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All mass has gravity

  1. Sep 5, 2003 #1
    All mass has gravity, which is obvious; But how does gravity effect long distances? This is sorta confusing and it'll make me seem like an idiot, but thats okay.

    Say we have 3 marbles, each close to each other, almost touching, but not quite. Lets say each were molecules. Wouldn't they attract each other, or their gravity adds up, into sets of gravitational waves?

    I'm probably misinterpreting gravity, or something, though. =(.
    Last edited by a moderator: Feb 5, 2013
  2. jcsd
  3. Sep 5, 2003 #2


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    All mass attracts all other mass in the universe.

    The force that each feels is equal to G*m1*m2/r^2, where G is the gravitational constant, m1 & m2 are the masses, and r is the distance between them.

    In your example, yes, they would be gravitationally attracted to each other, but the force is reeeealy small. Remember, the masses of molecules are tiny. In that domain, electrical forces dominate. The thing about gravity that causes everything to clump together is that it always adds. There is no 'negative' gravity.
  4. Sep 6, 2003 #3
    Gravity is my favorite force .
  5. Sep 7, 2003 #4
    Re: Gravity

    I wouldn't describe the fact that all massive objects gravitate other massive objects as "obvious" (in fact, it turns out, anything with *any* type of energy, not just "mass"-energy, gravitationally attracts other stuff with energy)...but that's not important here.

    The Newtonian picture of gravitation is that there is a field that massive objects interact with locally (that is, at their location). The field at that location, in turn, is created by massive objects around that location. This is a view that is in analogy to the view of electric and magnetic fields popularized by Faraday. Thus, massive objects feel a force to to other distant massive objects due to a local interaction with a field. This was the early 20th century view: that there were particles and fields that operated as described above.

    This notion of "field interaction" is essentially the viewpoint that exists to this day, although we have come a long way in our fundamental understanding as well as precision. See below if you want to know more.

    [Even with the revolutionary view that general relativity brought to gravitation, the concept of a "field" was essentially still there. The Newtonian gravitational field was replaced by a dynamical spacetime metric field (which describes the geometry of spacetime, be it flat or curved). In this Einsteinian picture, things like light and electrons are sources of local spacetime curvature. Once the distribution of energy and momentum is specified in a region of space, the local curvature (at the locations of the sources) is determined. This is called the "Ricci" curvature. Now, the curvature of the "in-between" region of spacetime can be determined given those local spots of "Ricci" curvature. This in-between curvature is called "Weyl" curvature. If you put an electron in this region of spacetime (neglect its contributions to the curvature of the spacetime, so it's called a "test particle"), the geometry of spacetime still tells it how to move (it did so even in Newtonian physics): it moves in what is sees as a straight path. But since the spacetime is curved, an observer on Earth watching the electron come toward Earth would see the path curve as it is "gravitationally attracted" to the earth. Even in this picture, then, there is a concept of a field that objects interact with *locally*, so that there is no non-local interaction between, say, the earth and the sun.

    Even more precisely, when you want to merge quantum mechanics with relativity, you get quantum field theory, where fields still play the ultimate role in a *local* interaction theory (though now, objects like atoms talk to the fields via discrete packets of information, like photons...or "beams" of coherent packets, like the electric field).]
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