Is Loop Quantum Gravity trying to bring quantum mechanics from it's principles? Or is it taking ideas from quantum mechanics and merging them with GR to find a theory of quantum gravity?
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$\begingroup$ LQG tries to quantize gravity whilst keeping with the main tenet of GR, that is, background independence (as opposed to string theory). But it isn't really 'adding ideas from quantum theory to GR' or vice-versa. $\endgroup$– EletieCommented Dec 24, 2020 at 11:44
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2 Answers
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LQG in principle is trying to discuss gravity as a quantum field theory from a geometric perspective. It is not a "theory of everything" so it does not want to explain qm.
It evolved into a theory which is capable of discussing the evolution/interactions of a small (planck sized) chunk of space and also apply its principles to early universe physics (loop quantum cosmology, LQC).
If we have a working and renormalizable theory of quantum gravity then we can merge it with the standard model.
LQG is currently not trying to be such a theory, as quantizing gravity in itself is already hard enough of a problem!
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LQG came from work on spin networks, which posed a simple question: Suppose you were unaware of space-time and looked at networks of interacting particles with spin, would space-time or something similarly arise naturally from their description?
Penrose only applied QM principles, he did not modify QM and did not bring in any GTR; rather he hoped that he could derive GTR-like results from his model. In this sense, LQG is much more natural than string theory which "generalises" QM. On the downside, not all the desired GTR-like results have yet been derived.
