1) The Planck mass is a unit, not a theoretical concept, so this is only useful with respect to derivation writing convenience, not more.

2) A key to quantum gravity would be something that gives us guidance how to get to an acceptable unification of quantum mechanics and general relativity. Staring at E=mc² won't help us to achieve anything in that regard because it is just the "zero momentum" case of E=(p²c²+m²c4)1/2. The only thing it will be useful for: a quantum gravity theory should also have E=mc² as the borderline case in the macroscopic case of zero motion.

Really both - Planck mass and Einstein mcc - aren’t some specific key to quantum gravity.

Planck mass is the rest mass of hypothetical now “Planck mass particle” , and this particle in this sense doesn’t differ from any other particle in Matter, if we say about Gravity – in Matter everything gravitationally attracts everything, tough with different strength. As well as there is no any principal problems with “quantum Gravity” at all - Gravity is fundamentally nothing else than some fundamental Nature force, and so, as all other, i.e. Weak, Electric, and Nuclear/Strong, Forces, acts in the Matter fundamentally absolute, fundamentally flat, fundamentally continuous, and fundamentally “Cartesian”, spacetime, which fundamentally cannot be curved, contain some energy/stresses, etc.,

- as the other Forces have no problems in this case. More about what really is Gravity [and Electric, and Nuclear Forces as well] Force see the Shevchenko-Tokarevsky’s Planck scale informational physical models of the Forces in

https://www.researchgate.net/publication/383127718_The_Informational_Physical_Model_and_Fundamental_Problems_in_Physics “Mediation of the fundamental forces in complex systems”

Though because of the Gravity Force’s extreme weakness all known now particles cannot compose some typical QM systems, say, “gravitational atoms” [an example see in

https://www.researchgate.net/publication/388922029_The_informational_physical_model_detection_of_dark_matter_particles ],

- though from this seems it is an unique exclusion – the Planck mass particle that has extremely large, comparing with al known particles , rest mass ~ 1019 GeV/c2, what is more than in 1019 times larger than nucleons rest masses; and so which rather probably can compose some “gravitational atoms” with protons, where will be some discrete energy QM levels, etc.; however i these particles exist, their density in Space is extremely small [again see the last link], and so this point really is rather inessential in ordinary matter.

However in high density cosmological objects the quantum nature of Gravity can be rather actual, more see SS posts in https://www.researchgate.net/post/Why_is_it_difficult_to_quantize_gravity/2

The fact that particles have energy E=mc2, has no specific relation to Gravity, so here only note, that there is no problem with both - macroscopic and QM cases of “zero motion”. More in detsil see in first sections [about what is QM] in]the first link.

Cheers

The Planck mass and Einstein's famous equation E=mc2E=mc2 are indeed essential components when discussing quantum gravity, but whether they are the "key" depends on the specific approach and perspective one adopts in the field.

Planck Mass: The Planck mass (mP=ℏcGmP​=Gℏc​​) is often considered a fundamental scale in quantum gravity theories. It's approximately 2.18×10−82.18×10−8 kg, which is around 1.22 × 10^19 GeV in energy terms. At this scale, quantum gravitational effects are expected to become significant, as both quantum mechanics and general relativity would likely need to be unified. The Planck mass sets a boundary between the macroscopic (classical) world and the quantum world, where a quantum theory of gravity would become necessary. It's also related to the energy scales at which phenomena like black holes and early universe physics would be important.

Einstein’s E=mc2E=mc2: This equation relates mass and energy, which is foundational to both special relativity and quantum mechanics. In quantum gravity, understanding how mass and energy interact within the context of spacetime curvature and quantum fields is crucial. While this equation itself isn't specifically a quantum gravity theory, the relationship it expresses underpins much of the behavior of matter in gravitational fields, particularly when combined with other relativistic or quantum frameworks.

However, quantum gravity is still an unsolved problem in theoretical physics. The key to understanding quantum gravity lies in developing a theory that successfully incorporates both quantum mechanics and general relativity.

Approaches like string theory, loop quantum gravity, and others attempt to incorporate quantum gravitational effects, but there's no single, universally accepted theory yet. The Planck mass provides a scale where quantum gravitational effects would become noticeable, but it's not the complete theory itself.

The Planck mass doesn't exist. It is based on big-G, which is not a thing, because big-G assumes that gravity depends on mass, which it doesn't.

Great question, and definitely one that sits at the intersection of known physics and what comes next.

The Planck mass and E = mc² do seem to define the energy scale where gravitational and quantum effects converge, but they don’t offer a mechanism for how spacetime behaves at that junction.

My own work explores an approach where time itself is a dynamical field, not just a parameter. When treated this way, the evolution of mass-energy becomes constrained by the structure of time, not just geometry. This leads to testable corrections in quantum systems (like hydrogenic energy levels), and suggests new ways to model irreversibility, decoherence, and memory in a gravitational context.

So yes, they’re foundational. But I believe we need to go beyond them and rethink how time, energy, and information co-evolve to truly unify the framework.