Consider the uncertainty principle: dp * dx = hbar For photons we have the relation: E = p c Substituting into the above uncertainty principle: dE = hbar c / dx (1) As we look at a smaller and smaller piece of the zero-point field the (positive) energy diverges. But that energy has a mass equivalent which therefore has a negative gravitational potential self-energy, dP. dP = - G dM^2 / dx (2) As dx -> 0 then dP -> -infinity as fast as dE -> infinity so they cancel each other out. If we have: dE = -dP = dM c^2 and substitute this relation into (1) and (2) we get a relation for the length scale dx: dx = sqrt(G hbar / c^3) This is the Planck length. I would guess that space-time quantisation is equivalent to the zero-point energy at each point being cancelled out by its negative gravitational potential energy.