Brane gravitational extension of Dirac's 'extensible model of the electron'

A gravitational extension of Dirac's 'extensible model of the electron' is presented. The Dirac bubble, treated as a three-dimensional electrically charged brane, is dynamically embedded within a four-dimensional Z₂- symmetric Reissner-Nordstrom bulk. Crucial to our analysis is the gravitational extension of Dirac's brane variation prescription; its major effect is to induce a novel geometrically originated contribution to the energy-momentum tensor on the brane. In turn, the effective potential which governs the evolution of the bubble exhibits a global minimum, such that the size of the bubble stays finite (Planck scale) even at the limit where the mass approaches zero. This way, without fine-tuning, one avoids the problem so-called 'classical radius of the electron'.