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Why Gravity Field exist?

  1. Nov 5, 2006 #1
    If we have two particles with mass m1 and m2 than there is the gravity force between its. Can anyone answer why this force exist?
     
  2. jcsd
  3. Nov 5, 2006 #2

    Danger

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    Science isn't about why, only how. And the exact mechanism of gravity is as yet undetermined. There could be a graviton interaction, or it could simply be space-time curvature, or a 'leak-down' effect from extra dimensions, or something that nobody has even considered yet. All that is known for sure is that it's there.
     
  4. Nov 5, 2006 #3
    Personally i think theres another sub particle halfway between matter and energy that causes gravity but i have no proof to that so dont take my word for it.

    for now just consider how not why like danger said.
     
  5. Nov 6, 2006 #4

    vanesch

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    To Ki Man: please, no half-backed "new theories".

    To the OP: as Danger said, "why" questions always end up unanswered. That doesn't mean, however, that "why" questions do not have a restricted meaning in science, but often, it means: "given these originally unrelated ideas, is there a connection?"
    For instance, you could get the answer: "it looks as if there is a force of gravity, but that's because spacetime is bend and makes it look that way".
    This is not a genuine answer to "why", but tries to explain the phenomenon using a deeper-lying idea. That begs the question of the reason for the deeper-lying idea, and in the end you question the most fundamental understanding as we have it at the moment, at which point the answer is only: "because things seem to be that way".
    Nevertheless, each time we understand a phenomenon because of a deeper-lying principle, this is a great "aha!" moment :cool:

    This is not only the case in science. All rational knowledge or argumentation is ultimately based upon some postulated truths, which cannot be argumented further ; or, if they are, they are being replaced by a new set of axioms. Sometimes it is simply: "because i feel it in my bones" :smile:
     
  6. Nov 6, 2006 #5
    And when the distance between is very small and almost zero then the force is infinitive. Is this mean infinitive energy? It is strange.
     
  7. Nov 6, 2006 #6

    vanesch

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    No, when the distance is very small, the force is "very" big but, in all observable circumstances , still way smaller than other forces.

    Mass points at zero distance are an idealised extrapolation from the scope of observable things, and hence do not necessarily have to make sense. The theories we have today are good summaries of our currently available observations, and they can, for our mental tranquility, be associated with certain ontological hypotheses, which help us remember the theories and reason in it. It is from this idealised mental picture that we can arrive at extrapolated idealised situations which may seem paradoxical: the idealisations of mass points, continuity of space and the theory of gravity (in this case, Newtonian gravity) and present us with the mental picture of mass points at 0 distance, giving a potentially paradoxical situation because of infinite available energy. But as this is an extrapolation, using a mental image associated with a theory, far outside of its tested scope, this is not really a problem when using the theory within realistic situations.
    The only thing that these mental paradoxes may illustrate, is that our picture is not entirely complete. But that's no surprise, is it ?
     
  8. Nov 6, 2006 #7

    jtbell

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    Only if the particles are very very small and they can get very very close to each other, according to current theories of gravity. But when you get to this scale you need quantum mechanics, and there is no generally accepted quantum-mechanical theory of gravity yet. We don't know what happens with gravity at such a small scale. It probably behaves very differently from classical theories, and even from general relativity, at those scales.
     
  9. Nov 6, 2006 #8
    And on what distance the Newtonian gravity forces don't work? What energy is correspond to this distance? What the maximum of this energy? May be mc^2?
     
    Last edited: Nov 6, 2006
  10. Nov 6, 2006 #9

    Office_Shredder

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    cartuz, the point is we don't know how gravity works on small scales like that, so there are no answers.

    and mc2 is such a random concept to throw out here, it's almost funny
     
  11. Nov 6, 2006 #10
    Office_Shreder, may be mc2 the maximum of all possible energy what the mass can to produce. I don't sure that the energy of this mass must be more.
     
  12. Nov 6, 2006 #11

    Office_Shredder

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    E=mc2 is the rest mass of an object. Just by sitting there, it has that much energy because it IS made of energy (basically).
     
  13. Nov 6, 2006 #12
    No, E=mc^{2}=\frac{m_{0}c^{2}}{\sqrt{1-v^{2}/c^{2}}}\approx \ m_{0}c^{2}+\frac{
    m_{0}v^{2}}{2}
     
    Last edited: Nov 6, 2006
  14. Nov 6, 2006 #13

    jtbell

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    We don't know. We don't have any experimental information about gravity on sub-atomic scales, as far as I know. All we see are the effects of the electromagnetic, strong and weak interactions. Gravity on this scale must therefore be much much weaker than those interactions.
     
  15. Nov 6, 2006 #14
    if we have theoreticaly predicted black holes, how then we dont know anything about gravity at small distances?
     
  16. Nov 7, 2006 #15

    jtbell

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    Someone who knows more about GR than I do should probably comment on this, but my understanding is that black holes are not small.
     
