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Electromagentic Field vs. Gravity Field

  1. Jun 22, 2008 #1
    I read somewhere that at small atomic levels (quantum levels) the electric force is some 100,000 stronger than the force of gravity. But long range (hundred of millions of miles to light years), gravity is the dominant force holding objects together or close by.

    Why is this?

    Could it possibly be that these forces exert a type of wavelength (much like light) and gravity's wavelength could be equated to a large length like a radio wavelength, while the electric wavelength is short and choppy like that of a gamma ray's wavelength?
  2. jcsd
  3. Jun 22, 2008 #2
    In other words, does the concept of wavelengths and wavelength size have anything to do with why certain forces are more effective long-range than short-range?
  4. Jun 22, 2008 #3
    Forces are carried by particles, and as a first approximation, the potential experienced by two objects charged under said interaction is given by the Fourier transform of something like [itex]\frac{1}{k^2 - m^2}[/itex], where m is the mass of the force carrier. For the electromagnetic and gravitational forces, the particles in question, the photon and the graviton, are massless, and the fourier transform of [tex]\frac{1}{k^2}[/tex] is roughly [tex]\frac{1}{r}[/tex]. On the other hand, the weak and strong forces are mediated by massive particles, and so [tex]m^2 \neq 0[/tex]. The Fourier transform is now roughly [itex]\frac{e^{-m r}}{r}\,[/itex], in other words it is a short range force. This is the correct meaning of short versus long range.

    Now there is an additional effect, which has to do with the spin of the force carrier. In the case of the spin 1 force carriers, the photon and W bosons, the potential depends on the sign of the objects in question. Thus positive and negative particles attract, positive and positive particles repel, etc. As nature would have it, matter in the universe is largely neutral, with equal number of positive and negative charges spread out. Thus, over long distances, the electric force is very feeble, because objects are approximately neutral. This is known as screening. On short distances, however, minor irregularities (rubbing amber rods against cats, e.g.) give rise to powerful forces.

    This is contrasted to the graviton, which is spin 2, and couples to mass. Barring things with negative mass, the gravitational force is always attractive, and so there is no way for forces to cancel eachother out. You can have electrically neutral galaxies but not gravitationally neutral ones. This is fortunate otherwise the electric force would surely dominate.
  5. Jun 22, 2008 #4

    D H

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    That's a bit too complex an answer and misses the point. Atoms are electrically neutral, having the same number of electrons as protons. At large scale distances the electrical force from the protons in the nucleus of an atom pretty much counters that from the electrons. The one exception are atoms/molecules with dipole (or quadrupole, or higher order moments). The dipole moments of a collection of atoms/molecules of course have to be co-aligned or these too will cancel at large distances. Even then, these higher order moments are inversely proportional to the cube of distance.

    Gravity dominates on a large scale because there is no such thing as negative mass. There is nothing to counteract the cumulative gravitational attraction of mass.
  6. Jun 22, 2008 #5
    Didn't bother to read it, then?
  7. Jun 23, 2008 #6
    I just read somewhere actually, that at shorts distances, the electrical force is some 10 ^40th power stronger than gravity. Also that electrical charge (attraction and repulsion) is mainly cause because of the exchange of photons between the nucleus and it's electrons.

    However, some genius by the name of Theodor E. Kaluza came up with a unifying theory that linked the relation between gravity and electromagnetism essentially by adding an extra (fourth spatial dimension) to Einstein's Gen. Theory of Relativity which helped to link the two forces together.

    He basically said that there was a 4th "rolled up and hidden" dimension that was so small we couldn't see it and some atomic particles went through this "hidden space" and that gravitational forces would either cause them to repel or attract depending on the direction such particles such as the electron or proton went though n this 4th dimension.

    Electromagnetism was basically just gravity acting in this "4th dimension".

    Is this true?
  8. Jun 23, 2008 #7
    Kaluza's ideas are pretty neat and are actually used in some string theory contexts. However, there have been many attempts to extend it to include known particles and such attempts have failed. For instance, at the time I don't think he knew about spin or nuclear forces.
  9. Jun 30, 2008 #8
    I heard Klein -Kaluza theories account for spin and elctromagnetic repulsion and attraction.

    Something tells me this guy and a woman by the name of Lisa Randall are on to something.
  10. Jul 3, 2008 #9
    At this time, there are no universally accept theories that unify gravity with the other three interactions (strong, weak, electromagnetic). The two "hot" areas are string theory (there's actually several string theories) and quantum gravity. People are working on it, but there's nothing solid yet (these things do take time). As with all new theories, there are supporters and there are opponents.
  11. Jul 3, 2008 #10
    So I guess what Kaluza was proposing would fall into the category of "quantum gravity"? Yes...no?
  12. Jul 3, 2008 #11


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    No, it's a classical extension of general relativity. There are five dimensions, one more than usual. The position in the extra dimension is equated to charge, and Maxwell's equations come out neatly in the EOM. It's not truly a unification, just good packaging in my opinion.

  13. Jul 4, 2008 #12
    What is quantum gravity then? Does it involve extra dimensions in it? I would think it has something to do with "gravitrons" then.
  14. Jul 4, 2008 #13


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    I should point out it's 'gravitons'. The difference between a classical theory and its quantised version is tricky to explain. In principle one can take a theory like GR or Newtonian gravity and apply a process to it that turns it into a quantum theory. So classical mechanics can be quantised to give quantum mechanics. But this goes wrong with Newtonian gravity and GR and we are left with no quantum theory of gravity. String theories have loads of dimensions but I don't understand them, so I can't comment.

    There is research in quantising space-time itself, also some gauge theories of gravity that might work out.

    My feeling is that peoples expectations of quantum gravity are too high, and there may be no way to do, or no need to do it.

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