For decades, physicists have tried to quantize gravity like the other three fundamental forces, but all approaches (string theory, loop quantum gravity, gravitons) remain incomplete or unverified. If gravity were truly a separate fundamental force, one would expect at least one consistent quantum theory by now. Its failure to unify suggests gravity may not be independent at all, but instead an emergent effect of atomic charges — a hidden form of electromagnetism.
The fundamental difference between gravity and the other interactions is that the gauge group of gravity, the group of diffeomorphisms, is a noncompact group, whereas the gauge groups of the other interactions are compact groups. It's not yet known how a gauge theory with a noncompact group can emerge as the classical limit of a quantum theory. That's the problem of quantum gravity.
The answer to
“Why has quantum gravity not succeeded if gravity is truly separate from other forces?”
[“bold” -SS]
- is as: “quantum gravity fundamentally cannot be succeeded if gravity is truly separate from other forces”;
- while really Gravity fundamentally truly is a separate fundamental Nature force as that 3 other, i.e. Weak, Electric, and Nuclear/Strong, Forces are; and so can be without any principal problem “quantized” - as that happens with the other Forces.
That is another thing, that the quantizations of other Forces in mainstream physics really are fundamentally strange, say, in QFTs the Forces mediators are “virtual” – while really, of course, the mediators are real, etc.
Really scientific models of Gravity, Electric, and Nuclear/Strong Forces are given in the Shevchenko-Tokarevsky’s Planck scale informational physical model, see https://www.researchgate.net/publication/391209088_The_Informational_Conception_and_Basic_Physics, section 6. “Mediation of the fundamental forces in complex systems”.
More see the link, to read SS post in
https://www.researchgate.net/post/Is_there_a_legit_branch_of_physics_called_physics_of_quantum_gravitational_particles ,
and a series of posts in https://www.researchgate.net/post/Do_Static_Gravitational_Fields_Propagate/2
it is useful as well.
Cheers
Who says quantum gravity hasn't been successful?
First of all, quantum gravity does not yet fully exist and has not been worked out in detail.
In my published article "Quantum Gravity" I have outlined a valuable path toward a detailed elaboration.
The article is to find by "figshare". To do this you have to enter by Google my name: "Friedhelm Jöge" .
Secondary, why should gravity be separate from other forces?
Who says that?
In my opinion it is linked to quantum theory.
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