  17. Nov 7, 2006 #16

    vanesch

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    Indeed, the only potentially observed black holes we know about (because indirect observations correspond to what one would expect if there was something there that is rather well modeled by the concept of black hole in general relativity) are stellar black holes, and bigger ones, in the centers of galaxies. This makes us believe that at least on this (stellar) scale, the theory of general relativity makes good predictions. We also know that GR makes other good predictions. It is hence a reasonable extrapolation to think that GR could describe also rather well "table top" black holes (in the range above some tenths of mm, say). Now, it isn't because GR could describe them, that they necessarily have to exist. It might be that no physical process exists which can produce them (there *is* a physical process which can produce stellar black holes).

    However, as said before, we have no clue of what would be "a black hole" on smaller scales. Sure, GR can describe them, but we're not sure that GR is the correct paradigm to do so. We have serious reasons to doubt this, because of quantum theory. So, the only "existance" of "black holes on a sub-micron scale" is given by, again, the use of a formal construction in a theory (in casu GR) potentially outside of the scope of its applicability. And even if it is not outside of its scope, it is not necessarily existing, if no process exists to produce them. For sure, we don't have any experimental or indirect indication that they exist.

    It is what I wanted to outline in my previous post here. Theories allow for idealisations, which are useful mental constructions to solve problems and to obtain an intuitive understanding of the functionning of the theory at hand. But these idealisations can sometimes be pushed far beyond the realm of their usefulness, and then they lead to "theoretical predictions" or "paradoxes" which are just chimera of the mind.
     
  18. Nov 7, 2006 #17

    vanesch

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    Because these predictions are nothing else but speculations.
     
  19. Jul 4, 2009 #18
    Why does gravity exist?

    Excellent question! Unfortuneatly no one knows.

    As of late, I've come the the conclusion that maybe gravity exist not because it's a force all its own, but because it's the left-over residual effect of another force--the electro-mangetic one. I'm of the opinion that the charges of the two primary electric players in our universe (the proton and the electron) are not equivalent though opposite in sign. Perhaps the charges of the proton and the electron are slightly off by a very tiny value -- and one is winning. This left-over charge effect shapes the field around it, but very weakly. However, when you accumulate enough 'neutral' matter into a concentrated region that weak residual charge effect adds up and gives rise to the phenomenon of gravity.

    This is a plausible answer to me for several reasons. (1) As of now, contemporary physics is having a hard time unifying gravity with quantum mechanics. Perhaps gravity isn't a force all its own with a symmetrical nature like the EM, weak and strong nuclear force, but a residual effect from slightly unbalanced (and I mean tiny!) charge magnitudes. (2) Protons and electrons are not natural particle/anti-particle partners, which means their charges might be off, even though our best instruments indicate their charges are identical to several significant digits. (3) Gravity is astonishingly weak compared to the other forces, which indicates that the difference in charge would have to be very, very tiny, which sits well with empirical evidence

    If we followed this line of reasoning, we would have to assume that this residual charge effect would certainly be a detriment to particles of the same sign of the charge that's out-performing the other, and when two such particles encounter each other, even when in the company of the neutralizing partner (of slightly smaller charge), the two would repel each other great distances. But it turns out there's a way around this, too. If the mass of the particle with left-over residual charge is significantly greater than the mass of the particle of slightly smaller charge, the inertia of the first particle would prevent it from being repelled great distances upon encountering a particle of identical kind, when in the company of the neutralizing partner.

    And it just so happens that the mass of one of the universe's primary electric players--the proton--is disproportionately greater than the mass of it's electric counterpart, the electron. Could the proton, then, have a slightly greater charge than the electron, yielding a residual postive charge effect on the surrounding field which materializes itself as space-time curvature and gravity?

    If this is indeed the case, then our understanding of gravity is way off. What we usually attribute to being responsible for gravity--mass--would no longer be correct. In a world of unbalanced charged partners, the residual positive charge of the proton would be the culprit. Nevertheless, this residual charge should still be proportional to the mass of 'nuetrally charged' arrangements of matter it emanates from, so that would explain how mass was identified as the instigator from early on, and still is. And at the level of the very small, where gravity is unverifiable anyways, the force would simply translate into the property of charge.

    It's an idea I've been promoting for a while, but unfortuneatly there's a lot of those floating around right now.

    (And if any of you use my idea and become famous, I expect to see my name somewhere in your tell-all book! J.P. Perezchica. Ha ha.)
     
    Last edited: Jul 4, 2009
  20. Jul 4, 2009 #19

    I don't even know where to begin with this. Its quite clever and totally batty.

    Shouldn't the Sun and the Earth repel one another by your logic? (Both having an excess positive charge).

    I should point out that nonstandard physics is disallowed in this forum as it is primarily a teaching website.
     
  21. Jul 4, 2009 #20

    vanesch

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    Locked.
     
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