Dirac Equation and Planck-Scale Quantities
  This work investigates in which form quantities with Planck dimensions occur already in the common quantum theory with local Lorentz symmetry. Since such Planck quantities as Planck length or Planck mass involve the Planck constant h, the velocity of light c and the Newton gravitational constant G, the relativistic Dirac equation (h, c) in the Newtonian gravitational potential (G) can be considered as a test theory. The evaluation of the break-off condition of the power series of the radial energy eigenfunctions of a purely gravitational atom leads to exact terms for the energy eigenvalues E for various special cases of the quantum numbers N, k and n = N + |k|. It turns out that a meaningful atom model, based solely on Newtonian gravitational forces, can result if, inter alia, the test mass m in the gravitational field of the mass M is selected to be smaller than the Planck mass. 
Rainer Collier
gr-qc hep-th quant-ph