“Starts to play a role” means when the effect of gravity becomes significant to the quantum equations which currently don’t have any placeholder for gravity.

Just like Newton’s equations are accurate to describe gravity and motion but only because the full equations Einstein worked simplify to them at small speeds.

So similarly it would make more sense that quantum equations and relativity are two sides of the same coin and there’s some missing mathematical bridge where at small particle sizes it simplifies to quantum and at large particle scales gravity matters and we get Newton and then further relativity.

Using Occam’s razor that’s at least my prior vs some undefined mechanism of action where all possible interactions of every particle actually creates an infinite superposition of universes where that did happen and you get an infinite number of such superpositions ad infinitum. Much easier to believe the huge gaping hole between two very accurate theories explains how quantum mechanics turns into classical mechanics, especially since we know gravity exists, what it is, and quantum mechanics can’t explain gravity.

This is more or less Roger Penrose's position: https://web.archive.org/web/20150327124054id_/http://image.s...

He argues that the wavefunction collapses on a timescale of h-bar / E_G, where E_G is the gravitational self-energy of the quantum system.