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I don't understand the nature of gravity

  1. Jul 14, 2012 #1

    Gravity is by far the weakest force in the universe, yet it is the only force with an infinite range.

    This fact about gravity doesn't make a whole lot of sense to someone like me; someone with little knowledge of Physics. Why doesn't it make sense? It doesn't make sense because weak and infinite are diametrically opposite. A force that is extremely weak cannot propagate indefinitely because surely it would be very sort lived and cease to exist after some period of time. However, this is not the case with gravity. It's intangible nature has shaped the cosmos over millions of years. That we know.

    What really baffles me about the function of gravity is when I apply it to complex frames of reference. Within a frame of reference there are millions of microscopic particles of dust. The dust particles appear to be falling towards the Earth. We know from Newton, that the strength of the gravity affecting each dust particles is the proportional to the masses of the Earth and the dust particles divided by the radius squared. In this reference frame, gravity pulls each dust particle with the same force as another dust particle next to it. We're assuming that each dust particle has the same mass in this situation. Because there are millions of dust particles intuitively I'd predict that gravity wouldn't exist at all in this reference frame. If there is just one dust particle, then the force of gravity would be let's say g. If there are two particles, gravity would be twice as weak. Three times as week with 3 particles and so on. In this situation there are millions of dust particles and so gravity would be millions and millions of times weaker than if just one dust particle was present in the reference frame.

    The fact is, gravity doesn't work in this way - or so it seems. Each dust particle can be treated as a reference frame in itself. The force of gravity interacts with the millions of dust particles in the same way as if there were just one particle of dust present.

    From this, it seems to me that there must be a gravity field akin to the Higg's field. The hypothetically proposed Higg's field is what gives particles their mass. The gravity field is woven to our Universe in the same why a Higg's field would be. Everything lies within this gravity field and although it's weak, it has an attractive nature. The gravity field may have existed within the Singularity before the universe came into existence. Inside the singularity, gravity would be infinitely stronger. However, some repulsive force, opposite to gravity, would also be infinite in strength and got the upper hand. This caused the big bang. Gravity, although beaten within the singularity, comes into existence in the universe we see today. The expansion of the big bang would also expand the gravity field, although stretch it out to the current size of the universe. This could explain why gravity is so weak in the universe today.
  2. jcsd
  3. Jul 14, 2012 #2


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    Wait, what?

    Have you ever looked at the E-field from a point charge, for example? Where does it end? So already, your starting point is faulty.

    This is very puzzling. Weak and infinite are not mutually exclusive. When we say "weak", we mean that in relation to something else. If I tell you that something is long, would you be able to tell me exactly how long it is? Considering that the force of gravity can keep planets and stars in orbits, I'd say that that's very damn "strong"!

    The rest of your post is rather incoherent, especially since it is based on wrong understanding. Please try to understand the basics first before extrapolating on top of something that you don't understand.

  4. Jul 14, 2012 #3


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    Where do you get this idea? Strength and range are not generally related at all, and often where they are related the relationship is opposite what you say. E.g. Consider a simple lever, the strong side is the side with the shorter range.
  5. Jul 14, 2012 #4
    1/R^2 can be a very small number, but never zero. Take it from a fellow amateur in this field, don't try to run before you've learned how to walk. Your post seems analogous to a child who has never learned basic math trying to understand calculus. Please don't take offense because I am in the same boat. Gravity can be a fascinating study, even at the Newtonian level. Learn where your limitations are and start there.
  6. Jul 14, 2012 #5


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    Isn't the issue with magnetism due to the fact that there are + and - charges so they tend to cancel each other over large distances?
  7. Jul 14, 2012 #6
    Even though this is true, people still seem to make analogies between gravitation and electric charge. I don't really know why because there's not a lot they have in common. From Max Jammer's book ``Concepts Of Mass In Contemporary Physics And Philosophy" he writes ``In many respects $m_{a}$ and $m_{p}$ can be conceived of as gravitational analogues to electrical charges and are therefore sometimes referred to as gravitational charges."
  8. Jul 14, 2012 #7


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    With weak fields, gravity is similar to the electric force, if there would be just one charge and similar charges would attract.
    With strong fields (or higher precision), General Relativity makes things more complicated. But the concept of gravitoelectromagnetism shows more similarities even there.
  9. Jul 15, 2012 #8
    If you use the general relativity concept of gravity you will find that it isn't a force at all. Falling rocks orbiting planets and the stars in the galaxy are symptoms of the deflections in time/space. This is why a light beam will deflect in the "gravitational" field of a massive object even though light has no mass. If Gravity was a force then light would have to have mass. Since light just follows the curves in time/space it doesn't need mass.
  10. Jul 17, 2012 #9


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    Light has energy, and GR is all about energy (mass is just one sort of energy).
    If you have a box with perfect mirrors inside and "fill" it with light, it will have a higher weight, and it will attract other objects with a larger force.
  11. Jul 17, 2012 #10
    Weak and infinite are not diametrically opposite. For example, I cannot run nearly as fast, as, say, a cheetah, but the human body is designed to run for much further distances than any animal, in this example I am comparatively WEAK but still have a much greater RANGE.

    This all has very little to do with gravity, though. The answer is very simply in Newton's Law. G, the gravitational constant, is a very small number, which makes the gravitational force very weak, (the analogous constant in E/M, which, by the way, is also infinite in range, is k, which is a pretty large number). Since the denominator of Newton's equation is r^2, no matter how big r is and regardless of how small G is, the force will never be exactly zero, so the gravitational field is infinite. This is not so trivial to demonstrate in general relativity, but it is easy to conceptually convince yourself that any mass at any point in the space-time continuum must be able to influence, to widely varying degrees, every point in space-time. This is true, gravity has an infinite range in GR too.

    There is also a quantum field theory explanation for why some forces must have an infinite range, but I'll leave that for people who are more knowledgeable on the subject than me to explain.

    Of course, there is a gravity field, it is called the space-time continuum. And there is thought to be a sort of force which opposes gravity that we call Dark Energy or Vacuum Energy, which is parametrized by the Cosmological Constant; something Einstein actually came up with, but didn't really understand fully. We don't know a lot about it yet, but it dominates gravity over large scales in our universe and it is the reason the universe is actually accelerating outward, not being accelerated inward by gravity like it should be. I doesn't quite work as you've described it, but the idea of gravity and dark energy in the Big Bang and early universe is far from understood on any level, so I wont comment on it.
  12. Jul 18, 2012 #11


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    Kind of ....no.....

    If you have two objects - calculating the force of gravity is straightforward but if you have gravity pulling in every direction it becomes more complicated. Gravity is a vector force. Between two objects - you and the earth you could express the force as a one dimensional vector. In a dust cloud, you'd have to sum all the vectors in all three dimensions. It gets tricky. The forces would sum to a net draw in a single direction - just as it would if there was a single equivalent object. But it's not really the same thing.

    If you had just a dust cloud and no earth. Initially, the dust would float aimlessly around. Then cloud would begin to coalesce around the heaviest dust. Momentum would be conserved, but eventually the dust cloud would spin like a spiral galaxy.
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