Determine when quantum gravity becomes necessary

Determine the characteristic scale at which low-energy quantum gravity (the effective field theory combining quantum mechanics and general relativity) becomes internally inconsistent and must be replaced by a high-energy theory of quantum gravity; in particular, ascertain whether this onset scale is approximately equal to the Planck length or differs from it.

Background

The paper defines “quantum gravity happens” as the scale at which present theories become internally inconsistent and require replacement by a high-energy theory. Across Sections 4.1–4.5, the author evaluates five common lines of argument—dimensional analysis, quantum black holes, generalized uncertainty principles, nonrenormalizability in effective field theory, and beyond-the-standard-model proposals—and concludes that none suffices to establish that the breakdown occurs at the Planck scale.

Given the insufficiency of these arguments, the author explicitly states that the timing (scale) of the onset of quantum gravity remains unknown, leaving a central unresolved question about whether the Planck length is the relevant boundary or merely a lower or upper bound.