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Gravity is not an EM wave?

  1. Feb 13, 2015 #1
    Why is gravity not electromagnetic radiation like attractive/repulsive force? I know the ans is related to shielding but I dont get it.
     
  2. jcsd
  3. Feb 13, 2015 #2

    Drakkith

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    It has nothing to do with shielding. Gravity is an entirely different force than electromagnetism and obeys very different rules. For example, there is only 1 'charge' in gravitation, unlike the two in electromagnetism. The current mainstream theory regarding gravitation is General Relativity. In GR you can have gravitational waves, but these waves are in the metric tensor of spacetime, not an EM field.
     
  4. Feb 14, 2015 #3
    Moreover, radiation carries energy away (and with that mass); if gravitation was like radiation then after enough time, nothing would remain.
     
  5. Feb 14, 2015 #4

    ShayanJ

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    Are you saying that gravitational waves don't carry energy? This isn't correct. See e.g. Sticky bead argument and mass in general relativity!
    The only point is that, the energy in gravitational waves can't be localized.(In contrast to EM waves.)
     
  6. Feb 14, 2015 #5

    Ibix

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    I think shielding comes into it to explain why EM forces dominate at short range but gravity dominates the long range. As Drakkith noted, there are no negative masses, so a large mass has a large gravitational effect. However, most matter is electrically neutral on average, so for every proton pulling at a charge there's an electron pushing at it. When distances are small enough that averaging out the charges is noticeably wrong, EM forces dominate.

    Why there are opposite charges but mass only has one sign is not known, as far as I am aware. It's just an observed fact.
     
  7. Feb 14, 2015 #6

    Drakkith

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    If that's what the OP meant by shielding, sure, that definitely has something to do with it. I thought they meant something else when I posted.
     
  8. Feb 14, 2015 #7

    Ibix

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    I think there should be an "an" after the "not" in the OP, and no "radiation", and am answering on that basis. I could be wrong...
     
  9. Feb 14, 2015 #8
    Well not sure what you have in mind by shielding (we can have electrostatic shielding but there is not a known way to shield from a gravitational field if thats what u mean) but untill scientists come up with the grand theory of unification that unifies all the known fields and all the known forces, gravity will be considered a different force from the electromagnetic force. The best theory that we currently have about gravity is General Relativity, while the best theory nowdays about electromagnetism is Quantum Electrodynamics.
     
  10. Feb 14, 2015 #9
    I think it is smth related to this which I myself dont really understand.

    The argument for no-shielding comes from the focusing property of gravity--- the basis of the singularity theorems and the area theorem.

    Static shielding would allow you to float over the shielding mat, because it blocks the gravity of the Earth. This is forbidden by the equivalence principle, and the positive mass theorem (itself closely related to the focusing property, although this is obscured in most proofs. see here for a simple argument for positive mass: Positive Mass Theorem and Geodesic Deviation ).

    In a free falling frame, the gravitational field of the Earth is no longer visible, and the shielding mat is just a gravitationally repelling surface. Such a surface cannot exist, because it would increase the area of a black hole, were it to fall in. The reason is that it would push outgoing light plane outward, allowing it to gain area, which violates the area theorem, the focusing property of null geodesics, which is the weak energy condition.

    So such a shielding mat does not exist.
     
  11. Feb 15, 2015 #10

    PeterDonis

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    I think you mean "decrease", correct? Any normal object that falls into a black hole increases the area of its horizon.

    If you are referring to the focusing theorem, this assumes the strong energy condition, not the weak energy condition. (Note that, despite the names, the strong energy condition does not imply the weak energy condition.)
     
  12. Feb 15, 2015 #11
    No. The OP asked not about gravitational waves but about gravity, which is a property of stable matter. At least locally measured that implies constant mass, if I'm not mistaken.
    Once more, if gravitation of stable matter was like radiation -which carries energy away- then after enough time, nothing would remain, following E=mc2.

    A simpler version of the same answer is that one should not confound a field with radiation.
     
    Last edited: Feb 15, 2015
  13. Feb 15, 2015 #12
    Im surprised no one is talking about general relativity here. In GR, it is explained that the gravity is not an actual "force" but a property of the space, and thats why it has ling ranges and thats why it "knows" exactly how much to pull each mass so that they can all (for example) orbit in the same orbit even tho they have different masses. If we look at it this way we can totally get the difference between it and the EM field which is described by electrodynamics. Also i have heard some stuff about gravitational waves but havent been able to study em. I think they can be super interesting.
    Btw, if ppl want an image from general relativity ( as in the imaginary kind of view) i strongly recommend them to read "uncle albert & the black hole" by russel stannard, which is written originally for children but is an amazing book in my view.
     
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