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Confusion about gravity and the fundamental forces

  1. Apr 15, 2013 #1
    I understand that there are 4 fundamental "forces" of nature that we've identified; gravity, electromagnetism, and the two nuclear forces. However, I have been under the impression that gravity isn't a "force" in the same way that the others are. From what I figured, a force is something that has a field which permeates space - while gravity is the curvature of space/time itself. I suppose I was thinking that gravity feels like a force, even though it isn't really.

    Furthermore, I've found out that there is a hypothetical particle called the graviton which is analogous to the photon and electromagnetism. Why would a graviton even be necessary? I don't see how a gravitational wave needs anything to carry it.

    Adding to my confusion, the term "gravitational field". This is just a tool for explaining the interactions between masses, right? There is no literal gravitational field, right?
     
  2. jcsd
  3. Apr 15, 2013 #2

    rbj

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    i don't think that the hypothetical graviton is necessary from the POV of General Relativity. from the GR POV, gravity isn't a force per se, but is the consequence of curved spacetime as we would observe from the POV of the normal Euclidian 3-dimensional space we normally think of ourselves in.

    but even so, it's an "interaction". the presence of mass curves spacetime.

    it's just as well to think of it as a force (in the sense that Newton did) when comparing to EM, weak, and/or strong.
     
  4. Apr 15, 2013 #3
    Welcome to the club.
    Nobody has ever seen a 'field' [wave]....it's a theoretical structure we imagine. The electromagnetic quantum is the photon, the gravitational quantum is the graviton which is many thousands of time weaker. Quantum are the local energy excitations of waves...stuff we can detect, like in a geiger counter.

    We think there are 'fields' [waves] involved but even in the famous double slit experiment reveals particles...but the pattern of particles strongly indicates underlying waves. So the photon 'carries' the EM field, the graviton the gravity field in quantum mechanics. We use fields to describe forces that interact at a distance. With a push or pull type force, classical physics doesn't require that.

    In general relativity there is no explicit agreed upon 'field'...different people mean different curvatures..... the curvature of spacetime. As Wheeler said "Mass[energy] tells spacetime how to curve; spacetime tells mass how to move." Gravitational curvature in Einstein's theory is spread among different mathematical terms, different pieces give different information. Curvature is complex.

    Gravity is different because it is the only fundamental 'force' that curves spacetime. The other forces have energy and that energy also has gravitational effects, but the characteristic forces themselves only act on certain particles...like the EM force acts only on charged particles.

    It's a good bet all the forces are more closely related than we currently understand as it's likely they all originated with the big bang...as did space,time, etc. Everything is thought to have been one high energy [unified] entity which decayed because it was unstable...out popped our universe and all it's building blocks... All these entities that we observe today appear separate; exactly how they are all related is a work in progress....one name for that idea is 'unification'.
     
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