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Why gravity?

  1. Aug 26, 2009 #1
    Why is gravity considered a fundamental force and not the production of electromagnetism and weak/strong nuclear forces?
     
  2. jcsd
  3. Aug 26, 2009 #2

    Danger

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    Welcome to PF 2keyla.
    Gravity is a result of mass, regardless of any other forces. While it is far and away the weakest of all forces, it's the only one that doesn't have an opposing counterpart. That's why it 'governs' the universe. That's just my layman's take on it, though. Wait for an expert to respond with something more tangible.
     
  4. Aug 26, 2009 #3

    negitron

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    The strong and weak nuclear forces are also only attractive forces, but they operate over very short distances, less than the diamater of an atomic nucleus.
     
  5. Aug 26, 2009 #4

    Vanadium 50

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    The first part of your statement is not so. The strong and weak forces can be either attractive or repulsive.
     
  6. Aug 26, 2009 #5

    negitron

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    Strike the weak force, then. I see it is only repulsive. It is true for the strong force, however:

    http://www.vias.org/physics/bk4_03_04.html

     
  7. Aug 26, 2009 #6

    Vanadium 50

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    That site is wrong. The force between nuclei has a repulsive component. This is why nuclei don't get infinitely close to each other, and you can get a hint that the force has a repulsive component because the two neutrons don't stick together.
     
  8. Aug 26, 2009 #7
    I'm not sure what experiments were done to exclude gravity from the other fundamental forces, but it might be noteworthy to point out that it is an attraction between masses - meaning that the theory works for measurements of mass. Unless some subtle relation between mass and charge is made, the argument tends to close around gravity being its own force. For a more in depth relation between gravity and the other forces, you'd have to look into higher level theories.
     
  9. Aug 26, 2009 #8
    The repulsive nature of strong force at very close distances can be discussed only with quantum mechanics using second quantisation, a fact that those sites can't share. We simply tell you guys that it has such a repulsing component at very short distances, this is (as a simplification) due to the fact that this force is due to the exchange of bosons between those particles, and when the flux of those bosons becomes very high (at small distances) it turns to be repulsive to conserve energy and stability of the matter. I hope this simplifies the matter.

    About the gravity being a fundamental force, I can tell you that in General relativity, the only force that bends (curves) the time over space is the gravity and it accepts the equivalence principle, and there is not a reason to consider it as other forces. Actually since it has been discovered that the universe is expanding and accelerating in the expansion, scientists think that the universe should be expanding in constant speed due to the big bang, and because of a repulsion component of gravity (dark energy which is proportional to the cosmological constant) the universe is accelerating. The topic is still under debate.

    I hope this answers :), if you look for more details, just ask :)

    Good luck :)
     
  10. Aug 26, 2009 #9
    Isn't the repulsive nature of the strong nuclear force just a manifestation of the Pauli exclusion principle ? At least that explains why 2 neutrons can't stick together.
     
  11. Aug 26, 2009 #10
    Pauli principle applies on energy and momentum spaces, since they are quantised, I don't think it can be applied that way on space.

    I forgot to say that strong forces where studied in the idea of exchanging bosons classically by the scientist "Yukawa", he created the potential of the form V(r)=1/r exp(-r/r0). This form is totally classical and doesn't support repulsion.
     
  12. Aug 26, 2009 #11

    Vanadium 50

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    No. I can have two neutrons with opposite spins. Indeed, if there were no strong force, the dineutron would be electromagnetically bound, so the fact that it is unbound indicates that the strong force must have a repulsive component.
     
  13. Aug 27, 2009 #12
    This is great. Thanks everyone for your input. I have another related question.

    So does gravity have a minimum threshold? That is, does gravity affect atoms?

    KG
     
  14. Aug 27, 2009 #13
    Gravity is produced and affects anything with mass, so yes, atoms too. In General Relativity gravity isn't regarded as a force but as the geometry of space-time itself. Mass and energy (which are equivalent) curve space-time, and this curvature is gravity.
     
  15. Aug 27, 2009 #14
    Then gravity would also exist between an electron's orbit and its nucleus, yes?
     
  16. Aug 27, 2009 #15
    Yes, but it's so weak, it's negligible compared to the electromagnetic force. It's negligible compared to all of the other forces. Even the weak nuclear force is 10^25 times stronger. The EM force is 10^36 times stronger than gravity.
     
  17. Aug 27, 2009 #16
    we believe in evidences. and so far there is no evidence of gravity being a result of smallish electrons ... but all evidence of it being a handiwork of giant galaxies. the day we see that gravity actually is a conspiracy of electrons and protons, we would abandon newtons /einstiens theory.
     
  18. Sep 6, 2009 #17

    Danger

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    :confused:
     
  19. Sep 7, 2009 #18
    I have a question to:

    Why does everybody keeps seeing space as distance?
    If you would see space as something that belongs to matter you can solve everything.

    The big bang as well as the excellerating expanding of the universe

    Space has nothing to do with distance, time has something to do with distance.
     
  20. Sep 7, 2009 #19

    Vanadium 50

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    That's nonsense. Distance is how we measure space.
     
  21. Sep 7, 2009 #20

    Danger

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    Quite right. Space is generally considered to be the 'nothing' between two 'things', although technically those two 'things' occupy the space. I tend to think of it as a lake. If there are two boats, one is inclined to comment about there being a lot of water between them. In fact, though, both of them are in the water. That's a pretty lame analogy, but the best that I can come up with on short notice.
     
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