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Gravity question, very small distances

  1. Aug 5, 2011 #1
    It's my understanding that it is believed, though not proven, that gravity may be comparable in strength to the other fundamental forces at very small distances. Somehow justified by string theory... the universe looping back on itself, or ... whatever. Not the point.

    My question is, if gravity were comparable in strength to the other fundamental forces at very small distances, wouldn't that complicate our understanding of the nuclear force?
    Specifically, I'm referring to how protons don't repel each other within a nucleus. The repelling force is electromagnetism, but if gravity were as strong as electromagnetism at this distance...

    I'm not under the delusion that I've made some kind of discovery here, I'd just like you to clear up whatever i've misunderstood, so the universe makes sense again.
  2. jcsd
  3. Aug 5, 2011 #2
    It isn't. Gravity is very very weak compared to electromagnetism. There is a distinction to be made between the magnitude of a force due to certain considerations (large masses, short distances will naturally lead to stronger gravity forces) and the fundamental strength of interaction due to that force, measured by the gravitational constant G. G is very small and that's what leads people to say "gravity is a very weak force", even though it can keep planets in orbit.

    At subatomic levels the electrostatic repulsion is still (far) stronger than gravity (classically, both obey the 1/r^2 law so distance shouldn't matter), and the strong force is stronger than that. This is what keeps protons bound to a nucleus.
  4. Aug 5, 2011 #3
    The scale at which gravity becomes strong for elementary particles is the Planck length (see Wikipedia), which is much, much smaller than the size of a nucleus. So while protons are "very small" compared to the human scale, as far as gravity is concerned they are actually very large. The lesson here is that qualitative phrases like "very large" or "very small" depend on context; when thinking about a problem like this, always make sure to get the actual numbers.
    Last edited: Aug 5, 2011
  5. Aug 5, 2011 #4
    If the strength of electromagnetic force and gravity depends on distance by the 1/r^2, how does the strong nuclear force relate to distance? It seems the strong nuclear force loses it's power quicker than the electromagnetic force when increasing distance. What is the relationship between the force of the strong nuclear force and distance of the nuclei?
  6. Aug 5, 2011 #5
    The general feature is confinement: the force does not decay with distance. It's similar to a spring, if you stretch it, the restoring force increases. That's why you can't ever see lone quarks- trying to split a meson (quark antiquark) requires a lot of energy, enough to form another antiquark-quark pair. There's a nice illustration here:
  7. Aug 5, 2011 #6
    @Mike Pemulis: 10^-20 the diameter of a proton, eh?
    So because gravity becomes strong at this distance, smaller distances than this do not make physical sense... elegant.
    I'll rest easy tonight.

    @MikeyW Interesting, I was thinking the constant G broke at a planck length, but that was incorrect. (I didn't actually know the term 'planck length', so I said 'very small')
    But if it's not the constant that changes, and gravity is always getting stronger as you get closer, why do they say it 'becomes strong' at this distance? Stronger than what?
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