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I 4 forces or 3?

  1. Sep 14, 2016 #1
    If what we call gravity is really the effect of mass-curved space time, then gravity, as a force itself, doesn't exist. The mis-definition is very similar to when people watched the sun arc across the sky and concluded that it must be revolving around the earth.

    So we have never been able to find the elusive forth force because, under the current theory, there is no force to find.

    How does all of this change our idea of what it is we are actually looking for in order to find the grand unifying force?

    And, what is the mechanism that makes mass distort space time? What is the connecting "thing" that makes space-time take more notice of a very massive object than a much less massive object?

  2. jcsd
  3. Sep 14, 2016 #2
    Hello, tex,

    I hope my opinion and limited information on this can add to your post. As far as I know (which is very little) gravity can be treated mathematically as an intrinsic property of objects which distorts spacetime, or a force that attracts masses to each other (with the appropriate relativistic compensations in the equations) to yield the same results. Matter distorts, bends, curves, generally interacts with spacetime to give forces that act on masses in the same way charges interact with the electromagnetic field. This is closely related to Quantum field theory, and how fields are bent to give forces. The graviton hypothesis relates the QM interaction of gravity and electromagnetism, maybe this is your "connecting thing". You may also, out of interest, want to research the Quantum Mechanical Lagrangian, a formulation that considers all forces using their potentials, maybe this is unifying the forces to some extent?

    I hope this helps, please pardon gaps in my knowledge, I am still learning.
  4. Sep 16, 2016 #3


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    Staff: Mentor

    I can't answer the other questions in your post, but the answer to this one is simple. We don't know. The field equations for General Relativity can be used to calculate how spacetime should curve under the influence of stress and energy, but they do not suggest any means by which spacetime "knows" to curve. At this point, all we can say is that this is just the way the universe appears to work.
  5. Sep 16, 2016 #4


    Staff: Mentor

    What do you mean by "the elusive fourth force"?

    If you mean the "grand unifying force" that is supposed to unify gravity with the other "forces", then you are incorrect that there is no such thing to find. Even though, in GR, we account for gravity as spacetime curvature, GR is not a theory of everything. We know it's incomplete, because it doesn't include quantum effects. So even if GR says that gravity isn't a force, we can't take that as the final answer; we have to look further.

    We already know how to model the other "forces" (a better word might be "interactions", which is actually the one that's more often used in particle physics) as quantum fields; and we also know that it is possible to model gravity (with some technical caveats that I don't think are relevant here) as a quantum field, and that the field equation for this field, in the classical limit, turns out to be the Einstein Field Equatino, i.e., the same equation that is used in GR. So it's entirely possible that the GR model of gravity as spacetime curvature is not fundamental, but a low energy classical approximation to some deeper theory. And given the history of how our quantum field theories of the other interactions have developed, it is reasonable to expect that such a deeper theory will also unify gravity with the other interactions.
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