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Space and time

  1. Jul 14, 2004 #1
    I am trying to understand how Space and time Relate with Gravity. In Physics class i saw a video showing planets Rolling around on a grid with the sun Making the deepest indentation and planets making smaller ones. Now I can but help thinking that this didnt give me the full picture .. Is it on a 3D scale with it also being on the vertical as well as the horizontal and rather then the gravity of the whole planet making the indentation is it the Gravity of each Atom; electron, Proton, nutron; quark;ect.. doing this?
    If this makes any sense i would like some help on this matter thanks.
  2. jcsd
  3. Jul 14, 2004 #2


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    From the little I know of the subject, you shouldn't take the "rubber sheet" analogy of GR too literally. It's really a higher dimensional surface than a plane that a planet (or large body) warps around it.
  4. Jul 14, 2004 #3
    Gravity acts towards the center of the object, while each Atom has it's own mass, the gravity is calculated from the Combined sum of each individual part, and is located in the Gravitational center, the place where half of the mass is on each side when calculated from any crossection.
  5. Jul 14, 2004 #4
    ok so Large objects have gravity that attracts them to each other. While on the other side of the size spectrum they use nuclear forces to bond togeather. but do these small particals also have their own gravitional field that sorround them that is just really minute? if so is that why they draw to the center, because thats where the most Matter is?
  6. Jul 15, 2004 #5


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    You seem to have the right ideas. Nuclear particle with mass do have gravity, but it is tiny as you say, and is ignored in most quantum physics. The thing that holds the nucleus together is the strong force, which is not gravity (it behaves different; the best theory of it is called QCD "quantum chromodynamics").

    And yes, all the atoms do attact each other with their gravity. It is weak but unlike other forces it adds and never subtracts. The huge number of atoms in a few kilograms of lead have enough gravity to be measured in a laboratory (Cavendish experiment).

    Since to a very good approximation, gravity propagates symmetrically in three dimensional space, a big enough mass of self gravitating material will form itself into a sphere. "Big enough" mean that the gravity is strong enough to overcome any rigidity in the material.
  7. Jul 15, 2004 #6
    well the weird thing that i dont understand about that is doesnt every force have an equal and opposet reaction? if thats the case what is gravities?
  8. Jul 15, 2004 #7
    Be careful, the law isn't every force has an equal but opposite reaction, it's every action. If the action happens to be a force, that just means that what ever apply the force to applies an equal force back on you. You push on the wall, it pushes back on you. Pull on a rope tied to a post and the rope pulls back on you with equal force. In the case of gravity, look at the standard apple-falls-from-tree view that Newton took. The earth pulls on the apple and the apple pulls back on the earth with an equal but opposite force. Since the earth is considerably more massive than the apple, the apple moves but the earth doesn't (well...not but any measurable amount). And it's also important to not that in general relativity, gravity technically isn't a force. It's just the curvature of space "telling" an object how to move. The object just follows the curve. Not to say a repulsive gravitational force is impossible. Actually, when you start considering things like the electroweak force and the resulting Higgs fields, repulsive gravity is a likely candidate to explain the rapid expansion the Universe that we call the Big Bang. Though I'll agree with what some others have said: be careful not to take the rubber sheet analogy too literally. Objects sink into a rubber sheet due to gravity. You can't really explain gravity WITH gravity. The fact is though that particles cause indentations in spacetime and...well, we really don't know why (or at least I don't know why, I'm not exactly in the loop of current advances in gravitational theory).

    edited to add: Beatrix Kiddo, are you saying you believe gravity is exclusively a repulsive force? So, if I bring a particle closer to another particle and they are attracted to each other, are saying this is because the act of bring them together suddenly increases the repulsive force of the rest of the Universe acting on them? I'm sorry, but that doesn't sound like much of a scientifically valid theory to me. Repulsive gravity, if it exists, is an extremely special case that breaks down quickly and in general relativistic terms is a product of an inverted curve in spacetime. The Higgs field specifically define how it would work. And in it's mathematics, such a curve would collapse in a time that would make the blink of an eye seem an eternity. That website you mention is written by someone who seems to be very fond of making himself sound important (he seems to think he deserves credit for a discovery because the word "really" was inserted before the statement) and he admits to having no education in physics and astronomy. All I can say is, be careful what you read.
    Last edited: Jul 15, 2004
  9. Jul 15, 2004 #8
    Ok i see what you are saying there. thanks for the info.
  10. Jul 15, 2004 #9


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    Also momentum conservation would be broken if forces didn't always come in pairs. Just think of forces in the dp/dt form of F=ma and it'll make sense.
  11. Jul 16, 2004 #10


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    There is a reaction in gravity, there has to be one as it's not a fictional force in Newtonian physics. Remember when two objects gravitate towards each other they are subject to equal and opposite forces.
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