Why has it been so hard to cominbe Gravitational Forces with

  • #1
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Main Question or Discussion Point

Why has it been so hard to combine gravitational forces with that of electrical forces?

I'm curious. What are the major underlying problems?
 

Answers and Replies

  • #2
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because i guess laws of gravitational forces apply to both large and small particles but the laws of electric forces apply to only point charges
 
  • #3
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I am in way over my head here, but perhaps because we can "find" electrons & protons, but we haven't found the elusive monopole, the "graviton".

Fish
 
  • #4
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I am in way over my head here, but perhaps because we can "find" electrons & protons, but we haven't found the elusive monopole, the "graviton".

Fish
 
  • #5
K^2
Science Advisor
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Electrodynamics is described properly with Quantum Electrodynamics. QED is a subset of RQFT which can be used to describe electromagnetic, strong, and weak nuclear forces. One of the main assumptions of RQFT is that fields obey the principle of superposition.

Gravity is described by General Relativity. One of the features of GR is that it is non-linear. That is, within GR principle of superposition does not work.
 
  • #6
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In General Relativity, gravitational force is caused by curvature in the space-time continuum which is supposed to be just that, a continuum. The Standard Model of forces by which the Electromagnetic, Strong and Weak forces abide states that forces are explained by quantum interaction of particles exchanged between objects acting on each other through one of these forces, thus, the strength of these forces come necessarily in quantized amounts, discrete bundles, if you will. This directly contradicts General Relativity. The reason we cannot simply adapt General Relativity to work with the Standard Model is because we have not found the particle which mediates gravity. There are also the problems of Dark Energy and Dark Matter yet to deal with.
 
  • #7
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In General Relativity, gravitational force is caused by curvature in the space-time continuum which is supposed to be just that, a continuum. The Standard Model of forces by which the Electromagnetic, Strong and Weak forces abide states that forces are explained by quantum interaction of particles exchanged between objects acting on each other through one of these forces, thus, the strength of these forces come necessarily in quantized amounts, discrete bundles, if you will. This directly contradicts General Relativity. The reason we cannot simply adapt General Relativity to work with the Standard Model is because we have not found the particle which mediates gravity. There are also the problems of Dark Energy and Dark Matter yet to deal with.
So then does Newtonian gravity able to blend in with the forces of electromagnetism?
 
  • #8
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So then does Newtonian gravity able to blend in with the forces of electromagnetism?
No, the Newtonian view of gravity also allows for any value of force, so it is not quantized, like the Standard Model predicts it should be.

It IS possible to unite gravity with the other three forces, and we have done it, in fact, but only in theory. If we could find evidence of gravitons, it would bridge the gap between quantum gravity and general relativity quite elegantly AND would allow gravity to fit in with the other three fundamental forces. It wouldn't mean GR is wrong, just as Newton's theory of gravity is not wrong, but only applicable on a certain scale. There are other theories which only attempt to reconcile GR at a quantum level but do not try to unite gravity with the other forces, like Loop Quantum Gravity. We do not necessarily know that Gravity must fit into the Standard Model, but many (I think probably most) scientists believe it should.
 
  • #9
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It IS possible to unite gravity with the other three forces, and we have done it, in fact, but only in theory.
We have? Can u please show me. :confused:

Oh, are you talking about the standard model?
 
  • #10
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The other forces are mediated by measurable quantities of energy. Gravity is mediated just by spatial distortions, which we are not able to measure as physical quantities.
 
  • #11
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We have? Can u please show me. :confused:
Sure, for example, String Theory has done it, mathematically. However, String Theory has not been experimentally tested yet, and there are still problems to work out. The discovery of the graviton would basically prove that gravity could be quantized, here's what wikipedia has to say about it:

contrary to the popular claim that quantum mechanics and general relativity are fundamentally incompatible, one can demonstrate that the structure of general relativity essentially follows inevitably from the quantum mechanics of interacting theoretical spin-2 massless particles [2][3][4][5][6] (called gravitons).

While there is no concrete proof of the existence of gravitons, quantized theories of matter may necessitate their existence.[citation needed] Supporting this theory is the observation that all other fundamental forces have one or more messenger particles, except gravity, leading researchers to believe that at least one most likely does exist; they have dubbed these hypothetical particles gravitons. Many of the accepted notions of a unified theory of physics since the 1970s, including string theory, superstring theory, M-theory, loop quantum gravity, all assume, and to some degree depend upon, the existence of the graviton.
 
  • #12
There's also the very pragmatic consideration of the impossibility of getting data to verify a theory. Gravity is extraordinarily weak relative to something like the EM force thus its effect at the quantum level is for all possible intent and purposes non-existent (any effect it would have would be ludicrously far back in terms of decimal places). Similarly, we know from Quantum Mechanics that as we move to larger and larger systems something called decoherence tends to wash out any quantum mechanicness such that at the scale of galaxies and solar systems (where GR is experimentally verified) its presence is just as meaningless as GR was at the quantum level. Any situation where both GR and QFT are important (like black holes or the big bang) are of course outside of our ability to ever experiment with. Thus, all mathematical considerations aside merging the two theories in such a way that we could verify our correctness is quite the tall order.
 
  • #13
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There's a conceptual gap too, whereas the Electronuclear forces act between objects, Gravity acts between objects using the space time continuum, so are we sure Gravity should conform to the Standard Model? It would seem to me as though Gravity was inherently different from the other three fundamental forces, this would also explain its weakness.
 

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