# Why two mass attracts each other?

Discussion in 'Physics & Math' started by hansda, Mar 19, 2013.

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1. ### hansdaValued Senior Member

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Considering the case with Planet Mercury, do you think the "interplanetary gravitational interactions" are "attractive" from "the Sun" point of view?

Also see this link.

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3. ### Markus HankeRegistered Senior Member

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I presume you mean whether or not a field can exist without a mass as its source, since the field itself obviously does not have mass. The answer is yes - one can construct metrics which are "held together" by their own self-interaction without the presence of any other sources; such topological constructs are called geons, and where first studied by Wheeler in the 1950s. See here :

http://en.wikipedia.org/wiki/Geon_(physics)

Whether or not such constructs would be stable is an entirely different issue again - but the very fact that they are valid solutions to the EFEs demonstrates the self-interaction of gravity in GR, since there are no other explicit sources of gravity present in these solutions ( i.e. the stress-energy tensor vanishes everywhere, both inside and outside the geon ). It is interesting to note that these geons are investigated as candidates for a purely geometric model of elementary particles; to the best of my knowledge though without much success.

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5. ### TachBannedBanned

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yes.

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7. ### Markus HankeRegistered Senior Member

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Why do you keep talking and quoting about "forces" and Newtonian mechanics, if we are dealing with GR ? There are no forces in GR, neither repulsive nor attractive. The perihelion precession is simply the result of a geometry which differs from the Newtonian one. It's that simple. The entire discussion is meaningless and a waste of time, simply because the cause of aforementioned precession has nothing to do with any kind of force.

8. ### hansdaValued Senior Member

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Geon is not yet proven. Here is the quote from your wiki link:

9. ### hansdaValued Senior Member

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This proves, GR has a repulsive effect.

Do you have some apathy with the term "force"? If some 'physical phenomena' fits well with the 'standard definition' of "force", whats wrong in calling this a "force".

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11. ### TachBannedBanned

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There is nothing about a "repulsive force". You clearly do not understand physics.

12. ### IncogNegroBannedBanned

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What do photons use to "push off" of?

and what direction do they go after?

13. ### wellwisherBannedBanned

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This is true, because GR ignores gravitationally induced pressures, where pressure = force/area. This pressure is missing from GR because GR is not concerned with the entire observed gravity phenomena, only space-time.

If you look at a star, the deepest point in the space-time well is where time is supposed to be the slowest due to time dilation. Yet the fastest frequency transitions within the star occur at the deepest point of the space-time well; nuclear reactions and gamma radiation. Time is supposed to slow as we go down the well, but material transitions get faster and faster, with faster and faster transitions out pacing space-time contraction.

This paradox is connected gravity induced pressure, which can alter the phases of matter regardless of the direction of space-time. In this case, gravity provides the force/area. GR cannot predict solid iron in the core of the earth since that is a function of pressure not space-time. When you talk about mass attracting mass, GR is not enough unless a first approximation is all you hope to achieve.

14. ### AlexGLike nailing Jello to a treeValued Senior Member

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GR is the theory of gravity.

Time is slowest deep in the gravity well but only when compared to a point higher up.

In the gravity well, time moves at the usual rate, and reactions occur at the usual tempo. It's only when compared to a different point higher up that there's any difference in the rate of time.

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15. ### Markus HankeRegistered Senior Member

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Wrong. Geons are valid solutions to the field equations, which can be proven quite easily; that is all that matters, since the question was whether or not those equations describe a self-interacting field. This is also precisely what the article says which I linked for you. I understand that geons may not physically exist, but that is irrelevant in this case since all we are talking about are mathematical properties of the field equations, i.e. self-interaction. If you don't wish to use geons, there are many other solutions to the field equations which demonstrate the same principle; I just picked geons as an intuitive example to illustrate what I meant to say.

No it doesn't. You really don't want to understand, do you.

But that is exactly the point; the phenomena of perihelion precession does NOT fit the standard forced-based model, being Newtonian mechanics. Only GR gives the correct prediction, and GR does not involve forces in the precession calculation. Why is this so difficult to understand ?

