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Spacetime and em

  1. Jun 2, 2008 #1
    In GR, gravity is seen as property of the geometry of spacetime (curvature) as opposed to a force field. Does this theory extend to electromagnetics ,i.e. could EM be described as a property of spacetime, or is EM strictly dealing with fields? Do light waves and gravitational waves travel in the same medium (spacetime), or separate mediums? Do gravitational waves travel at c, and if so , do all fundamental force waves travel at c, or just light? Also, is there any correlation between the ranges of EM and G being infinite as opposed to weak/strong forces (see link: en.wikipedia.org/wiki/Fundamental_forces#Overview)? I have no background in physics so any help/corrections would be greatly appreciated. thank you.
  2. jcsd
  3. Jun 2, 2008 #2
    well yes, gravity waves do travel at the speed of C. (don't get the waves confused with a gravitational feild itself tho o_O) i dunno about the rest. [:
    oh yeah forgot 2 add this too,
    well, i'm not sure if you really can say that the light uses the spacetime as a medium because when ther's no medium, light exists as photons, not waves (search planck's constant).
    Last edited: Jun 2, 2008
  4. Jun 3, 2008 #3
    By no medium do you mean no spacetime, or are you talking about a vacuum? If so isn't a vacuum still in spacetime?
  5. Jun 3, 2008 #4


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    Electromagnetic fields are treated like any kind of matter in GR, i.e. they appear in the stress-energy tensor, not in the metric. (The metric describes the geometrical properties of space-time, and that includes gravity. Actually that is gravity).

    A gravity wave is a propagation of a local distortion of the metric. An EM wave is a propagation of a local distortion of the EM field. It makes sense to say that space is the medium in which gravitational wave propagates, but it wouldn't make sense to say that about EM waves. Space is just a background for EM-waves, not a medium.

    Yes, gravitational waves travel at c (at least in GR).

    The range of the interactions correspond to the masses of the particles that that must be exchanged for (a quantum field theory version of) the interaction to take place. In the cases of gravity and electromagnetism, those particles are massless.

    I should mention that a quantum field theory of gravity isn't very useful since it lacks a mathematical property that the other QFTs have: It's not renormalizable.
  6. Jun 3, 2008 #5


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    In the 1920's people try to interpret electromagnetism as arising from curvature of spacetime the same way gravity arises from curvature of spacetime. The only problem is that nwo spacetime had to be five dimensional (four dimensiosn of space and one of time). It wa sthe curvature of the extra dimension of space that led to the electromagnetic force. This was an idea proposed by Kaluza. Then the question was: where is this fifth dimension of space that no one ever saw? Klein proposed that it was curled up to a very very small dimension. Look up "Kaluza-Klein" theories to learn more.
  7. Jun 5, 2008 #6
    If you want to have a geometric approach to gravitation, EM, weak and strong forces together you might want to lookup Connes' non commutative geometry. "A geometry without points" as one could say in popular terms. Connes models the uncertainty principle by using a noncommutative geometry. All 3 forces in effect become integrated with gravitation as 4 pseudo-forces just like gravity is a pseudo force under GR.

    For more particular inquiries about Connes' approach the "Beyond the Standard Model" subforum might be more appropriate.
    Last edited: Jun 5, 2008
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