- #1
inflector
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In another forum that I frequent, I have been having a discussion where the state of quantum gravity research came up. Another poster claimed that one of the first thing that any gravity scientist checks for the theory is the theory's prediction for the precession of Mercury's perihelion.
Now for cosmological extensions to GR like, MOND, TeVeS or STVG, I can see where this might make sense. But I can't see how this poster's claim holds for quantum-gravity theories that haven't been able to establish that they reproduce GR in the continuum limit.
So I feel like I'm either missing something important about quantum gravity theories, in general, or the poster's comment only applies to gravity modifications designed as an alternative to dark matter and not quantum gravity theories. Or perhaps a little of each.
How, for instance, would one compute the precession of Mercury's perihelion using CDT, or Rovelli et al.'s spin-foam version of LQG? It may be that the dynamics of any object are implied in the spacetime microstructure, I don't understand the math well enough yet to follow the implications to know if this is true. But how can you compute the orbit of a planet when you require mass and fermions in order to have a sun and a planet like Mercury?
I noted the recent paper Dec. 21 paper where the Marseille LQG group claims to incorporate Fermions, so my question applies to research prior to this event, not that going forward.
Now for cosmological extensions to GR like, MOND, TeVeS or STVG, I can see where this might make sense. But I can't see how this poster's claim holds for quantum-gravity theories that haven't been able to establish that they reproduce GR in the continuum limit.
So I feel like I'm either missing something important about quantum gravity theories, in general, or the poster's comment only applies to gravity modifications designed as an alternative to dark matter and not quantum gravity theories. Or perhaps a little of each.
How, for instance, would one compute the precession of Mercury's perihelion using CDT, or Rovelli et al.'s spin-foam version of LQG? It may be that the dynamics of any object are implied in the spacetime microstructure, I don't understand the math well enough yet to follow the implications to know if this is true. But how can you compute the orbit of a planet when you require mass and fermions in order to have a sun and a planet like Mercury?
I noted the recent paper Dec. 21 paper where the Marseille LQG group claims to incorporate Fermions, so my question applies to research prior to this event, not that going forward.