledge.
The latter approach is not very sexy and, for those who have not done the graft, may seem tedious and plodding. It does, however, work. It has produced more or less all the Science which we rely on for our technology (engineering, electronic, medical etc).
Of course, there are flashes of brilliance which occur throughout Scientific history but they were 'informed' flashes by people who knew the basics, by and large. Most of these people would readily acknowledge that all their basic groundwork was an essential ingredient in the process.
If I appear pedantic and pedestrian it is because I am suspicious of 'quick-fix' ideas and both sides of my brain tell me that this is likely to be a fruitful way towards a better understanding of where we're at.
Newton, Einstein, Planck and all the others, got there by hard, basic study, not by waking up one morning with a  vision and a theory which 'just happened to work'.

As for the singularity problem, Google it and find a whole range of ideas or read a Popular Science book like A brief History of Time or The Elegant Universe. Those two books contain very good treatments of such problems.
I warn you, though, that trying to read about a subject like Physics without some serious preparation is a bit like trying to read a Russian Novel with just a dictionary. It is very easy to get the wrong end of the stick if you don't speak the language. There are many more good translators of Russian than good translators of Science.
 

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Gravity pressure
« Reply #21 on: 14/02/2008 00:01:48 »
Reading through this discussion it seems to me that there is a problem with clarity, conventions and concepts on all counts maybe you will all understand it a bit better if I describe the whole physical processes involved in collapsing gravitating objects from a slightly different point of view.

All the matter in the universe is built up from indeterminately tiny subatomic particles  called leptons and quarks in extremis these can be compressed by incredible pressures to become arbitrarily compact.  These particles also interact tether to form larger particles like protons and neutrons and atoms.  Atoms can also clump together to form liquids and solids.

Now an arbitrary arrangement of atoms an other material spread out in small chunks and gas has the tendency to collapse and contract under its own gravity.  like a gas as it contracts the pressure will rise until that pressure stops the gravitational collapse. It will also heat up and spin faster because the energy of the collapse and the angular momentum must be conserved.

The pressure john lisk is taking about is the essential outwards pressure that stops the continued gravitational collapse of an object under its own gravity.  If you have a lump about as big as the earth and it is mostly reasonably heavy atoms like carbon silicon and iron you end up with a planet  with the inside hot an molten.  If you start of with a lot of hydrogen and helium and not much in the way of solids you can get a big gas giant like Jupiter  or a star like the sun or a very big bright star if you have a great deal of material.  If you start of with much more material than that you end up with nothing because as it collapses it gets so hot that the pressure get so great that it just evaporates again!

Now planets like the Earth and Jupiter are pretty stable and nothing more can happen to them but stars are only kept hot by the nuclear reactions going on inside them keeping up the pressure these eventually run out and the stars start to collapse under their gravity again.  In the case of small star like the sun this causes them to heat up inside and eventually blow off their outer layers leaving an incredibly dense small star (a white dwarf about the size if the earth with most of the mass of the sun)in which the atoms have been crushed into what is called degenerate matter by the gravitational collapse  these just then spend billions of years cooling down.  For larger stars this collapse may go further and more violent and squash the material right down to  the density of neutrons and form a neutron or quark star a few miles across with the mass a bit bigger than the sun. even bigger stars may go further and collapse into a black hole about a mile across with a mass a few times the mass of the sun because there is no outward pressure that will stop them collapsing to this critical limiting density.

 

lyner

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Gravity pressure
« Reply #22 on: 14/02/2008 09:17:03 »
Quote
It would appear ladies and gents or otherwise that we are entering into the realm of classical versus modernist,
Not at all.
We may be in the region of Science versus Non - Science. If you are going to have anything which can be called Science, it has to be based on observation, be verifiable by experiment and, essentially, be self consistent.
Jumping on the bus, half way through the journey, and wanting to draw valid conclusions is unlikely to advance understanding the physical nature of the World.
There are hours and hours of pleasure to be had in speculation about the nature of Things and that is fine; you may be surprised to learn that I indulge, frequently.
It does help, if one really wants to feel one is getting anywhere, at least to start with received wisdom and not just a melange of recently coined but little - understood scientific phrases (this is very common in forum contributions).
I am never surprised when my free-ranging and less substantial thoughts take me somewhere which is not sustainable, when referring to established observations. I use it as feedback with which to inform my next thoughts. I never feel inclined to feelings of resentment and pique; I just take on board the new experience and use it.
There is always the temptation to treat Science as an adventure game in which you build your own rules but, when you start to play seriously, most of these games fall over and disappoint the players.
« Last Edit: 14/02/2008 09:20:46 by sophiecentaur »
 

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Gravity pressure
« Reply #23 on: 14/02/2008 09:34:53 »
Sorry John your last statement is not clearly understandable you appear to refer to statements you have made earlier but I cannot tell which ones.  The subatomic particles like electrons and protons (which are made out of quarks but these are never seen alone) that I was referring to are easily detectable and well understood in laboratory experiments at high energy, their behaviour in conventional materials on the earth and in stars.
 

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Gravity pressure
« Reply #24 on: 14/02/2008 23:34:36 »
Surely this is why most asteroids 'tumble' as they move in their orbits?
 

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