16. ### Markus HankeRegistered Senior Member

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Incorrect, GR does not ignore anything. All the terms for pressure, momentum, density and flux are present in the stress-energy-momentum tensor, and are all accounted for in GR. The metric describing the processes for example in the interior of a star is called the interior Schwarzschild metric.

The point being...?

There is no paradox. GR correctly describes all gravitational phenomena associated with the interior of massive bodies, and that includes energy densities and fluxes. What it cannot do is describe the mechanics of phase transitions of chemical elements, since this is intrinsically quantum mechanical in nature; GR is simply a model for gravity, not chemistry or quantum mechanics.

GR predicts exactly all gravitational phenomena in the core; the rest is chemistry and quantum mechanics, and has nothing to do with GR.

Incorrect, GR gives the right numerical predictions for all gravitational phenomena connected to massive bodies, and is therefore "enough". See my GR primer thread for details.

17. ### Farsight

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Talking of geons, Wheeler was getting warm with that. Pity he didn't think of electromagnetic wave confined in a region by something a bit stronger than gravity.

18. ### Markus HankeRegistered Senior Member

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Personally I am fascinated by the idea of geons, both as mathematical artefact, and as a candidate for a geometric explanation of elementary particles. To the best of my knowledge though it would appear that they are not stable constructs; not sure though if the last word has been spoken here yet.
What specifically do you mean by "something a bit stronger than gravity" ? Wheeler was specifically working within the domain of GR ( i.e. gravitation ); an electromagnetic wave would in itself be a source of gravity, so we wouldn't be dealing with geons anymore.

19. ### Farsight

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The electromagnetic and strong interactions, see hyperphysics. Yes, Wheeler was working in the domain of gravitation. It was Wheeler who said "matter tells space how to curve, space tells matter how to move". But we know that a concentration of energy causes gravity, and it doesn't have to be in the guise of matter. We also know that gravity is associated with curved spacetime, not curved space.

To appreciate the distinction, imagine you're flying your plane over a flat calm sea. It's so calm it's like glass. Then you notice that there's a single oceanic swell wave traversing this sea. You follow it, and you notice that for some reason it isn't on a constant heading. It's veering North a little. When you plot your course, you realise that in following the wave you've taken a curved path. That's an analogy for curved spacetime. Now look at the surface of the sea where the wave is. It's curved. And this curvature is far more dramatic than your curved path. It's an analogy for curved space.

Now take a look at The Role of Potentials in Electromagnetism by Percy Hammond and pay special attention to "We conclude that the field describes the curvature that characterizes the electromagnetic interaction." This electromagnetic curvature isn't gravitational spacetime curvature. Note though that when it comes to electromagnetism, E and B are the spatial and time derivatives of four-potential, and the typical electromagnetic sine waves don't depict the curvature directly. You have to take the integral of a sine wave for that. Something like this:

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If you arrange for an electromagnetic wave to travel through curved space, you can confine it. And you're right, we aren't dealing with geons anymore. Because one way to do this is called gamma-gamma pair production.

20. ### Markus HankeRegistered Senior Member

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Yes, but that is exactly the point. There are no energetic sources of gravity anywhere in the geon solution, $T_{\mu \nu}$ vanishes everywhere both in the interior and the exterior regions of the geon. The entire metric is Ricci-flat. This topological construct is formed and held together purely by its own self-interaction, which is the fascinating part.

21. ### hansdaValued Senior Member

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See the following quote from the abstract of the above link. I have highlighted where "repulsive force" is mentioned.

22. ### hansdaValued Senior Member

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You are just arguing with a hypothetical evidence. Mathematics tries to explain physical evidence. So, from mathematics you can not conclude a hypothetical evidence.

Do you think GR has only attractive effect?

You are just misinterpreting my words and making your own statements. I have only said "force" and not "force based model". I am quoting my statement again for your reference.

23. ### TachBannedBanned

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Like I said, you do not understand what you are reading. There is nothing about a "repulsive force of the Sun".